Synthesis of Polycaprolactone Polymers for Bone Tissue Repair · Synthesis of Polycaprolactone...
Synthesis of Polycaprolactone Polymers for Bone Tissue Repair John Michael Colwell B. App. Sc. (Chemistry) (Hons) A thesis submitted for the degree of Doctor of Philosophy School of Physical and Chemical Sciences Queensland University of Technology December 2006
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Transcript of Synthesis of Polycaprolactone Polymers for Bone Tissue Repair · Synthesis of Polycaprolactone...
Synthesis of Polycaprolactone
Polymers for Bone Tissue Repair
John Michael Colwell B. App. Sc. (Chemistry) (Hons)
A thesis submitted for the degree of Doctor of Philosophy
School of Physical and Chemical Sciences
STATEMENT OF ORIGINAL AUTHORSHIP The work contained within this thesis has not been previously submitted for a degree or
diploma at any other higher institution. To the best of my knowledge and belief, the
thesis contains no material previously published or written by another person except
where due reference is made.
John Colwell
I would like to thank my supervisory team: Dr Edeline Wentrup-Byrne, Prof Franois
Schu, and Prof Graeme George, for their help and support throughout my PhD. It was
a pleasure working with all of them and I would especially like to thank Franois for his
hospitality during my stay in Montpellier.
It would have been impossible to conduct the necessary analyses of my materials
without the help of the various technical and research staff in both QUT, and UQ. My
thanks go to: Dr Llew Rintoul, Dr Barry Wood, Dr Greg Cash, Loc Doung, Dr Thor
Bostrom, Pat Stevens, Dr Chris Carvalho, and Karl Jacques.
I would also like to thank everyone at the Laboratoire de Chimie Macromoleculaire,
Universit Montpellier II, for their hospitality during my three month research visit,
April July, 2004.
My thanks also go to Freddy De Filipis and Rackel San Nicolas, two visiting students
from Polytech Montpellier, who helped with the model mineralisation studies.
Last, but not least, I would also like to thank my fellow postgraduate students who have
provided an interesting and supportive work environment during my time at QUT.
PUBLICATIONS AND CONFERENCE
PRESENTATIONS RELEVANT TO THIS THESIS
J. M. Colwell, E. Wentrup-Byrne, G. George and F. Schu, Synthesis of
polycaprolactone-block-poly(ethylene glycol)-block-polycaprolactone triblock
copolymers using a calcium-based initiator, Macromolecular Chemistry and Physics (in
preparation).
J. M. Colwell, E. Wentrup-Byrne, G. George and F. Schu, The effect of calcium
residues on the in vitro mineralisation of polycaprolactone, Biomaterials (in
J.M. Colwell, E. Wentrup-Byrne, and F. Schu, Synthesis of PCL/PEG/PCL Block
Copolymers Using a Calcium-Based Initiating System, 27th Australasian Polymer
Symposium, Adelaide, Australia (November 2004).
John Colwell, Edeline Wentrup-Byrne, Graeme George and Franois Schu, Synthesis
of polymeric tissue scaffolds using a biocompatible calcium initiator, 26th Australasian
Polymer Symposium, Noosa, Australia (July 2003).
J.M. Colwell, N. Guerrouani, F. DeFilippis, E. Wentrup-Byrne, A. Mas, G. George, and
F. Schu, Mineralisation of Polycaprolactone Effected by Calcium Initiator Residues, 7th
World Biomaterials Congress, Sydney, Australia (May 2004).
E. Wentrup-Byrne, J.M. Colwell, K.A. George, and F. Schu, Controlled Polymer
Synthesis for Craniofacial Applications, Australian Society for Biomaterials 14th Annual
Conference, Adelaide, Australia (March 2005)
Polycaprolactone (PCL) is a biodegradable synthetic polymer that is currently used in a
number of biomedical applications. A number of concerns have been raised over the
toxicity of initiators commonly employed for the synthesis of PCL. Therefore, more
biocompatible initiators have been studied. The biocompatibility of PCL, itself, is
adequate; however, improved bioactivity is desirable for several applications.
Copolymerisation, and incorporation of bioactive fillers can both be used as ways of
enhancing the bioactivity of PCL. Therefore, the global objective of this project was to
enhance the bioactivity of PCL by copolymerisation of PCL with poly(ethylene glycol)
(PEG) using a biocompatible calcium-based initiator. This calcium-initiator was
expected to leave potentially bioactive calcium-initiator residues in the synthesised
copolymers.
A study of the ring-opening polymerisation of -caprolactone (CL) in the presence of a
poly(ethylene glycol) (PEG) / calcium hydride (CaH2) co-initiation system was
performed. Polymerisation kinetics were monitored by following the degree of
conversion of CL by Fourier transform-Raman (FT-Raman) spectroscopy and 1H
nuclear magnetic resonance spectroscopy (NMR). Resultant PCL-b-PEG-b-PCL
(PCL/PEG/PCL) triblock copolymers were analysed by NMR and gel permeation
chromatography (GPC).
The observed rates of polymerisation for the synthesis of PCL/PEG/PCL triblock
copolymers using the PEG / CaH2 co-initiator were much lower than expected. 1H NMR
and Raman microspectroscopy analysis showed that the concentration of the active
calcium-PEG alkoxide was much lower than the initial feed concentration of PEG. Even
so, the molecular weight of PCL/PEG/PCL triblock copolymers could be predicted from
the CL : PEG feed ratio. This was found to be due to a fast reversible transfer process.
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis of
solutions containing acid digested, pure PCL/PEG/PCL copolymers showed calcium
concentrations at 77 % of the calcium feed concentration. These calcium-initiator
residues were isolated and their structures confirmed by Fourier transform infrared-
attenuated total reflectance spectroscopy (FTIR-ATR). They were found to be a mixture
of calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3).
The effect of calcium-initiator residues on the in vitro mineralisation of PCL/PEG/PCL
triblock copolymers, as well as the same effect on a model calcium-salt-doped PCL
homopolymer system, was studied by immersion in simulated body fluid (SBF). In the
model studied, PCL homopolymer was doped with low concentrations (0.2 2 w / w %
Ca) of Ca(OH)2, or CaCO3. Results from the model study showed calcium phosphate
(CaP) mineral deposition on Ca(OH)2-doped PCL, and not on CaCO3-doped PCL. This
was attributed to the higher solubility of Ca(OH)2, compared to CaCO3. Minimal CaP
deposition was observed on PCL/PEG/PCL triblock copolymers. This was attributed to
the low Ca(OH)2 concentration in these samples. For all mineralised samples in the SBF
studies, the formation of carbonated HAP was observed.
Overall, the synthesis of PCL/PEG/PCL copolymers using the PEG / CaH2 co-initiator
was found to be a suitable method for preparing reproducible materials. The calcium-
based initiator was also found to have potential for increasing the bioactivity of PCL-
based materials.
