Stereocontrol in Polymerization

In 1977 Deslongchamps formulated, on the basis of a number of observations of different reactions (e.g. ozonolysis, hydrolysis) of heterocyclic compounds the following rule of reactivity  

This rule, born as a purely experinKntal observation has been applied very recently to explain son phenomena in the jpolymerizatfon of cyclic amides by Bertalan and cyclic orthoesters by Hall l 

An alternative mechanism of the cationic polymerizatimi of lactams was postulated by Bertalan The protonated amino end-group existing in equilibrium with a tetrahedral intermediate 188d) was assumed as the growing species. 

In order to explain the termination reaction, leading to the cyclic amidate cation through removal of the water molecule, Bertalan proposed the antiperiplanar participation (structure 188b)  

It seems to us however that propagation, proceeding via tetrahedral intermediate should involve activated monomer. Nucleophilic attack of the amine on positively charged carbon atom in activated monomer molecule is followed by proton transfer either to oxygen or nitrogen atoms. The deavage of C—O bond (termination) in the intermediate is accelerated by two antiperiplanar orbitak at N atoms 188b). 

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Yoshio Okamoto was born in Osaka, Japan, in 1941. He received his bachelor (1964), master (1966), and doctorate (1969) degrees from Osaka University, Faculty of Science. He joined Osaka University, Faculty of Engineering Science, as an assistant in 1969, and spent two years (1970— 1972) at the University of Michigan as a postdoctoral fellow with Professor C. G. Overberger. In 1983, he was promoted to Associate Professor, and in 1990 moved to Nagoya University as a professor. His research interest includes stereocontrol in polymerization, asymmetric polymerization, optically active polymers, and enantiomer separation by HPLC. He received the Award of the Society of Polymer Science, Japan, in 1982, the Chemical Society of Japan Award for Technical Development in 1991, the Award of The Chemical Society of Japan (1999), and the Chirality Medal (2001), among others. 

Stereocontrol in polymerization can arise from either chain-end control, where the stereochemistry of the chain-end determines the configuration of the incoming monomer, or enantiomorphic site control, where the chirality of the ligand imparts stereoselectivity. However, these two mechanisms are not mutually exclusive, and often act in concert in a given system. 

Free radical propagation is poorly stereocontrolled, with nearly equal proportion of meso and racemic dyads in polymerization of monosubstituted alkenes and a preference for syndiotactic placement for disubstituted monomers such as methacrylates (rr = 0.62, mm = 0.04). The sequence distrihution follows a first-order Markov model with a slight deviation from Bernoulian statistics. However, for very bulky substituents, as in polymerization of triphenylmethyl methacrylate, the preference for isotacticity was observed (mm = 0.64, rr = 0.12). Recently, complexation with Lewis acids and acidic solvents enabled to enhance stereocontrol in polymerization of vinyl esters and acrylamides, and to a smaller degree in polymerization of methacrylates (127-129). 

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