Stereospecific Polymerization of Chiral Oxazolidinone-Functionalized Alkenes

Abstract

Acryloyl and vinyl monomers functionalized with a chiral oxazolidinone auxiliary have been successfully polymerized in a stereospecific fashion to highly isotactic, optically active polymers, through either the previously established isospecific coordination polymerization (for acryloyl monomers) or a novel isospecific cationic polymerization (for vinyl monomers). Specifically, conjugated chiral acryloyloxazolidinones, N-acryloyl-(R or S)-4-phenyl-2-oxazolidinone [(R or S)-AOZ], are readily polymerized by chiral ansa-zirconocenium coordination catalysts, (R,R-, S,S-, or R,R/S,S)-[C2H4(η5-Ind)2]Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− (1), in an isospecific manner through a catalyst-site-controlled mechanism, producing the corresponding optically active chiral polymers, (R or S)-PAOZ. Owing to the nature of stereocontrol dictated by the chiral catalyst site, even the coordination polymerization of the parent AOZ, without the chiral side group, also affords PAOZ with nearly quantitative isotacticity. A series of experiments have shown that the chiral polymers (R or S)-PAOZ exhibit no chiral amplifications, despite having stereoregularly placed stereogenic centers in the main chain, and the optical activity of the polymers arises solely from their chiral auxiliary, a consequence of adopting a random-coil secondary structure and thus having a cryptochiral chain. In sharp contrast, the chiral isotactic polymers derived from nonconjugated chiral vinyl oxazolidinones, N-vinyl-(R)-4-phenyl-2-oxazolidinone [(R)-VOZ] and its p-hexyloxyphenyl derivative (R)-HVOZ (designed to solve the solubility issue of the resulting polymer), exhibit substantial chiral amplifications by virtue of adopting a solution-stable, one-handed helical conformation. The synthesis of such helical vinyl polymers has been accomplished by the development of a novel isospecific cationic polymerization using Lewis and Brønsted acids, such as [Ph3C][B(C6F5)4], BF3·Et2O, and [H(Et2O)2][B(C6F5)4], through a chiral auxiliary-controlled mechanism. Noteworthy is the combination of the near-quantitative isotactic placement of the stereogenic centers of the polymer main chain with the chiral side groups located near those stereocenters that renders one-handed helicity of (R)-PVOZ and (R)-PHVOZ. Significantly, this novel cationic polymerization process, operating at ambient temperature, effectively assembles two elements of polymer local chirality—side-chain chirality and main-chain chirality—into global chirality in the form of excess one-handed helicity. Furthermore, the resulting chiral helical vinyl polymers exhibit considerably higher thermal decomposition temperatures and polymer crystallinity, in comparison to the random-coil chiral acryloyl polymers, having a similarly high degree of main-chain stereoregularity.