STATEMENT OF ORIGINAL AUTHORSHIP II
ACKNOWLEDGEMENTS III
PUBLICATIONS AND CONFERENCE PRESENTATIONS
RELEVANT TO THIS THESIS IV
ABSTRACT VI
TABLE OF CONTENTS VIII
LIST OF FIGURES XII
LIST OF TABLES XVI
LIST OF ABBREVIATIONS XVIII
1 GENERAL INTRODUCTION 1
2 SYNTHESIS OF PCL/PEG/PCL TRIBLOCK COPOLYMERS 6
2.1 Introduction 6
2.1.1 Polyester Synthesis 6
2.1.2 Living Polymerisation 7
2.1.3 Synthesis of Poly(-hydroxy acid)s 9
2.1.3.1 Initiators / Catalysts for the Synthesis of Poly(-hydroxy acid)s 10
2.1.3.2 Transesterification Reactions 13
2.1.3.3 Aggregation Phenomena 14
2.1.4 Methods for Studying the Kinetics of Poly(-hydroxy acid) Synthesis 15
2.1.4.1 Nuclear Magnetic Resonance Spectroscopy (NMR) 16
2.1.4.2 Gel Permeation Chromatography (GPC) 16
2.1.4.3 Dilatometry 17
2.1.4.4 Infrared Spectrosopy (IR) 17
2.1.4.5 Raman Spectroscopy 18
2.1.4.6 Methods-of-Choice For This Study 18
2.2 Experimental 19
2.2.1 Materials 19
2.2.2 Methods 19
2.2.2.1 Syntheses of PCLx/PEG45/PCLy Copolymers 19
2.2.2.2 Real-Time Monitoring 21
2.2.2.3 NMR Characterisation of Initiator 22
2.2.2.4 Headspace Analysis 22
2.2.3 Measurements 22
2.2.3.3 FT-Raman Spectroscopy 24
2.2.3.4 Raman Microspectroscopy 24
2.3 Results and Discussion 26
2.3.1 Polymerisation Scheme 26
2.3.2 NMR Characterisation 28
2.3.3 PCLx/PEG45/PCLy Synthesis 32
2.3.3.1 Real-Time Monitoring of Polymerisation Kinetics 39
2.3.3.2 Analysis of Polymerisation Kinetics 41
2.3.3.3 Activation Energy 49
2.3.3.4 Aggregation 52
2.3.4 Elucidation of the Active Species Structure 56
2.3.4.1 1H NMR 56
2.3.5 Raman Microspectroscopy 59
2.3.6 Reversible Exchange 61
3 IN VITRO TESTING OF PCL/PEG/PCL COPOLYMERS 67
3.1 Introduction 67
3.1.1 Bone Tissue Repair 67
3.1.2 Modification of Polymeric Biomaterials 69
3.1.3 Biocompatibility and Bioactivity Testing 72
3.1.3.1 Method-of-Choice for This Study 74
3.2.2.1 Calcium-Doping of PCL Homopolymer 76
3.2.2.2 Isolation of Calcium-Initiator Residues 77
3.2.2.3 Melt-Pressing of Polymer Films 77
3.2.2.4 Simulated Body Fluid Study 77
3.2.2.5 Ca2+ Release Study 78
3.2.2.6 Calcium Content Analysis 79
3.2.3.1 Tensile Testing 79
3.2.3.2 Differential Scanning Calorimetry (DSC) 80
3.2.3.3 Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-
AES) 80
3.2.3.4 Flame Atomic Absorption Spectrometry 80
3.2.3.5 Scanning Electron Microscopy (SEM) 80
3.2.3.6 Energy-Dispersive X-ray Microanalysis (EDX) 81
3.2.3.7 Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance
(FTIR-ATR) 81
3.2.3.8 Water Contact Angle 81
3.3.1 Tensile Properties 83
3.3.2 Thermal Properties 85
3.3.3 Calcium-Initiator Residues 87
3.3.4 Ca2+ Release 90
3.3.5 SBF Study 91
3.3.5.1 Model Study: Calcium-Doped PCL 91
3.3.5.2 PCLx/PEG45/PCLy Copolymer Study 100
3.4 Summary 107
4 CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS 108
4.1 Conclusions 108
4.2 Future Research Directions 110
5 REFERENCES 112
Figure 2.1: Typical nM versus conversion plots for the synthesis of a polymer under
living conditions (----) compared to ideal polycondensation for an X-R-Y-type system
( ). Adapted from Odian, p.852 .....................................................................................8
Figure 2.2: Some poly(-hydroxy acid)s and their respective lactone monomers..........10
Figure 2.3: Tin(II)2-ethylhexanoate / alcohol co-initiated polymerisation of lactones,
adapted from Albertsson and Varma.10 NB: R 4 C......................................................11
Figure 2.4: Intra- and inter-molecular transesterification reactions of a polyester .........13
Figure 2.5: Heating and stirring controllers / devices for real-time kinetic studies by FT-
Raman spectroscopy.........................................................................................................21
Figure 2.6: Proposed polymerisation scheme for the synthesis of PCL/PEG/PCL using a
PEG / CaH2 co-initiator....................................................................................................27
Figure 2.7: Typical 1H NMR spectra of a crude PCLx/PEG45/PCLy polymerisation
mixture (upper) and a pure PCLx/PEG45/PCLy copolymer (lower). Peak assignments are
given in Table 2.3.............................................................................................................31
Figure 2.8: Fraction of theoretical CL content, [CL]t / [CL]0, versus time ([CL] was
determined from 1H NMR using PEG45 as an internal standard) for the synthesis of
PCL50/PEG45/PCL50 under an argon atmosphere in a dry box (note that the point near 0
min was taken two minutes after addition of CL to the pre-heated vessel) .....................32
Figure 2.9: ln [CL]0/[CL]t versus time plot for the synthesis of PCLx/PEG45/PCLy
copolymers; [CL]0 : [OH]0 : [CaH2]0 = 50 : 1 : 0.67 (), and [CL]0 : [OH]0 : [CaH2]0 =
100 : 1 : 0.67 (x) in flame-sealed glass tubes at 70 C. Error bars: 1 S.D., n = 1 3.
Where n = 1, the error bars represent the error inherent in the analysis technique..........34
Figure 2.10: ln [CL]0/[CL]t versus time plot for the synthesis of PCLx/PEG45/PCLy
copolymers; [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 3 (), and [CL]0 : [OH]0 : [CaH2]0 = 100
: 1 : 0.67 (x) in flame-sealed glass tubes at 70 C. Error bars: 1 S.D., n = 1 3.
Figure 2.11: Conversion versus time plot for [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 0.67, at
70 C (); 96 C (); and 128 C () .............................................................................36
Figure 2.12: GPC chromatograms of PCLx/PEG45/PCLy copolymers synthesised at 133
C. Upper: [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 3; tmax x 3. Lower: [CL]0 : [OH]0 :
[CaH2]0 = 50 : 1 : 3; tmax x 4.............................................................................................37
Figure 2.13: FT-Raman spectra. Upper: CL. Lower: PCL50/PEG45/PCL50 ..................40
Figure 2.14: Comparison of kinetic curves for the synthesis of PCLx/PEG45/PCLy at 128
C, [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 0.67, using FT-Raman monitoring (), and oil
bath heating / 1H NMR analysis () ...............................................................................41
Figure 2.15: Typical first-order kinetic plots, ln [CL]0/[CL]t versus time, for the
synthesis of PCLx/PEG45/PCLy using FT-Raman monitoring. [CL]0 : [OH]0 : [CaH2]0 =
50 : 1 : 0.67 () [CL]0 : [OH]0 : [CaH2]0 = 50 : 1 : 3 (). [CL]0 : [OH]0 : [CaH2]0 = 100
: 1 : 0.67 () [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 3 () ...................................................44
Figure 2.16: Typical second-order kinetic plots, 1 / (1 ) versus time, for the synthesis
of PCLx/PEG45/PCLy using FT-Raman monitoring. [CL]0 : [OH]0 : [CaH2]0 = 50 : 1 :
0.67 () [CL]0 : [OH]0 : [CaH2]0 = 50 : 1 : 3 (). [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 :
0.67 () [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 3 ()...........................................................44
Figure 2.17: Normalised polymerisation rate versus [CaH2]0 : [OH]0 for [CL]0 : [OH]0 =
50 : 1 at 3 polymerisation temperatures. 70 C (); 106 C (); 133 C () ...................48
Figure 2.18: Arrhenius plots for the synthesis of PCLx/PEG45/PCLy. Upper: [CaH2]0 :
[OH]0 = 0.67 : 1. Lower: [CaH2]0 : [OH]0 = 3 : 1 ...........................................................51
Figure 2.19: ln kobs versus ln [OH]0 for [CaH2]0 : [OH]0 = 0.67 : 1 at 128 C................53
Figure 2.20: ln kobs versus ln [OH]0 for [CaH2]0 : [OH]0 = 3 : 1 at 128 C.....................55
Figure 2.21: Upper: 400MHz 1H NMR spectrum of pure, dry tetraethylene glycol in
benzene-d6. Integration a : b : c = 1.00 : 2.05 : 6.00. Lower: 400MHz 1H NMR
spectrum of tetraethylene glycol heated at 70 C with CaH2 for 24 hrs, benzene-d6
extract. Integration a : b : c = 1.00 : 1.96 : 5.86 ..............................................................58
Figure 2.22: Raman spectra from air, and the headspace of a reaction vessel: [CL]0 :
[OH]0 : [CaH2]0 = 100 : 1 : 3, 133 C; maximum conversion of CL ...............................59
Figure 2.23: Graph of H2 production versus time from the reaction of PEG45 and CaH2
in a vessel sealed with a Teflon key and heated to 130 C (spectra were collected after
cooling to room temperature)...........................................................................................60
Figure 2.24: GPC chromatograms plotted as a function of conversion for the synthesis
of PCLx/PEG45/PCLy, [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 0.67 at different
polymerisation temperatures. Upper: 70 C. Lower: 96 C...........................................62
Figure 2.25: nM (1H NMR) versus for [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 0.67. (x)
96 C; () 128 C; () theoretical prediction based on a living polymerisation model63
Figure 2.26: nM (1H NMR) versus , for [CL]0 : [OH]0 : [CaH2]0 = 250 : 1 : 0.67. ()
128 C; () theoretical prediction based on a living polymerisation model...................64
Figure 2.27: PDI versus , for [CL]0 : [OH]0 : [CaH2]0 = 100 : 1 : 0.67 at (x) 96 C and
() 128 C.......................................................................................................................64
Figure 2.28: PDI versus , for [CL]0 : [OH]0 : [CaH2]0 = 250 : 1 : 0.67 at 128 C.........65
Figure 3.1: Typical stress-strain plots for melt-pressed PCLx/PEG45/PCLy copolymer
films, and a melt-pressed PCL film..................................................................................84
Figure 3.2: DSC thermograms of melt-pressed PCLx/PEG45/PCLy copolymers, PCL and
PEG ..................................................................................................................................86
Figure 3.3: FTIR-ATR spectra of calcium-initiator residues isolated by filtration. Top
to bottom: 5 X, 5 S, 4 X, 4 S............................................................................................88
Figure 3.4: Relevant calcium-alkoxide, and CaH2 reactions ..........................................89
Figure 3.5: Ca2+ release from Ca(OH)2- and CaCO3-doped PCL samples immersed in
ultrapure water, over a 14 day period...............................................................................90
Figure 3.6: EDX spectrum from CaCO3-doped PCL (0.2 w / w % Ca) after 14 days
immersion in SBF, showing the presence of Cl, in the absence of CaP ..........................92
Figure 3.7: SEM image showing spherical, and plate-like CaP growth on a Ca(OH)2-
doped PCL sample, 1 w / w % Ca, after 14 days immersion in SBF...............................93
Figure 3.8: Advancing and receding water contact angles for PCL, Ca(OH)2- and
CaCO3-doped PCL...........................................................................................................95
Figure 3.9: FTIR-ATR for Ca(OH)2-doped PCL after 14 days immersion in SBF........98
Figure 3.10: Percentage mass change of PCLx/PEG45/PCLy copolymers as a function of
