Recent progress in synthesis of stereoblock copolymers

Abstract

The synthesis of polymers with tailor-made microstructures and desired properties by controlling the regio- and stereoselectivity of olefin polymerization is an important research area in polymer science. In recent years, achievements were made to synthesis stereoblock copolymer having different tacticity of each segment such as crystalline-amorphous blocks from a single monomer. A stereoblock copolymer consisting of different stereospecific sequences in one polymer chain, shows special physical properties from the original stereoregular polymer which could be applied as thermoplastic polymer. On one hand, it leads to much wider array of polymeric materials from a limited set of commodity monomer feed stocks; on the other hand, it combines the properties of two or more homopolymers and exhibits better performance than the corresponding homopolymer blends. In this paper, research and developments about several kinds of stereoblock copolymers such as polypropylene, polydienes, polyacrylate and polylactide were reviewed and discussed, in particular, the methods to synthesize stereoblock copolymers were also summarized. The successful development of oscillating catalysts for synthesizing isotactic-atactic multi-stereoblock polypropylene opened the door of research on preparing crystalline-amorphous stereoblock polypropylene. Plenty of coordinative catalyst systems and novel polymerization methods were developed for preparing stereoblock polypropylene. The most common way is using living/controlled polymerization system and changing the selectivity simply by varying polymerization temperature, solvent polarity and pressure. Besides, two catalysts with different stereospecificity were mixed together for the propylene polymerization, by successful cross chain transfer reaction, multi-stereoblock polypropylene with thermoplastic elastomeric properties was also obtained. Stereoblock polydienes were also paid much attention for their diverse microstructures which could dramatically influence the properties of resulting materials. Much effort has been attributed to the modification of polydienes microstructure by changing the active species through incorporation of additives such as barium salt, Lewis base (PPh3) and Lewis acids in a living/controlled polymerization system. In addition, chain shuttling polymerization was also applied for diene polymerization to obtained hard-soft multiblock polydiene on the condition of appropriate catalytic systems and chain transfer agent. Progress in stereospecific living polymerization of methyl methacrylate (MMA) made it possible to synthesize stereoblock poly (methyl methacrylate) (PMMA). Stereoblock PMMA should be of interest from the viewpoint of stereocomplex formation which would occur intramolecularly as well as intermolecularly. Preparation of stereoblock PMMA with narrow molecular weight distribution was achieved by living anionic polymerization with incorporation of unique additives in the early time. Isotactic-syndiotactic stereoblock PMMA were synthesized with efficient metallocene and Lewis acid hybrid catalysts capable of switching stereospecificity of the MMA polymerization. Except for the three kinds of stereoblock polymer, other types of stereoblock polymer such as polystyrene, polylactide and polyacrylamide were also introduced. At present, although it is successful in preparing all kinds of stereoblock copolymers, several problems emerged that the feasible catalytic systems are still limited, some polymerization process demanding strict conditions is not easy to control and the obtained stereoblock polymer is often with low stereoregularity. Besides, the study on the properties of stereoblock copolymers also requires more attentions. Therefore, it is necessary for researchers to develop more practical catalytic system and novel polymerization methods for the better control of macromolecular structure, and to deeply and systematically understand the relationship between macromolecular structure and physical properties.