RAFT Polymerization Of Methacrylates

Reversible addition-fragmentation chain transfer (RAFT) polymerization is one of the most versatile controlled/"living" radical polymerization techniques due to its compatibility with a wide range of monomers and reaction conditions. It was widely utilized to synthesize well-defined polymers such as block, graft, star and hyperbranched polymers. In this work, we studied the kinetics and mechanism of RAFT polymerization of methacrylates and elucidated one of the possible reasons of retardation effects and inhibition during the RAFT polymerization. Some well-defined block and random copolymers were synthesized by employing some methacrylates with functional groups as the comonomers. We studied the relationship between structures and properties of these synthetic copolymers and prospected their potential applications.First, thermal stability of dithioesters, classical chain transfer agents, and its effect on the RAFT polymerization was investigated. Dithioesters, cumyl dithiobenzoate (CDB) and poly(methyl methacrylate) (PMMA) end-capped with dithioester underwent thermal decomposition at 120℃. The thermal decomposition yielded unsaturated compound and dithiobenzoic acid, leading to some loss of living character of the polymerization, such as retarded reaction rate and broadened molecular weight distribution. Nevertheless, thermal decomposition of 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate, a model compound for PMMA dithioester, does not yield unsaturated product in spite of the resemblance of the chemical structures. Thermogravimetric analysis shows that PMMA dithioesters are more thermally unstable than the other two.Second, aminolysis of polystyrene and poly(methyl methacrylate) (PMMA) prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization was investigated. The product of the former contains predominantly double molecular weight species by the formation of disulfide bond, whereas the latter formed coupled species which consequently cleaved to unimolecular weight species. Matrix-assisted laser desorption inionization time-of-flight mass spectrometry, elemental analysis and NMR indicated that thiolactone terminus was formed after aminolysis of PMMA. We propose that the thiol end-groups generated during the aminolysis of PMMA tend to cyclize through "backbiting" to form thiolactone structure. Similar reaction was observed in the case of poly(N, N-dimethylaminoethyl methacrylate) and poly(laury methacrylate). In spite of this, the preparation of thiol-end functionalized PMMA was achieved by introducing a short block of polystyrene after the RAFT polymerization of MMA.Third, RAFT technique was applied to design and synthesize some well-defined poymers. Five monomers, lauryl methacrylate (LMA, L), (N,N-dimethylamino)ethyl methacrylate (DMAEMA, D), poly(ethylene glycol) methyl ether methacrylate (PEGMA, E), 4-(4'-cyanobiphenyl-4-yloxy) butyl methacrylate (CBPBMA, C) and 2-(cholestryl carbonate)ethyl methacrylate (CCBEMA) were chosen to RAFT homopolymerization or copolymerizations mediated by cumyl dithiobenzoate. The kinetics of polymerizations was comprehensively investigated.Fourth, three kinds of synthetic polymers, quaternized P(L-b-D), P(L-ram-D) and P(C-raw-D) (denoted as P(L-b-D)(+), PLD(+) and PCD(+) ), were synthesized by RAFT technique and their structure-property relationships were studied: (1) P(L-b-D)(+) was a kind of cationic diblock copolymer, which can self-assembly into micelles with core-shell structure. (2) PLD(+), a kind of cationic polysoap, has the typical solution properties of polysoap and could has the potential applications in emulsifier, stabilizer, dispersant and so on. (3) PCD(+) solution in THF/H2O mixed solvent is temperature-responsive due to the thermal-induced phase separation. The phase separation is accompanied by the processes of nucleation, growth, coarsening and Ostwald ripening. The change of temperature would induce secondary phase separation after macro-domain formed.