SBF immersion time ......................................................................................................100
Figure 3.11: Advancing and receding water contact angles for PCL and
PCLx/PEG45/PCLy copolymers ......................................................................................101
Figure 3.12: SEM images (left), EDX spectra (right), showing CaP coverage of non-Si
contaminated SBF-treated samples. Top to bottom: series 4 X; 3, 9, and 14 days. ...104
Figure 3.13: FTIR-ATR spectra from series 4X after different SBF immersion times
........................................................................................................................................105
LIST OF TABLES Table 2.1: Typical feed ratios and quantities of reagents used for copolymer syntheses20
Table 2.2: Separation range of GPC columns .................................................................23
Table 2.3: 1H NMR (CDCl3) peak assignments for CL and PCLx/PEG45/PCLy.............30
Table 2.4: MWD data for PCLx/PEG45/PCLy copolymers synthesised at 133 C ..........38
Table 2.5: Summary of pseudo-first-order rate constants, kobs, for all reaction conditions
used throughout this study................................................................................................45
Table 2.6: Temperature study of the homo-polymerisation of CL with CaH2 ................47
Table 3.1: Reagent Purities.............................................................................................75
Table 3.2: Composition of Ca(OH)2-doped samples.......................................................76
Table 3.3: Composition of CaCO3-doped samples..........................................................76
Table 3.4: SBF Reagents .................................................................................................78
Table 3.5: PCLx/PEG45/PCLy copolymers selected for bioactivity testing .....................82
Table 3.6: Tensile properties of PCL, and PCLx/PEG45/PCLy copolymer melt-pressed
films..................................................................................................................................85
Table 3.7: Concentration of calcium in PCLx/PEG45/PCLy copolymers synthesised with
the CaH2 / PEG co-initiator..............................................................................................88
Table 3.8: Atomic calcium to phosphorus, Ca / P, and magnesium + calcium to
phosphorus, (Mg + Ca) / P, ratios for Ca(OH)2-doped samples ......................................93
phosphorus, (Mg + Ca) / P, ratios for CaCO3-doped samples .........................................94
Table 3.10: Ca / P ratios of CaP minerals129 ...................................................................96
Table 3.11: CaP phase determined from FTIR-ATR spectra of SBF-treated, Ca(OH)2-
doped PCL........................................................................................................................98
Table 3.12: Characteristic infrared frequencies (cm-1) for CaP minerals128,130 ...............99
Table 3.13: Atomic calcium to phosphorus ratio, Ca / P...............................................103
Table 3.14: Atomic magnesium + calcium to phosphorus ratio, (Mg + Ca) / P ...........103
Table 3.15: CaP phase determined from FTIR-ATR analysis of SBF-treated
AR Analytical Reagent
CaP Calcium Phosphate
CL -Caprolactone
DCM Dichloromethane
nDP Number Average Degree of Polymerisation
DSC Differential Scanning Calorimetry
EDX Energy Dispersive X-Ray
FTIR-ATR Fourier Transform Infrared-Attenuated Total Reflectance
GPC Gel Permeation Chromatography
HAP Hydroxyapatite
ICP-AES Inductively Coupled Plasma-Atomic Emission Spectroscopy
nM Number Average Molecular Weight
wM Weight Average Molecular Weight
MWD Molecular Weight Distribution
NMR Nuclear Magnetic Resonance
OCP Octacalcium Phosphate
PCL Polycaprolactone
PDI Polydispersity Index
PEG Poly(ethylene glycol)
PGA Polyglycolide
PLA Polylactide
SBF Simulated Body Fluid
SEM Scanning Electron Microscope
TMS Tetramethyl Silane
UV Ultraviolet
Chapter 1 General Introduction
1 GENERAL INTRODUCTION
Polycaprolactone, PCL, is a hydrophobic, slow-degrading synthetic polymer that is
widely studied because of its potential in a wide range of biomedical applications.
Among the most commonly reported are controlled drug delivery systems, and implants
for orthopaedic surgery.1 The PCL-based materials being fabricated in this study are
being targeted at applications concerning bone tissue repair. From a public health
perspective, the importance of bone tissue repair is statistically significant. At the time
of the national health survey of Australia in 1995, there were approximately 100,000
people with fractures and a further 1.8 million people with conditions that could be
attributed to bone injuries.2
Bone is a complex, composite material, composed of 65 % inorganic hydroxyapatites
and 35 % organic matrix (mostly collagen).3 The nature of this inorganic / organic
combination in bone results in strong, load-bearing biological materials. The difficulties
in manufacturing a similarly structured material with the same properties are enormous;
hence, traditionally, metal pins and plates have been used for bone repair and
replacement due to their inherent beneficial properties such as high strength. However,
there are intrinsic problems associated with their use, such as the need for revision
surgery,4 refracture upon removal of the implant,4 wear,5 high thermal and electrical
conductivity, poor tissue bonding, or tissue rejection,6 and issues with permanency in
juvenile patients. Acrylic polymers, especially poly(methyl methacrylate), have been
used as alternative materials to metal implants since World War II.6 They show
enhanced tissue compatibility, and have poor thermal and electrical conductivity.6
However, they too, are permanent implants and hence do not allow for significant
growth or change around implant sites.
In recent years a much greater emphasis has been placed on developing biodegradable
polymers for use as biomedical implants because they offer the possibility of tissue
ingrowth during resorption of the polymer, leaving a repaired wound site exclusive of
foreign material. Much of this recent interest has been due to the advent of tissue
engineering, a concept that combines the principles of biology and engineering to the
development of functional substitutes for damaged tissue.7 One of the main aims of
tissue engineering is to grow functional tissue substitutes, including complete organs.
To do this, specific substrates, or scaffolds need to be designed such that cell attachment
and maintenance of cell function can be supported.8 For such substrates and scaffolds,
synthetic biodegradable polymers hold several advantages over other materials.
Tailorable mechanical properties and degradation kinetics are two key advantages of
synthetic biodegradable polymers for tissue engineering applications.9 Other advantages
include: ease of fabrication into scaffolds of various shapes and sizes, ability to design
desirable pore morphology, and the ability to functionalise such polymers to induce
tissue ingrowth.9 There are also some limitations to the use of polymers in tissue
engineering. For instance, in load-bearing applications metal materials are still preferred
as many polymers have a low modulus and cannot offer the load-bearing support that
metal materials can. Therefore, low modulus polymers are used in non-load-bearing
applications, or as a component in a metallic device in load-bearing applications, for
example, in a total hip replacement device.
There are a number of synthetic biodegradable polymers presently being used for
various biomedical applications. Such polymers include: poly(-hydroxy acids),10
polyalkanoates,11 polyurethanes,1,12 polyorthoesters,13 polycarbonates,14 or copolymers
of these.15,16 The most widely researched of these in the biomedical field are poly(-
hydroxy acids).17 They were originally employed as resorbable sutures in the 1960s,10
and their application in the biomedical arena has grown to include drug delivery
vehicles,18,19 tissue regeneration scaffolds20,21 and other fixation devices.22 For
biomedical applications there are three main poly(-hydroxy acids): polylactide (PLA),
polyglycolide (PGA) and polycaprolactone (PCL). PLA, PGA and PLA / PGA
copolymers constitute the majority of poly(-hydroxy acids) in use.22 However, there
has been increased interest in the use of PCL in recent times.
PCLs low degradation rate (up to two years in vivo1) allows for slow tissue ingrowth in
sites, such as bone, where long-term remediation (4 8 weeks22) may be required. With
respect to bone tissue repair, PCL has been used as an injectable bone substitute,23 a
scaffold for bone-tissue engineering,24 and as an in-situ-polymerised material for
maxillofacial applications.25 Although PCL is known to elicit only a rather mild
inflammatory response,26 cell attachment and growth on PCL is limited due to its
intrinsic hydrophobicity.24 Copolymerisation of PCL with hydrophilic blocks such as
poly(ethylene glycol), PEG, has been performed as a means to enhance the
hydrophilicity of the parent, PCL homopolymer.27-30 This approach has led to enhanced
cell attachment, and accelerated hydrolytic degradation. At certain PCL : PEG
compositional ratios, PCL/PEG/PCL triblocks constitute an excellent support for human
endothelial cell adhesion and growth.28 In this study, two PCL/PEG/PCL copolymers
were compared, and the material with a higher PEG content was found to be a better
support for endothelial cell growth. PCL-PEG copolymers have also been shown to
support both human, and rat marrow stromal cell proliferation.31 It was found that the
PCL-PEG copolymers were better at supporting cell proliferation than the PCL
homopolymer itself. This was attributed to the greater hydrophilicity, and lower water
contact angle, of the PEG-containing copolymers. Since, PCL-PEG copolymers are
more hydrophilic than PCL homopolymer, it is expected that they should show a faster
rate of hydrolytic degradation, and this has been shown. Compared to PCL
homopolymer, the degradation of PCL-PEG copolymers has been shown to be
significantly faster, although still rather slow (60 weeks in PBS at 37C for a weight loss
of 7%).31 The major degradation products from the hydrolytic degradation of PCL-PEG
copolymers, have been found to be 6-hydroxyhexanoic acid (a naturally occurring
metabolite1) and a large amount of PEG-rich species,31 (PEG of relatively low molecular
weight is readily excreted through the kidneys).32 Since it is, in practice, not possible to
completely remove catalyst / initiator residues from synthetic polymers,33 the release of
such residues also inevitably occurs during degradation. Therefore, it is of great interest
and importance to synthesise polymers using catalysts / initiators that leave biologically
compatible residues.
