Syntheses of alternating head-to-head copolymers

Abstract

In the study of synthesis and property of head-to-head alternating copolymer of various monomers, 2-methoxy-6-ethoxy-5-cyano-3,4-dihydro-2H-pyran ($\underline{1}a$), 2,6-diethoxy-5-cyano-3,4-dihydro-2H-pyran ($\underline{1}b$) and 2-n-butoxy-6-ethoxy-5-cyano-3,4-dihydro-2H-pyran ($\underline{1}c$) were prepared by [4+2] head-to-head (H-H) cycloaddition reaction of ethyl α-cyanoacrylate with the corresponding vinyl ethers. 2-Methoxy-6-methyl-3,4-dihydro-2H-pyran ($\underline{1}d$), 2-ethoxy-6-methyl-3,4-dihydro-2H-pyran ($\underline{1}e$), 2-n-butoxy-6-methyl-3,4-dihydro-2H-pyran ($\underline{1}f$) and 2-ethoxy-6-phenyl-3,4-dihydro-2H-pyran ($\underline{1}$g) were prepared from the corresponding vinyl ketones and alkyl vinyl ethers. $\underline{1}a$, $\underline{1}b$, $\underline{1}c$, $\underline{1}d$, $\underline{1}e$, $\underline{1}$f and $\underline{1}g$ were ring-opening polymerized by cationic or anionic catalysts to obtain high molecular weight polymers of H-H alternating sequence. For the comparison the copolymers of head-to-tail (H-T) structures were also prepared by free radical copolymerization of the mixtures of the corresponding monomers. The H-H copolymers exhibited only minor difference in their $^1H$ NMR and IR spectra but in $^{13}C$ NMR spectra H-H copolymers showed significant difference exhibiting the signal attributable to the extra backbone methylene carbon. Also to note was that Tg values of H-H copolymers were always higher than those of the corresponding H-T copolymers. Decomposition temperatures of H-H copolymers were usually lower than those of H-T copolymers. All the H-H and H-T copolymers were soluble in common solvents.