Trityl methacrylate anionic polymerization

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Sterically Restricted Poly (methacrylate ester)s. It was recognized by Okamoto and coworkers150 that the anionic polymerization of tri-phenylmethyl methacrylate (TrMA, 41) (Chart 8) at low temperature in the presence of an optically active initiator results in the formation of an isotactic, optically active polymer. The helical conformation of the backbone in these macromolecules is the result of steric interactions between the bulky trityl groups, as was shown by the loss of optical activity upon their conversion to methyl ester groups. This class of bulky... 

During the two decades after this important discovery, a tremendous amount of research has been directed toward the polymerization of sterically demanding achiral monomers with chiral initiators to create enantiomerically pure helical polymers (also known as helix-sense selective or screw-sense-selective polymerization ). These polymers, known as atropisomers, are stable conformational isomers that arise from restricted rotation about the single bonds of their main chains. Key aspects of these reactions are enantiopure initiators that begin the polymerization with a one-handed helical twist, and monomers with bulky side-chains that can maintain the helical conformation due to steric repulsion. Notable examples of this fascinating class of polymers that are configurationally achiral but conformationally chiral include [8, 38, 39] poly(trityl methacrylate), polychloral, polyisocyanates, and polyisocyanides. Important advances in anionic and metal-based enantioselective polymerization methods have been reported in recent years. 

Optically active polymers can be prepared by free-radical additions that give polymers whose chirality is the result of an excess of one single-screw sense. Most polymers will not maintain a helix screw conformation in solution unless the chain backbone is rigid or the polymer side-chains are very large and prevent conformational relaxation. Polymers derived from trityl and related methacrylates have this apparent capacity, i.e. they display excess helical content in solution. Comprehensive reviews of helix-sense-selective anionic polymerizations have appeared [12], and in this section, we highlight some of the recent developments in this field related to radical polymerizations of these highly hindered methacrylates. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

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