A number of catalysts have been employed for the synthesis of PCL-PEG copolymers.
They range from the most commonly employed catalysts for the synthesis of poly(-
hydroxy acids) such as tin(II)2-ethylhexanoate,1,34,35 to biologically compatible initiators
such as calcium36 and zinc.37 Even though tin-based initiators are considered to be some
of the best initiators for the synthesis of poly(-hydroxy acid)s, concerns have been
raised over the potential toxicity of their residues.38,39 The toxicity of various organotin
compounds is well known,40 with the high toxicity of compounds such as tributyl tin
leading to restrictions on their use in anti-fouling paints.41 Tin(II)2-ethylhexanoate is
considered less toxic than organotin compounds and has been approved for use as an
anti-microbial agent by the FDA (United States food and drug administration) for use in
food stuffs.42 However, strict control over the exposure of people to this compound is
required and even though it has been approved for use in minimal quantities in food;
from an industrial perspective, there is a growing awareness about the dangers of using
compounds such as tin(II)2-ethylhexanoate in large scale production.42 Also, where the
materials are employed for biomedical applications it is particularly important to
maintain safe levels of toxic compounds, or more appropriately, exclude them
completely. As a result, there has been a move towards the development of potentially
more biocompatible initiators than those traditionally used for the synthesis of poly(-
hydroxy acid)s. This has resulted in the study of initiators for the synthesis of poly(-
hydroxy acids), based on metals (calcium, magnesium, iron and zinc) that participate in
human metabolism.33 The emergence of new initiation systems has in turn led to the
need to study the mechanism of such polymerisations in depth such that the
polymerisation processes can be exploited to give controlled, reproducible materials.
In this study, a previously described calcium hydride / PEG co-initiating system used by
Li et al.43 and Rashkov et al.44 for the synthesis of PLA/PEG/PLA block copolymers has
been applied to the synthesis of PCL/PEG/PCL block copolymers. The effect of varying
synthetic parameters such as temperature and reactant concentration has been studied in
more detail, as a means to elucidate the mechanism of polymerisation. Chapter 2 will
focus on the synthesis of PCL/PEG/PCL copolymers prepared with a PEG / calcium
hydride co-initiator. The mechanism of polymerisation will be discussed in terms of the
kinetics of polymerisation, formation of by-products, and resultant PCL/PEG/PCL
copolymer structure.
With the intention of using the synthesised PCL/PEG/PCL copolymers for bone tissue
repair, the effectiveness of these materials at nucleating hydroxyapatite, or other
calcium-phosphate (CaP) mineral phases in vitro has also been studied. By
incorporating a potentially active, biologically compatible, CaP-mineral nucleating agent
(calcium from the calcium-initiator) through a one-pot synthetic process this study not
only removes the problem of residual catalyst / initiator toxicity, it has also turned the
persistence of initiator residues into an advantage. Calcium ions, Ca2+, released from the
polymer matrix (due to the presence of calcium-initiator residues) in media, such as
simulated body fluid (SBF), can cause the nucleation of a CaP mineral phase,45 with the
formation of bone-like apatite (CaP mineral) on artificial materials being an established
condition for bonding to living bone.24 The enhanced water permeability of
PCL/PEG/PCL copolymers in aqueous media, compared to PCL homopolymer,27 should
aid in the diffusion of Ca2+ from the polymer matrix. Ultimately, it is hoped that the rate
of diffusion of Ca2+ from these materials will be sufficient to cause nucleation of a
uniform, bone-bonding or bone-growth-promoting, CaP mineral coating in vivo. A
preliminary investigation into the effect of calcium-initiator residues on the in vitro
(SBF) mineralisation of PCLx/PEG45/PCLy copolymers, synthesised in this study, and
commercially available PCL homopolymer (doped with calcium hydroxide, or calcium
carbonate) is presented in Chapter 3. The use of calcium-doped PCL homopolymer
served as a proof-of-concept for the action of calcium-initiator residues on the
mineralisation of the synthesised copolymers. Diffusion of Ca2+, from the calcium-
doped PCL, in an aqueous environment was also studied as a means to understand the
mechanism of CaP mineral formation on these materials.
2 SYNTHESIS OF PCL/PEG/PCL TRIBLOCK COPOLYMERS
Polymer syntheses can be classified through two general phenomena: step-growth
(condensation) and chain-growth (addition) polymerisation. Chain-growth
polymerisation involves the sequential addition of monomers to form a polymeric
species.46 Such examples include the polymerisation of styrene, by both free radical and
anionic mechanisms, and the ring-opening of cyclic monomers, such as trioxane to give
polyoxymethlyene.46 Under certain conditions, this can lead to a living polymerisation,
which will be discussed later. In contrast to chain-growth polymerisation, the
mechanism of step-growth polymerisation involves the random reaction of two, di-
functional molecules that may be any combination of monomers, oligomers or higher
molecular weight intermediates.46 The consequence of this is that polymers of high
molecular weight are only obtainable at high conversion, due to the greater probability
of the reactive ends of larger molecules meeting at this stage than when a large amount
of monomer is present. Some common examples of condensation polymers are: nylon-
6,6, polycarbonate and poly(ethylene terephthalate). Polyesters, which are of particular
interest for this study, can be prepared by either a step-growth or chain-growth
2.1.1 Polyester Synthesis
Step-growth polymerisation to prepare polyesters typically involves reaction between
difunctional monomers of the type X-R-X / Y-R-Y or X-R-Y, where X and Y represent
different functionalities that react to form an ester linkage. These functionalities ( X / Y,
or Y / X) are usually acid chlorides / alcohols or carboxylic acids / alcohols. Therefore,
the by-products of these reactions are respectively hydrochloric acid, or water. Being an
equilibrium reaction, these by-products often need to be removed to achieve high
conversion, hence high molecular weight.47 In contrast to condensation polymerisation,
ring-opening polymerisation (a chain-growth mechanism) can produce polymers of
relatively high molecular weight, without significant by-products, and may yield well-
defined structures if a judicious choice of initiator or catalyst is made.10 Polyester
synthesis by ring-opening polymerisation involves polymerisation of cyclic monomers
(lactones) through an ionic or coordination-insertion mechanism. Ionic polymerisation
may proceed by either a cationic or anionic ring-opening mechanism; however,
polymers of high molecular weight have only been obtained by anionic ring-opening
polymerisation.10 The sensitivity of ionic species to contaminants is the major drawback
of this technique. It is necessary to provide an oxygen-, water- and carbon dioxide-free
environment;48 ensuring that these contaminants are kept below parts-per-million levels.
Similarly to ionic polymerisation, coordination-insertion polymerisation catalysts can
also be poisoned by the introduction of contaminants such as water,10 but are generally
considered more robust than their ionic counterparts. If strict experimental conditions
are observed, then many anionic and coordination-insertion polymerisation catalysts
show the characteristics of a living polymerisation.
2.1.2 Living Polymerisation
The area of living polymerisation has expanded in recent times to meet the demand for
elegant polymers with tailored architectures that supply function through their structure.
Such examples, with respect to biodegradable polymers, include the synthesis of well-
defined copolymers and polymers with controlled architectures for drug delivery
applications.19
In the ideal living polymerisation, there are only two processes that occur: initiation and
propagation.49 When these are the only two processes that occur, the ideal
characteristics of a living polymerisation can be summarised as below.50,51
1. The polymerisation proceeds until all the monomer has been consumed, with further addition of monomer resulting in continued polymerisation
2. Number average molecular weight ( nM ) is a linear function of conversion
3. The number of polymer molecules (and active centres) is constant, which is independent of conversion
4. Molecular weight can be controlled by the stoichiometry of the reaction 5. Polymers with a narrow molecular weight distribution are produced 6. Block copolymers can be prepared by sequential monomer addition 7. Chain-end functionalised polymers can be prepared in quantitative yields 8. The kinetics of propagation should follow pseudo-first-order behaviour
Figure 2.1 illustrates one of the advantages of living polymerisation. It depicts typical
nM versus conversion plots for polycondensation and living polymerisation, showing
linearity for living polymerisation, and the dependence of high conversion for obtaining
high molecular weight in the case of polycondensation.
