New Strategy For Preparation Of Multiblock Polymer

Since 1990s, considerable efforts have been expended to develop free radical polymerizations with living characteristics. Several controlled/living radical polymerization (LRP) methods have been developed. In general, they are nitroxide-mediated polymerization (NMP), metal catalyzed atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) radical polymerization. Polymers, not only with predetermined molecular weight and narrow molecular weight distribution, but also with various compositions and architecture, can be prepared by living polymerization. RAFT polymerization has several advantages compared with NMP and ATRP, since it can be easily adapted to a wide variety of monomers under mild conditions. The most effective RAFT agents are dithioesters and linear trithiocarbonates. Up to now, no cyclic trithiocarbonate (CTTC) has been reported as RAFT agent.In this paper, two kinds of CTTC, o-xylenyl-1,3-dithiolane-2-thione (XDTT) and 4,7-diphenyl-[1,3]-dithiepane-2-thione (DPDTT), were synthesized by two different methods reported in literature. In the existence of radical initiator, XDTT can't carry out ring-opening polymerization, but DPDTT can be polymerized through ring-opening polymerization.Homopolymerization of styrene and n-butyl acrylate were carried out in the presence of CTTCs. The molecular weights of polymers were much higher than calculated ones. After treated with n-butylamine, the molecular weight of polymer declined rapidly and was close to calculated one. It is proposed that polymer prepared in the prensence of CTTCs have multiblock structure with more than one trithiocarbonate function per chain, compared with polymers prepared in the presence of linear trithiocarbonate, which have only one trithiocarbonate function per chain.Based on the study of the effects of CTTC concentration on styrene polymerization and the kinetics of styrene polymerization, and the mechanism of RAFT polymerization, it has been found that the mechanisms of formation of multiblock structure in the presence of XDTT and DPDTT are different.Styrene polymerization was carried out in the presence of various concentrations of XDTT. With the increase of XDTT concentration in a certain range, the number of trithiocarbonate unit per polymer chain increased, while the conversion of monomer and molecular weights of resulting polymers before and after treatment with rt-butylamine decreased. Under certain conditions, polymers with blocks having controlled molecular weight and narrow molecular weight distribution were obtained. Kinetic study of styrene polymerization in the presence of XDTT was carried out. At the early stage of polymerization, XDTT functions as comonomer in normal radical polymerization. After it is incorporated into the polymer chain, the derived trithiocarbonate function keeps its merits as RAFT agent. Therefore ln([M]o/[M]) increases linearly with the reaction time and the molecular weight of polymer before and after treated with rc-butylamine grows linearly with monomer conversion.With the increase of the concentration of DPDTT in a definite range, the number of trithiocarbonate unit per polymer chain increased, while the monomer conversion and molecular weights of resulting polymers before and after treated with H-butylamine decreased. GPC curves of original polymers show bimodal molecular weight distribution. With increase of the concentration of XDTT, polymers with lower molecular weight increase. Based on the study of kinetic of styrene polymerization in the presence of DPDTT, we found that at the early stage of polymerization, DPDTT carry out ring-opening polymerization by radicals transfer to DPDTT rapidly. We proposed that poly(DPDTT) might be the real intermediate which controls the polymerization. Therefore, once polymerization of monomer starts, the polymerization is carried out via RAFT process. When initiated by TEMPO-Phenylethane, polymer chains can be obtained with controlled molecular weight and molecular weight distribution, and the number of trithiocarbonate unit per polymer chain keeps almost constant.We also investigated the kinetics of ?-butyl acrylate polymerization in the presence of CTTCs. There is an induction period of about half an hour which resulted from the high speed of n-butyl acrylate polymerization and the slow fragmentation of the intermediate RAFT radical in the preequilibrium. The rate of propagation oftt-butyl acrylate can be slowed down by adding a small amount of styrene to eliminate or lower the induction period.The homopolymers containing trithiocarbonate units were used as macro-RAFT agent for copolymerization. The molecular weights of copolymers after treated with n-butylamine increased compared with their parent polymer. It is proposed that copolymers with structure of (BAB)n have been prepared by the copolymerization. The two kinds blocks of copolymers exhibit compatible due to the multiblock structure, because just one glass transfer temperature was detected, which increases with the styrene content of copolymers.In conclusion, through ring-opening process of novel CTTCs and RAFT polymerization, multiblock homopolymer and copolymer with blocks having controlled molecular weight and narrow molecular weight distribution were prepared respectively via one- or two-step process. The above polymerization method provides a novel and convenient strategy for preparation of multiblock polymers.