Stereospecific Radical Polymerization of α-(Alkoxymethyl)acrylates Controlled by Lewis Acid Catalysts:  Mechanistic Study and Effect of Amino Alcohols as Ligand for Zinc Bromide

Abstract

The stereospecific radical polymerization of methyl and benzyl α-(methoxymethyl)acrylates was performed in the presence of Lewis acids, such as ZnBr2 and Sc(OTf)3. The stereochemistry of the polymerization was significantly affected by the coordination of the Lewis acids to the polar groups of the monomer and polymer. The polymerization systems in the presence of ZnBr2 and Sc(OTf)3 produced syndiotactic- and isotactic-rich polymers, respectively, showing different ESR signals. When amino alcohols were added to the polymerization system with ZnBr2, heterotactic-rich polymers were obtained. Therefore, syndiotactic-, isotactic-, and heterotactic-rich polymers can be synthesized by the radical method in the presence of a catalytic amount of ZnBr2, Sc(OTf)3, and the ZnBr2−amino alcohol complex, respectively, without changing the other polymerization conditions, although an atactic polymer was produced in the absence of a Lewis acid. The detailed tacticity (tetrad) of the obtained polymer was estimated by 13C NMR analysis of the main-chain methylene carbon.

Controlled/Living Radical Polymerization of Methacrylic Monomers in the Presence of Lewis Acids: Influence on Tacticity

“…triflates, which interact simultaneously with several coordination sites (Scheme 1b), can increase the isotacticity of polyacrylamides, [12,13] polymethacrylamides, [14,15] poly-(a-alkoxymethyl acrylates), [16] and polymethacrylates. [17] However, stereocontrol strongly depends on the monomer structure, including nature of the a-substituent R 1 and of the coordination function COR 2 (Scheme 2).…”

Stereocontrol during the free‐radical polymerization of methacrylamides in the presence of Lewis acids

“…However, for homopolymerization, although Lewis acids were found to affect the monomer reactivity,25 the effect on the stereocontrol has been rarely reported 1, 5–8, 26–28. Recently, we found that the radical polymerization of α‐(alkoxymethyl)acrylates proceeded in a stereospecific manner with a catalytic amount of Lewis acids 29–31. We also reported that a catalytic amount of rare earth metal trifluoromethanesulfonates (triflates), such as Yb(OTf) 3 and Y(OTf) 3 (where OTf is OSO 2 CF 3 ), effectively increased the isotacticity of the obtained polymers during the radical polymerization of acrylamides 32–34…”

Synthesis and chiral recognition ability of optically active poly{N‐[(R)‐α‐methoxycarbonylbenzyl]methacrylamide} with various tacticities by radical polymerization using Lewis acids

“…However, radical polymerization, although it can be applied to a wide variety of monomers, is not suitable for preparing stereoregular polymers 7–12. Recently, we reported that the addition of catalytic amounts of Lewis acids significantly affected the tacticity during the radical polymerization of α‐(alkoxymethyl)acrylates,13–16 methacrylates,17 and (meth)acrylamides 18–22. For example, N ‐isopropylmethacrylamide (IPMAM) afforded an isotactic‐rich polymer [up to 58% meso triad ( mm )] in the presence of yttrium trifluoromethanesulfonate [Y(OTf) 3 ] at 60 °C in tetrahydrofuran (THF), whereas a syndiotactic‐rich polymer ( rr = 68%) was obtained in the absence of the Lewis acid under the same conditions 19–22.…”