0 0.2 0.4 0.6 0.8 1
conversion,
Figure 2.1: Typical nM versus conversion plots for the synthesis of a polymer
under living conditions (----) compared to ideal polycondensation for an X-R-Y-
type system ( ). Adapted from Odian, p.852
In practice, not all polymerisation systems show all the living characteristics listed
above. In the case where no irreversible chain-breaking reactions occur they may be
more appropriately described as pseudo-, or quasi-living.49 Deviation from ideal living
characteristics can be caused by a number of phenomena. Such phenomena include,
reversible chain-breaking reactions (chain transfer and termination) and reversible
aggregation.49 Most commonly, for the case of living anionic polymerisation, ionic
aggregates will form, leading to an equilibrium between propagating, polymeric ions and
less reactive, non-propagating aggregates.49 The rate of polymerisation can then be
expressed, including the equilibrium constant (K) for such processes.49
[ ][ ] [ ][ ]( )KMN
kMPkR pppp +==
Equation 2.1
Where kp, [P*], [M] and [Np] are the rate constant of polymerisation, the concentration
of propagating species, the monomer concentration and the total concentration of
polymer molecules in the polymerisation system, respectively. As K approaches zero,
the system can be seen to become more living in nature.53,54 Depending on the nature of
the quasi-living system; ie, aggregative, terminative, or transferative,49 a number of
conditions can influence the magnitude of K. These may include monomer and solvent
type as well as the initiator used for the polymerisation.
2.1.3 Synthesis of Poly(-hydroxy acid)s
Poly(-hydroxy acid)s are generally synthesised by ring-opening polymerisation from
the respective lactone. In most cases, control of the polymerisation is obtained by
choosing the most appropriate initiator, or catalyst.
*O
O *
polycaprolactone polylactide polyglycolide
caprolactone lactide glycolide
Figure 2.2: Some poly(-hydroxy acid)s and their respective lactone monomers
2.1.3.1 Initiators / Catalysts for the Synthesis of Poly(-hydroxy acid)s
Initiator selection plays an important role in the control of polymer syntheses. This, in
turn, can affect both physical and chemical polymer properties, including: crystallinity,
melt and glass transition temperatures, molecular weight, molecular weight distribution,
end groups, sequence distribution, and the presence of residual monomer.55 Of
particular interest for the syntheses of poly(-hydroxy acid)s are initiators / catalysts that
show an anionic, or coordination-insertion ring-opening mechanism, due to their ability
to produce polymers of high molecular weight.10 For the ring-opening polymerisation of
(di)lactones, such as lactide, glycolide and caprolactone; tin- and aluminium-based
initiators are the most widely used.56 In the past, aluminium systems were preferred
where a high degree of control was desired56 because tin-based initiators are good
transesterification catalysts compared to aluminium-based initiators.56,57 However, the
greater hydrolytic stability,56 low cost,58 and the development of tin-based initiators to
provide greater control over macromolecular architecture56 has resulted in their more
widespread use as initiators for the synthesis of poly(-hydroxy acid)s.
Tin(II)2-ethylhexanoate, or stannous octoate (Sn(Oct)2) as it is also known, is the most
commonly employed tin-based initiator for the synthesis of poly(-hydroxy acid)s.58
Ring-opening polymerisation using tin(II)2-ethylhexanoate requires an active hydrogen
compound as co-initiator,10 usually an alcohol. The mechanism of poly(-hydroxy acid)
synthesis using tin(II)2-ethylhexanoate has been extensively studied. The formation of a
tin(II) alkoxide active centre, formed by the reaction of tin(II)2-ethylhexanoate with
added alcohol,58,59 followed by a coordination-insertion, ring-opening polymerisation
mechanism (Figure 2.3) is considered the most likely mechanism of polymerisation.10
The consequence of such a mechanism is that alkoxy end groups from the polymer may
be covalently bound to tin post-synthesis, even after purification, if complete hydrolysis
of the tin-alkoxy bond does not occur. Purification by dissolution and precipitation has
been shown to be inadequate for the complete removal of tin residues in polymers
synthesised using tin(II)2-ethylhexanoate. PLA synthesised by Schwach et al.60 using
tin(II)2-ethylhexanoate as the initiator was shown to contain 306 ppm tin after
purification.
R1O
R1O R
OSn
Sn(Oct)2 + R1-OH OctSnOR1 + OctH
Tin alkoxide formation:
Ring-opening:
Figure 2.3: Tin(II)2-ethylhexanoate / alcohol co-initiated polymerisation of
lactones, adapted from Albertsson and Varma.10 NB: R 4 C
Because it is impractical to completely remove all traces of initiator from polymeric
products,33,39 a number of groups have been investigating more biocompatible initiators,
based on metals that participate in human metabolism. Such studies have involved the
use of initiators based on: calcium,33,38,61,62 iron,39 magnesium33 and zinc.42,63 Zinc-
based initiators are highly studied, and include zinc metal, which has been used
industrially in France for the polymerisation of D,L-lactides.60 Zinc salts such as zinc
octoate,64 zinc stearate,42 and zinc lactate42,60 have also been used as initiators for the
ring-opening polymerisation of lactones. Ring-opening polymerisation of D,L-lactide by
zinc metal is known to produce polymers with high molecular weight, and high
conversion ratios, after long polymerisation times (approximately 4 days) at
temperatures about 140 C.63 Zinc / PEG and CaH2 / PEG co-initiation for the ring-
opening polymerisation of L-lactide were compared and both showed good control over
the degree of polymerisation.43 However, some racemisation was noted for the CaH2 /
PEG co-initiator and not for the zinc / PEG co-initiator, leading to a preference for the
zinc-based initiator.
Other calcium-based initiating systems that have been studied for the synthesis of
poly(-hydroxy acid)s include, calcium dimethoxide,65 bis(tetrahydrofuran)calcium
bis[bis(trimethylsilyl)amide] / alcohol co-initiatior,33,38 calcium -diketonate
complexes,62 and calcium acetylacetonate.66 Copolymerisation of glycolide with -
caprolactone and L-lactide using calcium acetylacetonate afforded high conversion, only
after several days at temperatures of 150 and 200 C. Calcium dimethoxide-initiated
polymerisation of -caprolactone and L-lactide in the bulk at 120 C showed quantitative
consumption of monomer after 5 10 min for the case of -caprolactone, but longer
reaction times were required for L-lactide polymerisation using the same initiator. In all
cases polydispersity was greater than 1.25, with initiator efficiency in the range 0.41
0.54, possibly due to aggregation and insolubility of the calcium dimethoxide initiator.
Greater control in the ring-opening polymerisation of -caprolactone and L-lactide has
been obtained with a bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] / alcohol
co-initiatior. The in situ generated calcium alkoxide formed from the reaction of the co-
initiators during polymerisation helped circumvent the problems of aggregation and
insolubility of pre-made calcium alkoxides. This, in turn, allowed for polymerisation
under mild conditions that proceeded in a living manner. In the synthesis of poly(-
hydroxy acid)s, quasi-living systems are often observed due to the co-existence of
processes such as transesterification.
2.1.3.2 Transesterification Reactions
For the case of the ring-opening polymerisation of poly(-hydroxy acid)s, the ionic
nature of the initiator / propagating species is an important factor for determining the
rate of intra- and inter-molecular transesterification.
Intramolecular transesterification:
Intermolecular transesterification:
Figure 2.4: Intra- and inter-molecular transesterification reactions of a polyester
As the propagating species moves from anionic towards covalent in nature, the
formation of macrocycles due to intra-molecular transesterification is less prevalent at
early polymerisation times.67 This has been shown experimentally for the synthesis of
PCL using sodium trimethylsilanolate, an anionic initiator, compared to
aluminiumdiethylmethoxide, a pseudoanionic / coordination-insertion-type initiator.68 It
is also important to note that steric hindrance can play a role in the transesterification
process. Where respective initiators have the same propensity to polymerise a particular
monomer, ie, where the kp (actual rate constant of polymerisation) of respective
initiators is the same, greater steric hindrance around the active centre has been shown to
decrease the rate of transesterification.67 One example is the synthesis of PCL using
aluminiumdialkylalkoxide initiators.69 In this case, the selectivity co-efficients for
diisobutyl and diethyl aluminiumdialkylalkoxides differed by a factor of 1.67, with the
more sterically hindered isobutyl initiator showing greater selectivity in producing linear
polymer, rather than cyclic, intra-molecular transesterification products. As well as
transesterification side-reactions, polymerisation systems that utilise anionic and
pseudoanionic / coordination-insertion-type initiators commonly show aggregation
phenomena.
2.1.3.3 Aggregation Phenomena
Evidence for aggregation may be observed through kinetics; specifically, through the
external order in initiator.67 An external order in initiator of less than one is indicative of
aggregation phenomena. Penczek et al.67 have derived a method for determining the
aggregation degree in systems where aggregation dominates. This can be performed by
plotting the left-hand-side of Equation 2.2 against ln[I]0, to obtain a slope of x1 , where x
is the aggregation degree.
001 ]ln[1ln
][][
lnln Ix
A = kp(nKa)-1/x, Ka is the equilibrium constant of aggregation, [M]0 and [M]t are
respectively the initial and instantaneous monomer concentrations, [I]0 is the initial
concentration of initiator and t is the polymerisation time. For the complete derivation
of Equation 2.2 see Section 2.3.3.4.
