Polymerization of exo‐methylene styrenic monomers: Effect of α,α‐cyclization on reactivity and stereoregularity

Abstract

Abstractα‐Substituted styrene derivatives are reluctant to undergo radical homopolymerization in general due to steric hindrance. However, α‐acetoxystyrene (ACOST) polymerizes by radical initiation, which is a slow equilibrium process with a ceiling temperature of 47 °C at 1 mol/L of monomer. Cyclic analogs of ACOST such as 4‐methylene‐4H‐1,3‐benzodioxin‐2‐one (MBDOON) and 3‐methylenephthalide (MP) provide a high yield of polymer in a short period of time in solution whereas the radical polymerization of ACOST is slow in bulk. The overall activation energy for radical polymerization of MBDOON with α,α′‐azobis(isobutyronitrile) in N‐methylpyrrolidinone is 75.7 kJ/mol, much smaller than that for ACOST (116 kJ/mol) and even smaller than the value for styrene (100 kJ/mol). The cyclic acetophenone enol ester polymers are heterotactic, which is in contrast with the sydiotactic poly(ACOST). α‐Methyleneindane (MI) undergoes radical homopolymerization in contrast to its acyclic analog, α‐methylstyrene. Anionic and cationic polymerizations of MI are facile. The both radical and anionic poly(MI)s are heterotactic as opposed to syndiotactic poly(α‐methylstyrene). Thus, the steric hindrance in the transition state of propagation is very much reduced by α,α‐cyclization, resulting in increased reactivity and reduced stereoregulation.