theoretically speaking , if any kind of environmentally sensitive moiety , such as a thermally cleavable or photo - labile linkage were incorporated into polymerizable resin monomers , such resin or its resulting polymeric material would become command - responsible , more specifically enable them thermo - cleavable or photo - cleavable . the chemistry of some classical photo - initiators could be adopted as the base for designing such photopolymerizable and photocleavable resin monomers , because such an initiator was explored as polymerizable photoinitiator or macroinitiator . however , none of them were really incorporated into polymer chain or polymeric network to make the polymeric chain or network breakable one way or another . it is the another objective of this investigation to develop a new resin system for next generation low shrink and low stress restorative materials by incorporating a photocleavable or thermally liable moiety as part of a photopolymerizable resin monomer . it was expected with such an unusual approach it would enable a conventional polymerized network should be selectively cleavaged , thus to disperse the stress from postpolymerization and furthermore to result in a self stress - relief , ultimately to minimize the overall stress concentration . in order to make a polymerized network cleavable - on - command by light or photocleavable , a light responsible moiety should be stable towards standard light exposure process such as visible light curing until additional exposure to specific light with distinguished energy level . in particular , such energy source can be anything other than the standard visible blue light . near uv light would be one of typical examples among the many possible choices . furthermore , it was expected that compounds derivated from ortho - nitrobenzyl segment or from α - hydroxyalkylphenone should be ideal candidates for this new class resin monomers that be photopolymerized by visible light and be triggered to be breakable by extra uv light if needed . its feasibility of this approach allows a rapid exploration on its versatility for a new class of resin . accordingly , accordingly , a variety of such polymerizable and photocleavable resin monomers were successfully prepared with wide range of viscosity as illustrated in scheme ii . furthermore , such new resin monomer was formulated with other conventional resin monomers like bisgma , tegdma , udma or experimental resin monomer like macrocyclic resin in a variety ratio in order to have overall performance got balanced for the resulting composites . as showed in the following examples , remarkable low shrinkage , low stress and excellent mechanical property plus the good handling characteristics were demonstrated by those composites based on such new class p & amp ; p resin monomers . table i polymerization shrinkage and stress for various activated resin mix shrinkage (%) by stress ( mpa ) helium pycnometer by tensometer denfortex resin 10 . 2 4 . 1 tph resin / 999446 6 . 8 4 . 5 tph resin / 999447 7 . 3 4 . 3 harpoon resin / xj5 - 12 5 . 5 3 . 1 harpoon resin / xj5 - 26 5 . 8 3 . 2 lb5 - 158 - 1 5 . 2 1 . 4 lb5 - 158 - 2 5 . 7 2 . 0 lb5 - 167 - 2 6 . 5 1 . 9 lb5 - 167 - 3 6 . 2 1 . 5 lb5 - 167 - 4 6 . 9 1 . 5 according to the present invention there is provided a composition of matter that can be polymerized via an energy source , containing portions within the new composition of matter that are reactive to a second energy source . the invention also provides a composition of matter that can be polymerized via an energy source , containing portions within the new composition of matter that are reactive to a second energy source and that upon activation of the second source of energy , de - polymerize and / or degrade . a composition of matter is also provided that can be polymerized via a first energy source , containing portions within the new composition of matter that are reactive to a second energy source and that upon activation of the second source of energy , de - polymerize and / or degrade without substantially effecting the structural properties of the material polymerized by the first energy source . a further composition of matter is provided that can be polymerized via a first energy source , containing portions within the new composition of matter that are reactive to a second energy source and that upon activation of the second source of energy , de - polymerize and / or degrade to elevate stress created during the polymerization of the composition of matter created via the first energy source without substantially effecting the structural properties of the material polymerized by the first energy source . according to another aspect of the invention , a composition of matter is provided that comprises monomers , prepolymers and / or polymers that can be polymerized via an energy source ( thermal , photochemical , chemical , ultrasonic , microwave , etc . ), containing portions within the new composition of matter that are reactive to a second energy source ( thermal , photochemical , chemical , ultrasonic , microwave , etc .). thus , certain limitations of the heretofore known art have been overcome . polymer networks with cross - linking are desired for strength properties , but lead to higher degree of shrinkage and stress . this invention allows formation of cross - linking , while at the same time , providing a mechanism ( the second form of energy application ) that relieves the stress created while maintaining the structural integrity of the polymer network created . relief of stress during polymerization has been desired and typically approached through attempt to relieve the stress during the “ pre - gel ” state of polymerization , prior to the “ post - gel ” state , wherein the polymer network has now been established , cross - linked set up and , due to the more rigid state , stress is created . the invention substantially eliminates the stress during this “ post - gel ” state . there are prior known systems for materials that are reversible — that is , once polymerized , some form of post - polymerization energy is applied to fully decompose or