Potential solutions for the problem of aggregation have been investigated using a variety
of approaches. Such methods include the use of polymerisation solvents of varying
polarity and the introduction of specific chelating agents, such as crown ethers and
cryptates to enhance the solubility of ionic species. Aggregation of ionic and
pseudoanionic initiators has been shown to be solvent dependent. For the
polymerisation of -caprolactone, CL, initiated with diethylaluminium ethoxide,70 the
determined equilibrium constant of deaggregation was found to increase with increasing
polarity of the polymerisation solvent. That is, the free initiator / propagating species
was favoured over the aggregated form for increasingly polar solvents. Also, generally
for the case of anionic polymerisation of styrene and dienes, aggregation is negligible in
polar solvents such as tetrahydrofuran and dimethoxyethane for initiator concentrations
of less than 5 x 10-3 M.71 Specific chelating agents, such as crown ethers and cryptates
have been used to both reduce aggregation and alter ionic character. Specific binding of
metal ions to these chelating agents results in the destruction of aggregates.72
Furthermore, the trapping of metal ions by such agents allows for higher dissociation
into free ions.72 It was generally thought that a higher dissociation constant would
increase the rate of polymerisation; however, this is not true for all cases. Instead, the
process is more complex and can be better explained through three main factors: the
interaction of the monomer with the counter-ion, the charge localisation on the anion,
and the polarisability of the monomer.73 These factors, as well as the other previously
mentioned phenomena (transesterification, aggregation), that help to account for the
mechanism of polymerisation manifest themselves in the kinetics of polymerisation.
Hence, kinetic studies are an essential component of any in-depth investigation of
polymerisation mechanisms.
2.1.4 Methods for Studying the Kinetics of Poly(-hydroxy acid) Synthesis
The ring-opening polymerisation of poly(-hydroxy acid)s from lactones has been
studied by a number of techniques, including: nuclear magnetic resonance spectroscopy
(NMR), gel permeation chromatography (GPC), dilatometry, and infrared spectroscopy
(IR). Due to the atmospheric sensitivity of many of the initiators / catalysts employed
for the synthesis of poly(-hydroxy acid)s, and the subsequent difficulty in handling,
monomer conversion is usually evaluated by analysing crude reaction mixtures after
quenching by an appropriate method. Even so, manual sampling techniques are often
laborious and time-consuming leading to a preference for minimising data collection.
So, for further, more complete determination of polymerisation kinetics, a number of
real-time monitoring techniques have also been applied for the synthesis of poly(-
hydroxy acid)s.
2.1.4.1 Nuclear Magnetic Resonance Spectroscopy (NMR)
The ring-strain inherent to lactones gives rise to differing chemical shifts between
monomer and polymer signals in both 1H and 13C NMR. Complete separation of some
monomer signals and the respective signal from the polymer makes kinetic analysis
easier and reduces error associated with overlapping peaks. Along with this, 1H NMR
spectra are directly quantitative,74 allowing determination of the degree of conversion by
a ratio of polymer / total monomer and polymer peak areas. 1H NMR is commonly used
for determination of lactone conversion by manual sampling, but has also been used for
the in situ analysis of the enzyme-catalysed polymerisation of CL.75 In this case, poor
mixing led to the requirement for the sample to be removed from the spectrometer every
7 9 min and shaken. This made the process more arduous and highlights the problem
of poor-mixing for in situ NMR analysis. This problem of poor mixing is especially
prevalent for heterogeneous systems.
2.1.4.2 Gel Permeation Chromatography (GPC)
GPC (with refractive-index and UV detectors) has been used to determine the
conversion of CL with a variety of tin(II)2-ethylhexanoate / ROH co-initiating
systems.58 Monomer conversion was established by peak height comparison to a
standard UV absorption curve of known CL concentrations and then normalising to
injection volumes. As with other manual sampling techniques, the removal of aliquots
and subsequent sample preparation are time-consuming. GPC has the additional
hindrance of chromatographic separation, which often takes tens of minutes. Even so,
one of the advantages of using GPC in such a way is that the molecular weight
distribution of the growing polymer species may also be studied at the same time, giving
some insight into the mechanism of the polymerisation.
2.1.4.3 Dilatometry
Vacuum dilatometers have been used by Penczek et al.76 in their extensive studies of the
polymerisation of lactones. The use of dilatometry has the advantage of allowing real-
time monitoring of conversion. However, special dilatometers are required in order that
kinetics may be studied in a sealed, inert environment. One other major drawback of
dilatometry is that bulk polymerisation cannot be studied by this method due to the very
high increase in viscosity during polymerisation. In contrast, vibrational spectroscopic
methods lend themselves to the study of polymerisation in the bulk. Therefore, they
have more wide-spread applications ranging from laboratory research to industrial-scale
synthesis.
2.1.4.4 Infrared Spectrosopy (IR)
In situ FT-IR has been applied to the real-time monitoring of the synthesis of PLA by
two groups. Both Hillmyer et al.77 and Messman and Storey78 followed the reaction
progress by measuring the reduction in the absorbance of the 1240cm-1 peak (C-O-C
stretch) of D,L-lactide or L-lactide. However, the overlapping polymer C-O-C stretch at
1185 cm-1 limited the use of peak-area quantitation. Therefore, Messman and Storey78
used peak-heights to overcome this problem. The overlapping peaks due to the
inherently broad width of infrared absorption peaks plus the expense of the diamond
ATR probe used in this case seem to be the major disadvantages of this method.
Overall, the data obtained using this method were comparable to other techniques.
However, the complimentary vibrational spectroscopy technique, Raman spectroscopy,
may be better suited to the study of the polymerisation of poly(-hydroxy acid)s.
2.1.4.5 Raman Spectroscopy
Raman spectroscopy offers a number of advantages over IR for the study of the
synthesis of poly(-hydroxy acid)s. Raman spectroscopy permits the use of glass, or
quartz optics as opposed to the infrared-transparent optical materials required by IR.74
This allows for analysis of polymerisation mixtures in sealed-glass vessels (commonly
employed for poly(-hydroxy acid) synthesis), reducing the need for expensive probes
(as used for in situ IR) or other expensive infrared-transparent vessels. Secondly, there
is no rotational broadening of Raman peaks. Raman bands are generally very sharp and
narrow,74 minimising problems associated with overlapping peaks. There are, however,
a few disadvantages associated with using Raman spectroscopy. Raman scattering is a
weak effect, which has the consequence that spectra can be dominated by more efficient
phenomena such as sample fluorescence.46 FT-Raman is one well-recognised method
for overcoming this effect. Fourier transformation provides improved signal to noise
compared to conventional Raman spectroscopic techniques, therefore allowing the use
of longer wavelength light sources. This helps to reduce fluorescence by decreasing the
energy of the incident radiation.46
2.1.4.6 Methods-of-Choice For This Study
Polymerisation kinetics were monitored by manual sampling using 1H NMR and real-
time monitoring using FT-Raman spectroscopy. 1H was also used to determine
molecular structure and give insight into the polymerisation mechanism. In addition,
molecular weight distributions as a function of conversion were monitored by GPC to
further elucidate the mechanism of polymerisation.
Chapter 2 Experimental
2.2 Experimental
2.2.1 Materials
Argon (high purity) for the dry box was purchased from BOC gases, Australia. The
argon was dried through a commercially available drierite laboratory gas drying unit,
capable of drying gases to 0.005 mg L-1 H2O, purchased from W.A. Hammond Drierite
Co. Ltd, USA. Oxygen was subsequently removed through an OXY-TRAP oxygen
removal column purchased from Alltech Associates inc., USA. Calcium hydride (CaH2)
(99.99%) (Sigma Aldrich, Australia) was stored in a dry box under an argon atmosphere.
-caprolactone, CL, (Sigma Aldrich, Australia) was dried and purified before use by
storage over CaH2 for at least 48 hours, followed by distillation under vacuum at ~ 2 torr
and 90C. It was then sealed under argon and stored in a dry box until required. PEG45,
nM = 2 000, (Sigma Aldrich, Australia) was dried and purified by dissolution /
precipitation in chloroform / diethyl ether, followed by azeotropic distillation with
dichloromethane (DCM). After removal of the DCM, it was dried under vacuum at 100
C for three hours to remove all traces of solvent and water. It was then stored under
argon in a dry box until required. All other materials were of analytical reagent, AR,
grade or better.
2.2.2 Methods
2.2.2.1 Syntheses of PCLx/PEG45/PCLy Copolymers
The syntheses of all polymers were performed in vacuum-sealed, glass tubes equipped
with two 6 x 3 mm Teflon-coated magnetic stirrer bars. All reagents were added to the
tubes in a dry box under argon atmosphere employing standard dry box techniques. All
glassware and other utensils (spatulas, mortar and pestle) were dried overnight in an
oven at 100 C before use. The syntheses were performed in the bulk at three different
temperatures, (oil bath thermocouple reading: 70, 100 and 130 C; actual temperature in
tube: 70, 96 and 128 C respectively), with a summary of the [CL]0 : [PEG]0 feed ratios
and quantities commonly used in Table 2.1. CaH2 was powdered by crushing, using a
mortar and pestle for a period of five minutes before addition to the tube. PEG was then
added, followed by CL and the tube stoppered with a Teflon key, then sealed with
laboratory film. The tubes were then removed from the dry box and taken to a high
vacuum line.
