UV-INITIATED REACTIONS OF STYRENES WITH ELECTROPHILIC OLEFINS HAVING A BETA-LEAVING GROUP - PARTITIONING BETWEEN FREE-RADICAL COPOLYMERIZATION AND CATIONIC HOMOPOLYMERIZATION
Mg. Mikhael et al., UV-INITIATED REACTIONS OF STYRENES WITH ELECTROPHILIC OLEFINS HAVING A BETA-LEAVING GROUP - PARTITIONING BETWEEN FREE-RADICAL COPOLYMERIZATION AND CATIONIC HOMOPOLYMERIZATION, Macromolecules, 28(22), 1995, pp. 7339-7342
Three electrophilic olefins with various beta-leaving groups, namely,
1-cyano-1-carbomethoxy-vinyl 2-chloride (I), 1-cyano-1-carbomethoxyvin
yl 2-iodide (II), and 2-cyano-2-carbomethoxyvinyl p-toluenesulfonate (
III), reacted under the photochemical conditions with three moderately
electron-rich aromatic olefins, styrene, p-methylstyrene, and p-metho
xystyrene, in 1,2-dichloroethane solution at 0 degrees C. Reaction con
ditions were adjusted to minimize both the thermal dark polymerization
and the photo-self-initiation of the styrenes. The reactivity of thes
e olefins depends on the nature of the leaving group, tosylate > chlor
ide > iodide. The electron-rich character of the styrene monomers also
controls the tendency toward cationic homopolymerization: p-methoxyst
yrene > p-methylstyrene > styrene. Also the stability of the leaving g
roup anion (tosylate > iodide > chloride) controls the distribution be
tween cationic homopolymerization and free-radical copolymerization. A
mechanism was proposed to show the interaction between the excited el
ectron-rich styrene derivative and the electron-poor olefins which lea
ds to the formation of tetramethylene polar 1,4-diradicals. These eith
er initiate the free-radical copolymerization or eject the leaving gro
up anion and create carbocations which initiate the homopolymerization
of styrenes.