Living radical polymerization, where, unlike conventional radical polymerization, the polymer growth terminus is active so as to be chemically convertible, can freely control the molecular weight, monomer residue order, dimensional structure and the like of the polymer; therefore, it has attracted much attention for the last ten years. Examples include atom transfer radical polymerization (ATRP) (Non-patent Reference 1), nitroxide mediated radical polymerization (NMP) (Non-patent Reference 2) and reversible addition chain transfer (RAFT) radical polymerization mediated by sulfur compounds (Non-patent Reference 3). Among these, atom transfer radical polymerization (ATRP) that uses combinations of metal complexes and halogen compounds shows particular applicability to a wide range of monomer types. Methods that precisely control the polymer using this have spread not only to polymer synthesis, but also to chemical modification of substrate surfaces and interfaces and device construction.
The metal catalysts used in ATRP normally have copper or ruthenium as their central metal. They are not a well-defined metal complex, and they are used after the compounds formed from the metal ions and their ligands (amines for example) are mixed into the polymerization reaction. In such polymerization systems, the metal catalytic activity occurs after the ligands bind in the system and the complex is formed. When the coordinating force of the ligands is not very strong, metal that does not form the complex arises, and this metal cannot show catalytic activity. Therefore, the catalytic efficiency of the metal is reduced, and there is the demerit of having to increase the concentration of the metal or having incompatibility with production of high molecular weight polymers. Increasing the metal concentration places most of the burden on the process for eliminating the metal after the polymerization reaction or gives rise to the possibility of environmental contamination due to metal toxicity. On the other hand, to prevent a decrease in the catalytic efficiency of the metal, an excess of amine ligands (see Patent Reference 1 and 2, for example) may be used. However, if the type or the like of the monomer changes in the polymerization reaction because of the use of an excess of amine ligands, a large number of problems, such as the reaction becoming difficult to control and polymer purification becoming difficult because compounds other than the monomer are mixed in arise.
Typically, an organic compound of an activated halogen is used for the polymerization initiator in ATRP. Replacing the activated halogen compound initiator with a conventional radical generator (for example, a peroxide radical generator or azo radical generator) in the polymerization is called reverse ATRP (R-ATRP). With R-ATRP, a reactive residue may be introduced at the end of the polymer by adding a metal catalyst to the conventional radical polymerization process, thereby making the synthesis of a block copolymer possible. Therefore, R-ATRP is a useful production method for obtaining polymers where the structure is controlled in an existing production process. Most R-ATRP methods basically use copper ion complexes with amines as the ligands. The same problems as those in ATRP, such as increasing in metal ion concentration, increasing ligand concentration, reduction of catalytic efficiency, difficulty in polymer purification and polymer coloration, are involved.
The production of polymers using a safe, inexpensive iron catalyst with living radical polymerization that uses a metal complex has received much attention from the standpoint of being environmentally friendly (Non-patent Reference 4).
With ATRP, polymer production methods that are carried out after mixing iron ions and ligands (amines, phosphines and phosphite ester compounds) with polymerizable monomers and production methods for polymers that are carried out after mixing synthesized iron complexes and polymerizable monomers have been disclosed (Non-patent Reference 5). For example, a methyl methacrylate polymerization method where divalent iron ions and an amine ligand are mixed with a monomer and a halogen initiator is used therein (Non-patent Reference 6) and a methyl methacrylate polymerization method where an iron complex of divalent iron ions and a phosphorous compound ligand are used with a halogen initiator have been reported (for example, Non-patent Reference 7, Patent Reference 3).
There have also been investigations into using environmentally friendly iron ion compounds as catalysts in R-ATRP. For example, methyl methacrylate polymerization using a mixture of FeCl3 and triphenyl phosphine as the catalyst (Non-patent Reference 8) and methacrylate or styrene polymerization using a metal complex formed from organic onium cations and a ferric chloride compound that is anionic as the catalyst (Non-patent Reference 9) have been reported. However, in R-ATRP using these iron complexes or iron ion compounds, there are many problems that should be improved, such as the difficulty of controlling block copolymers.
On the other hand, in living radical polymerization systems that use metal catalysts, there is a large problem with methods for eliminating the metal from the polymer after polymerization. In a certain sense, the elimination of the remaining metal from the polymer is more of a real problem for practical application of living radical polymerization than the polymerization reaction itself. Methods that make use of complexing agents in the purification process have been examined for removing the metal (Patent Reference 4 and 5). The use of environmentally friendly iron ion compounds as catalysts produces no toxicity compared with other metals such as copper, cobalt, ruthenium and the like, and the merits for all of the processes in polymer production, including post-processing and other processes, are great. However, in living radical polymerization that uses iron ions, one can hear about problems in production processes such as the instability of iron catalysts and the difficulty of reusing iron catalysts before problems such as the polymerization efficiency being low.
In a living radical polymerization reaction, the polymerization reaction is carried out using an iron complex with high catalytic activity, and the elimination of that complex from the polymerization reaction and recovery by a simple method without disposing of it is considered to be an extremely important problem.
Patent Reference 1: Published Unexamined Patent Application H08-41117
Patent Reference 2: Published Unexamined Patent Application No. 2002-80523
Patent Reference 3: U.S. Pat. No. 2,946,497
Patent Reference 4: Published Unexamined Patent Application No. 2002-356510
Patent Reference 5: Published Unexamined Patent Application No. 2005-105265
Non-patent Reference 1: J. Wang et al., Macromolecules, Vol. 28, 1995, p. 7901
Non-patent Reference 2: C. J. Hawker, et al., Macromolecules, Vol. 29, 1996, p. 5245
Non-patent Reference 3: Chiefari et al., Macromolecules, Vol. 31, 1998, p. 5559
Non-patent Reference 4: Matyjaszewski et al., Chemical Review, Vol. 101, 2001, p. 2921
Non-patent Reference 5: Sawamoto et al., Polymer preprints, Japan, 2005, vol. 54, No. 2, p. 136
Non-patent Reference 6: Matyjaszewski et al., Macromolecules, Vol. 30, 1997, p. 8161
Non-patent Reference 7: Ando et al., Macromolecules, Vol. 30, 1997, p. 4507
Non-patent Reference 8: G. Moineau et al., Macromolecules, Vol. 31, 1998, p. 545
Non-patent Reference 9: Teodorescu et al., Macromolecules, Vol. 33, 2000, p. 2335