Table 2.1: Typical feed ratios and quantities of reagents used for copolymer
syntheses
Desired Copolymer [CL]0 : [OH]0 CL (mL) PEG2000 (mg) CaH2 (mg)*
PCL50/PEG45/PCL50 50:1 0.78 138 4 / 18
PCL100/PEG45/PCL100 100:1 1.56 138 4 / 18
* 1.33 / 6 times the amount of CaH2 required to completely react with the number of
hydroxyl groups present in the PEG45, i.e., [CaH2]0 : [OH]0 = 0.67 : 1 and 3 : 1,
respectively
After degassing, tubes were flame-sealed under vacuum (0.2 0.3 torr), then completely
immersed in a thermo-stated oil bath (stirrer setting: approximately 700 rpm). Tubes
were removed periodically from the oil bath and frozen at -20 C to give cream coloured
products. Once opened, the reaction mixtures were dissolved in chloroform and
transferred to round-bottomed flasks. The chloroform was evaporated to give crude
products. Samples of the crude products were then taken for analysis by 1H NMR and
GPC. The crude samples were purified by dissolution using chloroform followed by
precipitation in either cold ether or cold ethanol, depending on the molecular weight.
Molecular weights of crude products were estimated by 1H NMR. If the number average
degree of polymerisation, nDP , of PCLx/PEG45/PCLy, (x + y), was found to be less than
100 in total per PEG45, then the copolymer was precipitated in ether, above this nDP ,
copolymers were precipitated in ethanol. After precipitation, the white solid was filtered
and washed, then dried under vacuum to constant mass. 1H NMR and GPC analyses
were performed on the pure copolymers.
2.2.2.2 Real-Time Monitoring
FT-Raman spectroscopy was used for real-time monitoring of polymerisation kinetics.
Samples were prepared in 50 mm high, 12 mm diameter, flat-bottomed, flame-sealed
glass tubes. The preparation of samples was similar to the method described in section
2.2.2.1. A thermo-stated aluminium heating block equipped with a magnetic stirring
device (Figure 2.5), mounted on a moveable stage inside the spectrometer, was
preheated (heating block thermocouple reading: 100, 126 or 130 C; actual temperature
in tube: 106, 128 or 133 C respectively) prior to sample introduction. Spectra were
recorded as soon as the sample was introduced into the preheated heating block. A
macro created using Macro Mania version 5.0 was used to collect spectra every 10 or 15
min over a set period. Work-up of samples was as per section 2.2.2.1.
Figure 2.5: Heating and stirring controllers / devices for real-time kinetic studies by
FT-Raman spectroscopy
2.2.2.3 NMR Characterisation of Initiator
Samples were prepared in a dry box under argon atmosphere using standard dry box
techniques. All glassware and other equipment were pre-dried in an oven overnight at
100 C. CaH2 was powdered by crushing, using a mortar and pestle for a period of five
minutes before adding with other reactants. The initiator was prepared by heating PEG45
and CaH2 in a stoppered flask inside the dry box. Samples were extracted with benzene-
d6 and transferred to 5 mm diameter NMR tubes, which were capped and sealed with
laboratory film. NMR analyses were then performed on a Bruker FT-NMR
spectrometer (400.13 MHz for 1H and 100.61 MHz for 13C). All spectra were referenced
to tetramethyl silane, TMS, using the benzene-d6 solvent residue at 7.16 ppm as an
internal calibration.
2.2.2.4 Headspace Analysis
Raman microspectroscopy analysis was performed on the headspace of flame-sealed
tubes and a Teflon-key-sealed vessel. Both flame-sealed vessels and the Teflon-key-
sealed vessel were individually mounted on their side to a metal plate and set in the
Raman microscope stage. The microscope was focussed onto the under side of the
centre of the top face of the glass tube. The focus was then advanced 400 m into the
headspace of the tube. Spectra were then recorded.
2.2.3 Measurements
Samples were prepared at a concentration of approximately 0.5 w / v % for 1H NMR in
CDCl3 using 5 mm diameter tubes. 1H NMR spectra were recorded on a Bruker Avance
FT-NMR spectrometer equipped with a 9.39T magnet equivalent to 400.162 MHz for 1H. 32 scans were taken using a 4.70 s pulse, a dwell time of 100 s, an acquisition
time of 3.27 s, and a relaxation delay time of 1.0 s. The greatest T1 for any of the
samples studied was 840 ms. All spectra were referenced to TMS using the CHCl3
residue at 7.26 ppm as an internal calibration.
2.2.3.2 Gel Permeation Chromatography (GPC) Table 2.2: Separation range of GPC columns
Column Type Effective Molecular Weight Range*
Waters, HR1 100 5 000
Waters, HR3 500 30 000
Waters, HR4 5 000 500 000
Phenomenex, phenogel 5 - 50 100 3 000
Phenomenex, phenogel 5 - 103 1 000 75 000
Phenomenex, phenogel 5 - 104 5 000 500 000
*Polystyrene
A Waters GPC system equipped with a Waters 1515 isocratic HPLC pump, 200 L
injection loop, column heater and a Waters 2414 refractive index detector (analysis
temperature, 30 C) was used. Due to corrosion problems within this GPC system over
the course of this work, overall it was equipped with one of the three following column
arrangements. The first consisted of three consecutive Waters styragel columns (HR4,
HR3, HR1) operating at 30 C using chloroform as eluent at a flow rate of 0.8 mL min-1.
The second configuration utilised two consecutive Waters styragel columns (HR4, HR3)
operating at 30 C using tetrahydrofuran as eluent at a flow rate of 1 mL min-1. The
solvent was changed to THF to avoid corrosion problems, which may have been caused
by the use of chloroform as eluent. The third configuration consisted of three
consecutive phenomenex, phenogel 5 columns (104 , 103 , 50 ), preceded by a
guard column, operating at 30 C using tetrahydrofuran as eluent at a flow rate of 1 mL
min-1. All systems were calibrated with seven polystyrene standards in the range 1350
450 000 g mol-1. A relative calibration (analyte molecular weight uncorrected to
polystyrene) was used for all configurations, with a third-order polynomial fit employed
for the calibration curve (R2 0.998 in all cases). Due to the switching of solvent, a
correlation between the absolute nM , and the nM obtained by GPC measurements
was made. It was found that there was a linear correlation of absolute molecular weight
as measured by 1H NMR, and molecular weight as measured by GPC for both THF and
chloroform as eluent. These linear correlations were found over a broad molecular
weight range, 4400 13 300 g mol-1 for chloroform as eluent ( HNMRnM 1 = 0.5271 x
GPCnM + 556, R2 = 0.987), and 7400 60 300 g mol-1 for THF as eluent ( HNMRnM 1 =
1.6396 x GPCnM - 7426, R2 = 0.977). Due to the difference in slope for the linear
regressions for chloroform and THF correlations, direct comparison of molecular weight
obtained by GPC using THF as eluent and GPC using chloroform as eluent cannot be
made. Therefore, no direct comparison of data obtained using THF as eluent and data
obtained using chloroform as eluent has been made throughout the rest of this thesis.
2.2.3.3 FT-Raman Spectroscopy
All FT-Raman spectroscopic measurements were carried out on a Perkin-Elmer System
2000 NIR FT-Raman spectrometer, equipped with a diode pumped Nd-YAG laser ( =
1064 nm) as an excitation source and a room temperature InGaAs photoelectric detector.
The backscattered radiation was collected at 180 to the excitation. Typically, spectra
were recorded in the range 200 - 3800 cm-1 at a laser power of 320 mW. 32 co-added
scans were taken for each spectrum with a spectral resolution of 8 cm-1. Grams/32 AI
(6.00) was used for spectral analysis.
2.2.3.4 Raman Microspectroscopy
All Raman microspectroscopy measurements were undertaken using a Renishaw InVia
Raman microscope equipped with a Leica microscope and a frequency-doubled, diode-
pumped Nd-YAG laser ( = 532 nm). Calibration was performed by referencing to the
520.5 cm-1 band of a silicon wafer. Spectra were recorded in the following ranges: 100 -
4300 cm-1 (full scan); 4000 4300 cm-1 (hydrogen-specific scan); 2100 - 2500 cm-1
(nitrogen-specific scan). Spectra were acquired at 100 % laser power (120 mW) using a
long working distance X 50 objective, and 4 spectral accumulations at 60 s per
accumulation. Grams/32 AI (6.00) was used for spectral analysis.
Chapter 2 Results and Discussion
2.3 Results and Discussion
2.3.1 Polymerisation Scheme
The synthesis of PCL using hydroxy-functional and alkoxy initiators has been widely
studied. It is generally accepted that due to its relatively low ring strain, the ring-
opening of CL with such initiators occurs through acyl oxygen bond cleavage, leading to
an alkoxy or hydroxy end-functional propagating species.10
In this study, the initiating species is expected to be a calcium-PEG alkoxide formed by
the reaction of PEG, which contains two terminal primary alcohol groups, with calcium
hydride, CaH2. As calcium prefers to exist as a divalent ion and PEG is a di-functional
molecule, chain extension may occur. For simplicity, the chain extension phenomenon
has been expressed through the use of a * adjacent to the calcium atoms in the
following scheme. Figure 2.6 presents a summary of the proposed polymerisation
scheme for the synthesis of PCL/PEG/PCL using a PEG / CaH2 co-initiator. This
scheme has been based on a living polymerisation model.
