Elsevier

Polymer

Volume 47, Issue 4, 8 February 2006, Pages 1073-1080
Polymer

Synthesis of poly(vinyl alcohol) combs via MADIX/RAFT polymerization

https://doi.org/10.1016/j.polymer.2005.12.004Get rights and content

Abstract

Poly(vinyl acetate) combs have been prepared via macromolecular design via interchange of xanthate (MADIX)/reversible addition–fragmentation chain-transfer (RAFT) polymerization using xanthate functionalized polymer cores. The comb backbones were prepared using well-defined poly(vinyl alcohol) PVA polymers with a degree of polymerization of 20, 100 and 170, respectively. Functionalization with xanthates via R-group or a Z-group approach resulted in the formation of macromolecular MADIX agents. While Z group designed macromolecular xanthate agents appeared to inhibit the polymerization of vinyl acetate (VAc), R group designed macromolecular xanthate agents achieved to mediate efficiently the bulk polymerization of VAc affording PVAc combs. However, the growth of the combs was accompanied at low conversions by the formation of linear polymer chains as a result of the constant initiation (AIBN) and shoulders, which can be attributed to intermolecular coupling reactions. The proportions of single chains and termination products were observed to increase with the degree of polymerization of the macromolecular MADIX agents broadening the molecular weight distribution. As a result of a stable ester link between the branches and the PVA backbone, the branched PVAc architectures were finally hydrolyzed to afford poly(vinyl alcohol) combs.

Introduction

Over the past decade, the design of novel macromolecular architectures with well-defined structure has received substantial interest. Due to their interesting intrinsic properties (low bulk and solution viscosity, high functionality…), increasing research activity has been devoted to the preparation of branched polymers such as stars [1], [2], [3], [4], [5], [6], [7], [8], combs [9], [10], [11], [12] or dendrigrafts [13], [14], [15], [16]. Because of the ease and the versatility of the free-radical process, the synthesis of branched polymers via living/controlled free radical polymerization techniques, i.e. nitroxide-mediated polymerization [17], [18] (NMP), atom transfer radical polymerization [19], [20] (ATRP) or reversible addition-fragmentation chain transfer [21], [22], [23], [24] (RAFT) has been extensively investigated.

Among these free radical techniques, the MADIX/RAFT process appears to be particularly attractive since it can be successfully applied to the preparation of well-controlled polymers using a wide range of monomers.

Aiming at preparing narrow dispersed poly(vinyl alcohol) chains (PVA), a water-soluble, non-toxic and non-carcinogenic polymer with many biomedical applications [25], [26], [27], [28], we recently started a research program on using RAFT chain transfer agents to synthesize well-defined poly(vinyl acetate) (PVAc), a precursor to PVA. We and others demonstrated the ability of xanthates to mediate efficiently the living radical polymerization of vinyl acetate (VAc) [29], [30], [31]. Narrow dispersed linear poly(vinyl acetate) chains up to 50 K were prepared in our laboratory. In further works, we reported the synthesis of well-defined three and four arms PVAc stars from multifunctional xanthate cores [5], [6], [32]. Two categories of multifunctional xanthate cores (R-group and Z-group approach) were designed to prepare PVAc stars via (1) the R-group approach and (2) the Z-group approach. The R-group approach attaching the xanthate functionality to the core via a fragmenting covalent bond was found to be suitable for preparing narrow-dispersed PVAc stars while the distribution of molecular weights tended to broaden via the Z-group approach (xanthate functionality attached to the core via a non-fragmenting covalent bond) due to the steric congestion and the decreased chain transfer activity between the linear macroradical and the xanthate groups. Moreover, while the hydrolysis of the polymers prepared via Z-group approach resulted in the destruction of the architecture as the process also cleaved the xanthate linkage between the core and the arms, the hydrolysis of polymers prepared via the R-group approach afforded well-defined poly(vinyl alcohol) (PVA) star polymers.

In this paper, we describe an extension of the procedure leading to star polymers towards the preparation of well-defined poly(vinyl acetate) combs. We report here the synthesis, from narrow dispersed linear PVAc and subsequently well-defined linear PVA after hydrolysis, of two categories of xanthate functionalized polymer cores designed to synthesize well-defined polymer combs via a R-group or a Z-group approach (Scheme 1). The capacity of the synthetic approaches (R or Z) to provide a quantitative xanthate functionalization of the PVA backbone and the ability of the resulting polymeric cores to mediate the polymerization of VAc are discussed. The impact of the degree of polymerization of the xanthate functionalized polymer cores on the control of the polymerization of VAc is stressed.

Section snippets

Materials

Potassium carbonate (Finechem Ajax, 95%), 2-bromopropionyl bromide (Aldrich, 97%), triethylamine (Aldrich, 99%), O-ethyl xanthic acid potassium salt (Aldrich), carbon disulfide (Aldrich, 99,9%), methyl bromoacetate (Aldrich, 97%) and trioxane (Aldrich, 99%) were used without further purification.

Dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), chloroform and acetone were obtained from Ajax FineChem (Australia) and were dried over activated molecular sieves (4 Å) before use. Methanol

Synthesis of the R and Z designed xanthate functionalized polymer cores

R and Z designed xanthate functionalized polymer cores have been prepared from linear poly(vinyl alcohol) (Scheme 1). The strategy required the presynthesis of well-defined PVAc precursors via MADIX polymerization in the presence of methyl (ethoxycarbonothioyl)sulfanyl acetate. A series of linear precursors with degree of polymerization (DP¯n) ranging from 20 to 170 was thus prepared aiming at evaluating the influence of the polymer core chain length on the control of the polymerization of the

Conclusions

Poly(vinyl acetate) combs were elaborated using xanthate functionalized polymer cores. Contrary to the Z designed macromolecular xanthate agents which were shown to inhibit the polymerization of VAc owing to the presence of impurities within the polymer cores, xanthate functionalized polymers obtained via the R approach were suitable for the synthesis of PVAc comb polymers. However, the comb distributions were accompanied at low or moderate conversions by a linear polymer chains distribution

Acknowledgements

We like to thank the Australian research council (ARC) for a scholarship for JB and a professorial fellowship for TPD. We also would like to acknowledge the excellent management of the research centre (CAMD) by Leonie Barner and Istvan Jacenjik.

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    1

    Present Address: Laboratoire de Chimie des Polymères, Université Paris 6, UMR 7610, 4 Place Jussieu, Tour 44, 1er étage, 755252 Paris, Cedex 05, France.

    2

    Present Address: UMR 6517, CNRS-Université de Provence et Aix-Marseille 3, Avenue Escadrille Normandie-Niemen, 13397 Marseille, Cedex 20, France.

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