Synthesis Of Cyclic Trithiocarbonates And Related RAFT Polymerization

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. For example, nitroxide-mediated polymerization (NMP), metal catalyzed atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) radical polymerization. With these methods, Polymers with predetermined molecular weight and narrow molecular weight distribution, various components and complex architecture can be prepared. RAFT polymerization has several advantages compared with NMP and ATRP, it can be easily adapted to a wide variety of monomers under mild conditions. The most effective RAFT agents are dithioesters and linear trithiocarbonates. However, there is no report on cyclic trithiocarbonate (CTTC) as RAFT agent until our laboratory carried on the study.In this paper, two kinds of CTTC were synthesized by two different methods reported in literature. They are 4,7-diphenyl-[l,3]dithiolane-2-thione, DPDTT and dimethyl-biphenyl-[l,3]-dithiolane-2-thione, DMBDT. Among them, DMBDT was first synthesized by this paper.Homopolymerization of styrene and t-butyl acrylate (t-BuA) were carried out in the presence of CTTCs. The molecular weights of polymers were much higher than the calculated values. After treated with n-butylamine, the molecular weight of polymer declined rapidly and was close to the calculated value. It is proposed that polymer prepared in the presence of CTTCs have multiblock structure with more than one trithiocarbonate (TTC) group per chain, while polymers prepared in the presence of linear trithiocarbonate, have only one TTC group per chain. If TTC groups were regarded as nodes, the polymer would have multi-block structure.With DPDTT as chain transfer agent and S-TEMPO as initiator, polymerization of styrene was carried out. Compared with experiment with AIBN as initiator, the polymerization has no retarding period. For polymer got in the initial stage, the content of low-molecular-weight fraction decreased and polydispersity of the wholepolymer is narrower. Not only molecular weight and polydispersity of the blocks can be controlled, but also the number of blocks contained in each chain is controllable.Polymerization of t-BuA with DPDTT as chain transfer agent and AIBN as initiator was carried out in solution, and typical living polymerization was observed. A small amount of styrene (5% Vol.) can help to lower polymerization rate of t-BuA and realize controlled polymerization. After treated with n-butylamine, polydispersity of polymer is a little higher than those of polystyrene prepared under the same conditions. It indicates that t-BuA polymerization is poorly controlled than styrene by DPDTT.Styrene polymerization with DMBDT as chain transfer agent was studied. Typical living polymerization was also observed. Polymer with multiblock structure was obtained, molecular weight and polydispersity of blocks in the chain was also controlled.In addition, stability of multiblock polymer containing trithiocarbonate (TCC) group was also studied in this paper. In the existence of radicals, TTC groups in polymer will break and lead to recombination of the blocks. The recombination will result in lower molecular weight of the polymer and affect polydispersity of the final polymer. TTC groups in polymer are basically stable under strong base, but will be destroyed to some degree in the presence of acid.In conclusion, through ring-opening process of novel CTTCs and RAFT polymerization, multiblock homopolymer containing blocks with controlled molecular weight and narrow molecular weight distribution can be prepared via one-step process. This provides a novel and convenient strategy for preparation of multiblock polymers.