In addition, a reversible exchange reaction between alkoxide and hydroxy species,
proposed by Zhong et al.33 for the synthesis of PLA using a
bis(tetrahydrofuran)calciumbis[bis(trimethylsilyl)amide] / alcohol co-initiator, has been
included. The reactivity of the alkoxide towards protic species can be exploited for the
termination of active centres, where the addition of a suitable acid should produce
hydroxy-terminated polymers and the respective calcium-salt of the acid.
Ca*n *Ca
PEOO
Ca*
OCa*n
*Ca OO
*Ca
Ca*y
*CaOR + R'OH *CaOR' + ROH
CaH2 + + 2 H2
+ [(x-1) + (y-1)]
+ 2HX+ Ca(X)2
Initiating Species Formation
Reversible Transfer
Figure 2.6: Proposed polymerisation scheme for the synthesis of PCL/PEG/PCL using a PEG / CaH2 co-initiator
2.3.2 NMR Characterisation
1H NMR is commonly used for determination of the degree of conversion for lactone
polymerisations, as well as an aid for the elucidation of polymerisation mechanisms by
end-, and junction-group analysis. 1H NMR was used to determine the degree of
conversion, , of CL for crude polymerisation mixtures as well as the determination of
nM for pure PCLx/PEG45/PCLy copolymers. Equation 2.3 was applied for the
determination of the degree of conversion, where 2.3 is the 1H NMR signal arising
from the methylene group adjacent to the carbonyl of ring-opened CL (see Hc, Table
2.3) and 2.6 is the 1H NMR signal arising from the methylene group adjacent to the
carbonyl of CL (see Hc, Table 2.3). The degree of conversion has been taken as a ratio
of the polymerised CL signal, Hc, integral to the sum of the ring-opened CL, Hc, and
CL, Hc, signal integrals.
( )3.26.23.2
AHNMR +
nM was calculated using PEG45 as an internal standard (Equation 2.4). The following
logic was used to arrive at Equation 2.4. For purified PCLx/PEG45/PCLy copolymers,
from which nM values were calculated, there was assumed to be one PEG molecule per
PCLx/PEG45/PCLy triblock copolymer molecule. Therefore, a ratio of the PEG signal
(see Hd, Table 2.3) to a signal from the PCL segments, in this case Hc, was used to
calculate nM for the PCLx/PEG45/PCLy copolymers. The nM of PEG was known,
nM = 2000 g mol-1, therefore, it was calculated to have a nDP of 45. Since there are
four hydrogens per monomer unit in PEG, the total number of hydrogens per molecule
was calculated to be 180. Therefore, the signal integral for PEG, Hd - 3.6, was set to
represent 180 hydrogens per PCLx/PEG45/PCLy copolymer molecule. Using this
method, the ratio of the signal integral for PEG, and the signal integral for PCL, Hc -
2.3, was used to calculate the relative number of PCL hydrogens per PCLx/PEG45/PCLy
triblock copolymer molecule. Since Hc represents two hydrogens per monomer unit, the
total number of CL repeat units per PEG was calculated by taking a ratio of the signal
integrals of PCL and PEG then multiplying this ratio by the number of PEG hydrogens
per PCLx/PEG45/PCLy copolymer molecule, 180, and dividing by two. Hence, arriving
at: (A2.3 / A3.6 x 180) / 2, which is equal to A2.3 / A3.6 x 90. To calculate the nM of
the PCL segments, the number of repeat units was multiplied by the molecular weight of
each repeat, 114 g mol-1. Finally, to calculate the nM of the PCLx/PEG45/PCLy triblock
copolymers, the nM of the PEG segment, 2000 g mol-1, was added to the calculated
value for the nM of the PCL segments.
( ) 2000114906.33.21 += AAM HNMRn
The ratio of the PEG / CL junction-group (Hf, 4.20) to the PEG signal (Hd, 3.60) for
purified copolymers was used as an indication of whether or not all PEG hydroxyl
groups had been consumed. At a ratio 3.60 : 4.20 = 44 : 1, it was assumed that
quantitative consumption of PEG hydroxyl groups had occurred.
Table 2.3: 1H NMR (CDCl3) peak assignments for CL and PCLx/PEG45/PCLy
Signal Position Assignment*
[(CH2CH2O)43CH2CH2O(C(O)CH2CH2CH2CH2CH2O)yH]2
-C(O)CH2CH2CH2CH2CH2O-
* peak assignments were made according to the relevant literature35
nsity
, a.u
HaHb
HeHc
mixture (upper) and a pure PCLx/PEG45/PCLy copolymer (lower). Peak
assignments are given in Table 2.3
2.3.3 PCLx/PEG45/PCLy Synthesis
In an initial pilot study, the syntheses of PCLx/PEG45/PCLy copolymers were undertaken
in a dry box under an argon atmosphere. This approach was adopted in order to
facilitate the study of the polymerisation kinetics, which were to be monitored by
sampling the polymerisation mixture at set intervals and establishing the degree of
conversion by 1H NMR. This method proved, however, to be problematic. Even
heating of the reaction vessel proved impossible, resulting in monomer condensing on
the cooler, upper portions of the vessel. This affected the concentration of monomer
present in the reaction mixture (Figure 2.8) making reproducibility difficult.
0 50 100 150 200 250 300 350 400 450 500
time, min
frac
of th
eore
tical
PCL50/PEG45/PCL50 under an argon atmosphere in a dry box (note that the point
near 0 min was taken two minutes after addition of CL to the pre-heated vessel)
The reaction procedure was modified in order to overcome these problems by adopting
synthesis in flame-sealed, glass tubes. This ensured a more reproducible environment
for the polymerisation and also meant that the temperature of the entire reaction vessel
could be kept constant, throughout, by complete immersion in a thermo-stated oil bath.
For the first series of these experiments polymerisation kinetics were monitored at 70
C. This temperature was chosen to ensure both PCL (Tm = 60 C) and PEG (Tm = 55
C) remained molten throughout the polymerisation. The use of this relatively low
temperature is in contrast to other studies of the bulk polymerisation of lactones, where
polymerisation temperatures are generally in the range 100 150 C.10 Such high
temperatures, as well as long reaction times, can lead to transesterification reactions in
the ring-opening polymerisation of lactones.10 Therefore, by keeping the polymerisation
temperature low, it was hoped that competitive processes such as inter- and intra-
molecular transesterification would be minimised.
GPC analysis showed no evidence for the formation of low molecular weight intra-
molecular, cyclic, transesterification products for PCLx/PEG45/PCLy copolymers
synthesised at 70 C. This was indicated by the lack of low molecular weight peaks in
the chromatograms of crude polymerisation mixtures.
The extent of inter-molecular transesterification was determined by analysis of
molecular weight distributions (MWDs) obtained from GPC analysis. Inter-molecular
transesterification has been shown to lead to an increase in polydispersity index
( nw MM ), PDI.10 Since PDIs were generally between 1.1 1.3, for
PCLx/PEG45/PCLy copolymers synthesised at 70 C, it can safely be assumed that inter-
molecular transesterification was not significant for these reaction conditions.
Even though polymerisation at 70 C had beneficial outcomes such as minimal inter-
and intra-molecular transesterification, the rate of propagation was much lower than
expected for a calcium-alkoxide system. A number of days were required to obtain
PCLx/PEG45/PCLy copolymers at 70 C. In contrast, it has been reported that for the
synthesis of PCL using calcium dimethoxide as the initiator at 120 C in the bulk ([CL]0
: [calcium dimethoxide]0 = 100 : 1), complete consumption of CL occurred after 10 min
at an initiator efficiency of 0.54.65 As this polymerisation was performed at a much
higher temperature, the much faster rate of polymerisation could be due to a very high
activation energy for the ring-opening of CL by calcium alkoxides. However, for the
synthesis of PCL using a calcium dimethoxide initiator formed in situ at room
temperature ([CL]0 : [OH]0 = 50 : 1), quantitative consumption of monomer also
occurred after 10 min.33 Even though this polymerisation was performed in THF with
an initiator that was formed in situ, thereby slightly changing the nature of the reaction,
the fast rate of polymerisation at this temperature would suggest that the cause of the
low rate of polymerisation at 70 C is not due to a very high activation energy for the
ring-opening of CL by calcium-alkoxides.
time, hrs
ln ([
CL]
0/[C
L]t)
copolymers; [CL]0 : [OH]0 : [CaH2]0 = 50 : 1 : 0.67 (), and [CL]0 : [OH]0 : [CaH2]0
= 100 : 1 : 0.67 (x) in flame-sealed glass tubes at 70 C. Error bars: 1 S.D., n = 1
3. Where n = 1, the error bars represent the error inherent in the analysis
Another possible reason for the observed low rate of polymerisation at 70 C could be
the incomplete formation of the calcium-PEG alkoxide initiator. As shown in Figure
2.9, the kinetic curves obtained from the synthesis of PCLx/PEG45/PCLy show
induction periods before reaching a steady-state region. This induction period is
most likely due to the slow formation of the initiator compared to the much faster rate of
propagation. One possibility is that once a critical concentration of initiator has been
formed, the polymerisation proceeds much quicker, finishing before the reaction of PEG
with CaH2 has gone to completion. To increase the rate of calcium-PEG al
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