Copolimeros

Living characteristics of vinyl acetate polymerization induced by PMMA macroinitiators: Preparation of block and gradient copolymers

H. C. García-Valdez, H. Maldonado-Textle and R. Guerrero-Santos(
Centro de Investigación en Química Aplicada, Polymer Chemisty Department
Blvd. Enrique Reyna Hermosillo 140; Saltillo Coahuila, Mexico 25100

* Corresponding author. Tel. +52 844 4389830;E-mail address: ramirog@ciqa.mx

Abstract
Methylmethacrylate polymerizations were carried out using tetraphenyl-1,2-di-trimethylsilyloxyethane (TPSE) as an initer to produce thermolabile end-functionalized polymers. These polymers were then heated in the presence of vinylacetate (VAc) to regenerate the polymethylmethacryloyl radical which resumed polymerization to form block copolymers essentiallyfree of homopolymers. First-order kinetics were observed and the molecular weight of the block polymers increased linearly with conversion indicating that the concentration of the mediating free radical diphenylmethyltrimethylsilyloxy (DPSM) remain steadily low allowing polymerization to proceed as a living-like process. Methylmethacrylate/VAc mixtures were then used to prepare copolymers incopolymerizations that exhibited enhanced living characteristics. The methylene region on the 13C and 1H NMR spectra of products gave evidence about the formation of gradient copolymer. Products containing up to 68 mol% of VAc, with an initial broad molecular weight distribution that narrowed with increasing conversion were obtained.

1. Introduction
The search for a polymerization system to formblock copolymers composed of vinyl acetate (VAc) has long been of keen interest due to the enormous commercial importance of its hydrolyzed form; poly(vinyl alcohol). Because VAc chain polymerization is restricted to free-radical additions, only the ‘living’ process known as Controlled Radical Polymerization (CRP) [1] could, a priori, be used to produce block copolymers or other polymers withcontrolled monomer arrangements. However, CRP of VAc has only very limited success due to its highly reactive propagating species and its ability to induce chain transfer reactions.
Initial attempts on ‘living’ radical polymerization of VAc were reported in 1994 [2-3]. These earlier studies postulated that a persistent free radical generated from an organoaluminum compound was the speciesresponsible for the reaction control. However, it was proved later that this initiating system was unable to trigger a polymerization based on a series of experiments carried out under high-purity conditions which only yielded traces of polymer [4]. Other metallic catalysts, such as CCl4/Fe(OAc)2/PMDETA [5] and (Fe(Cp)(CO)2(2 [6], were evaluated, but the experimental molecular weight of polymers did notcorrespond precisely with the theoretical one. Furthermore, the Atom Transfer Radical Polymerization (ATRP) mediated by Cu-complexes had been shown to be ineffective. Our research group had also confirmed that Radical Addition Fragmentation Transfer (RAFT) polymerizations with dithioesters or trithiocarbonates were adversely affected by slow fragmentation which resulted in virtually zero monomerconversion. Nevertheless, other groups had discovered that some particular xanthates [7-8] were proven successful in polymerizing VAc in a controlled manner. An example was the use of a tetrafunctional form of methyl(ethoxycarbonothioyl) sulfanyl acetate to synthesize four-branch stars of poly(vinyl acetate) (PVAc) [9]. Another successful work was the degenerative iodine transfer process mediatedwith alkyl iodides to synthesize PVAc with low polydispersity [10–12]. Recently, these compounds were combined with fluoroalcohols to control the MWD and to enhance syndiotacticity [13]. Despite a certain degree of control was achieved in the CRP of VAc with xanthates or alkyl iodides, the high reactivity of the propagating radicals would induce side reactions that broaden the molecular-weight...