22 - Ring-opening Polymerization

Abstract

Although the ionic ring-opening polymerization of heterocyclic compounds, such as ethylene oxide, tetrahydrofuran, ethyleneimine, β-propiolactone and caprolactam, as well as the Ziegler–Natta ring opening of cyclic alkenes, such as cyclopentene and norbornene, are well known, free radical ring-opening polymerizations are rather rare. In fact, unstrained five- and six-membered carbocyclic rings are usually involved in ring-closing reactions rather than ring-opening ones. For example, Butler and Angelo1 reported in 1957 that when diallyldimethylammonium bromide was polymerized by a free radical mechanism, a soluble polymer containing five-membered rings was obtained by an inter–intramolecular polymerization as indicated in Scheme 1. Apparently the reaction is kinetically controlled to form the five-membered ring rather than the thermodynamically favored six-membered ring. The few examples of free radical ring-opening polymerization that are reported in the literature involve cyclopropane derivatives or highly strained bicyclic alkenes. For example, Takahashi2 studied the free radical polymerization of vinylcyclopropane and reported that the cyclopropane ring opened to give a polymer containing about 80% 1,5-units and 20% of undetermined structural units. Apparently the radical adds to the vinyl group to give the intermediate cyclopropylmethyl radical which opens at a rate faster than the addition to the double bond of another monomer as in Scheme 2. Somewhat similar results were obtained with the chloro derivatives. More recently, Cho and Ahn3 studied the related malonic ester derivative with similar results. Errede4 found the dimer of o-xylylene would undergo free radical ring-opening polymerization to give the corresponding poly(o-xylylene). In this case the driving force for the ring-opening step is the formation of the aromatic ring (Scheme 3). Hall and co-workers5 showed, as in equation (1), that derivatives of bicyclo[1.1.0]butane would polymerize via free radicals by cleavage of the highly strained central bond. Of course, the ring-opening polymerization of S8 has been postulated to involve free radicals.6