degrade the polymer network to a state that renders the material unusable . in the present invention , there is provided only partially , in a controllable manner , degrading or decomposing a portion of the polymer network and maintaining the integrity of the polymer network . as discussed above , according to one embodiment of the present invention , a photopolymerizable and photocleavable resin monomer ( hereinafter referred to as the “ p & amp ; p ” resin ) offers unique combination of low curing stress and good mechanical strength . the inventive p & amp ; p resin features by incorporating a photoresponsive moiety within the resin monomer and is a ( meth ) acrylate based resin and capable of being polymerized as any other conventional ( meth ) acrylate monomers . however , the presence of such a photoresponsive moiety enables p & amp ; p resin to polymerize in a way different from those conventional ( meth ) acrylate monomers . more specifically p & amp ; p resin polymerize with a unique curing kinetic , which allow stress relief through the relatively slow curing process without compromising the overall mechanical strength . consequently substantially low polymerization shrinkage stress results from p & amp ; p resin and p & amp ; p resin based composite , as compared to those conventional resin like bisgma / tegdma or ebpadma , and other conventional composites . typical posterior composites based on the inventive p & amp ; p resin and loaded 80 - 82 % ( wt / wt ) of inorganic fillers offer shrinkage stress of 1 . 3 - 1 . 7 mpa . they can also demonstrate good mechanical strength . the present invention is extended application of p & amp ; p resin . it was unexpectedly discovered that an exceptionally low curing stress remained even with lowering filler loading , which paved a way to low stress flowable composite . the filler level varies from 1 % to 70 %, wt / wt , preferably , 10 - 60 %, wt / wt , and more preferable 50 - 60 %, wt / wt . the conventional resin monomers can also be incorporated by up to 40 - 50 %, wt / wt with p & amp ; p resin , depending upon the nature of such conventional resin monomer and the end use . the filler composition can be adjusted as well . as showed in table i through ii , an exceptionally low shrinkage stress was revealed from these new flowable compositions . similar flowable pastes were also formulated by using tph resin ( 999446 and available from dentsply international ) with the same filler loading and composition as a control . as expected a much higher shrinkage stress resulted , 3 . 6 mpa vs . 0 . 9 - 1 . 3 mpa . fig1 shoes a comparison between the typical experimental flowable composites ( lb6 - 109 , 110 , 111 and xj5 - 196 ) and some of commercially available flowable materials , such as dyractflow ( dentsply international ), admiraflow ( voco , germany ), flow it ( jeneric / pentron , inc . ), esthetxflow ( dentsply international ), revolution ( kerr corporation ), and tetric flow ( ivoclar vivadent , inc .). there is up to 60 - 80 % ( percent ) stress reduction achieved by the experimental flowable composite as compared with esthexflow and dyractflow . in addition , the new flowable material still offers moderate mechanical strength , which is comparable to most flowable products ( see also in fig2 ). it is expected that the mechanical strength can be further improved by refining the filler compositions . the low stress nature demonstrated by p & amp ; p resin and its composites is attributed to the unique curing kinetic as discussed above . pdc study further confirmed this unique , moderately slow polymerization rate as compared to tph resin or its composite . tetricflow also demonstrated a slow polymerization rate ( under same curing condition ) due to the presence of a stable radical compound . tetricflow has a relatively lower stress than other commercially available flowable materials ( 3 . 3 - 4 . 6 mpa ), but it still generates a much higher shrinkage stress ( 2 . 4 - 3 . 2 mpa ) than the experimental flowable composites based on p & amp ; p resin ( 1 . 0 - 1 . 4 mpa ). the present invention provides flowable composites with an exceptionally low polymerization stress of 0 . 9 - 1 . 3 mpa , which is about 60 - 70 % less than that of typical esthetxflow ( 3 . 4 mpa ) or dyractflow ( 4 . 6 mpa ). more importantly , the new flowable material can still offer moderate mechanical property . this unique property combination regarding low curing stress and handling character enable to be used as dental restoratives like liners , sealants , et al and other application field where curing stress and flowability is critically concerned . table i general physical property for activated neat p & amp ; p resin systems 100 % p & amp ; p resin 100 % p & amp ; p resin 100 % tph resin ( lb6 - 71 ) ( ebr6983 ) 100 % tph resin ( 999452 ) ( w / tegdma ) ( w / tegdma ) ( 999446 ) 0 . 15 % cq 0 . 15 % cq 0 . 15 % cq 0 . 165 % cq 0 . 20 % edab 0 . 20 % edab 0 . 20 % edab 0 . 30 % edab 0 . 02 % bht 0 . 02 % bht 0 . 02 % bht 0 . 025 % bht lot # lb5 - 187 - 1 lb6 - 106 - 1 lb6 - 114 030804 viscosityat 20 ° c ., 150 500 1020 150 poise uncured density , 1 . 1206 1 . 1129 1 . 1162 1 . 1210 g / cm 3 cured density , 1 . 2077 1 . 1888 1 . 1867 1 . 2099 g / cm 3 shrinkage @ 7 . 2 6 . 4 5 . 9 7 . 4 24 hrs ., % stress @ 60 min ., 4 . 5 1 . 8 1 . 4 4 . 7 mpa δh 1 in n2 @ 110 mode 1 t o , seconds 15 t max , seconds 31 δh 1 in n2 @ 138 120 107 133 mode 2 t o , seconds 13 17 17 10 t max , seconds 31 35 36 29 thus , it should be evident that the invention as disclosed herein carries out one or more of the objects of the present invention set forth above and otherwise constitutes an advantageous contribution to the art . as will be apparent to persons skilled in the art , modifications can be made to the preferred embodiments disclosed herein without departing from the spirit of the invention , the scope of the invention herein being limited solely by the scope of the attached claims .
a photopolymerizable and photocleavable resin monomer is derived from a reactive photoresponsible moiety via various linkages to form photopolymerizable monomers and / or oligomers .