Free radical graft polymerization and copolymerization at higher temperatures

Abstract

Recent work on the graft polymerization of vinyl monomers to polyolefins and model hydrocarbons is reviewed. For practical reasons, these reactions are performed at high temperatures, typically 160–200 °C, with tertiary peroxides such as di- tert-butyl peroxide (DBP) as initiators. Vinyl monomers of particular interest include maleic anhydride, vinyl silanes and methacrylates. These monomers have low ceiling temperatures and are less prone to undergo concurrent homopolymerization at 160–200 °C than monomers such as acrylates and styrene which have normal enthalpies of polymerization. Initiation of grafting typically involves hydrogen abstraction from the hydrocarbon/polyolefin by tert-butoxy radicals. At high temperatures, β-scission of tert-butoxy radicals also occurs to give alkyl radicals which primarily react with monomers to initiate homopolymerization. There are two graft propagation steps—addition of monomer to a hydrocarbon/polyolefin radical, R and hydrogen abstraction by the resulting RM adduct to yield a singly attached monomer unit and a new R type radical. It is argued that the otherwise slow hydrogen abstraction occurs readily in these grafting systems for two reasons. One is the high temperature, which favors the process of higher activation energy. The second is intramolecular hydrogen abstraction with linear and branched hydrocarbons. The grafting of a number of vinyl monomers to hydrocarbon substrates is described in terms of the proposed mechanisms. The recently developed application of living free radical polymerization to the preparation of block copolymers is discussed. At temperatures above 100 °C, polymer radical–nitroxide complexes partly dissociate thus enabling addition of the same or a different vinyl monomer. Some recent applications are described.