Graft copolymers of natural rubber and poly(dimethyl(acryloyloxymethyl)phosphonate) (NR-g-PDMAMP) or poly(dimethyl(methacryloyloxyethyl)phosphonate) (NR-g-PDMMEP) from photopolymerization in latex medium

Abstract

Graft copolymers of natural rubber and poly(dimethyl(acryloyloxymethyl)phosphonate) (NR-g-PDMAMP), and natural rubber and poly(dimethyl(methacryloyloxyethyl)phosphonate) (NR-g-PDMMEP), were prepared in latex medium via a “grafting from” methodology based on the photopolymerization of dimethyl(acryloyloxymethyl)phosphonate (DMAMP) and dimethyl(methacryloyloxyethyl) phosphonate (DMMEP), respectively, used as phosphorus-containing monomers. The grafting polymerization was initiated from N,N-diethyldithiocarbamate groups previously bound in side position of the rubber chains. The effects of monomer concentration on monomer conversion and grafting rate were investigated, showing that conversion and grafting rate increased with increasing monomer concentration and reaction time. Highest conversions and grafting rates were obtained with a molar ratio [DMAMP]/[initiating units] = 7 for a reaction time of 180 min. Calculation of the graft average length ($\overline{\text{DP}}\text{n}$) from ¹H NMR spectra of the synthesized graft copolymers showed $\overline{\text{DP}}\text{n}$ values were in the range of 9–73. Visualizations of NR-g-PDMAMP and NR-g-PDMMEP latices by Transmission Electron Microscopy (TEM) showed that they exhibit core-shell morphologies. Degradation of NR-g-PDMAMP and NR-g-PDMMEP occurred in two steps: decomposition of dimethylphosphonate-functionalized grafts took place prior to the second step corresponding to the decomposition of NR backbone, but the degradation temperature of this last step was higher than that of pure NR.

Introduction

Natural rubber (NR) is an interesting material with commercial success due to its excellent physical properties (i.e., high mechanical strength, low heat build up, excellent flexibility, and resistance to impact and tear), but also to the fact that NR is a renewable resource. However, NR has also some drawbacks as, for instance, low flame resistance, sensitivity to chemicals and solvents (ozone and weathering), mainly due to its unsaturated hydrocarbon chain structure and its non-polar character, which cause limitation in variety of applications. Therefore, chemical modification of NR has been widely studied to improve for instance gas permeability, oil resistance, and flame resistance [1], [2]. Various types of well-known modified NR products were thus prepared such as epoxidized natural rubbers [3], [4], maleated natural rubbers [5], [6], and graft copolymers of NR and PMMA [7], [8] and NR and polystyrene [9], [10].

Chemical modification of NR macromolecules is one possible approach to prepare new materials. One of the convenient ways is to perform graft copolymerization by incorporating phosphorus-containing monomers onto NR chains. This type of monomer was successfully grafted onto polyethylene terephtalate fiber initiated by gamma radiation [11]. Several types of vinylphosphonate monomers were used including N-(dimethylphosphonomethyl)acrylamide, dimethyl(vinylmethoxy)phosphonate, dimethyl(vinylacetoxy)phosphonate, dimethyl(acryloyloxymethyl)phosphonomethyl, dimethyl(vinyl)phosphonate, dimethyl(allyl)phosphonate, and diethyl(vinyl)phosphonate. Furthermore, the low-pressure plasma technique was used to confer a fire-resistant character to polyacrylonitrile (PAN) textiles by grafting and polymerization of various acrylate phosphorus-containing monomers, such as diethyl(acryloyloxyethyl)phosphate, diethyl-2-(methacryloyloxyethyl)-phosphate, diethyl(acryloyloxymethyl)phosphonate, dimethyl(acryloyloxymethyl)phosphonate, diethyl(acryloyloxyethyl)phosphoramidate, and acryloyloxy-1,3-bis(diethylphosphoramidate)propane [12], [13]. All these works found that the flame-retardant effect correlated mostly with the chemical structure of the monomer and the grafted phosphorus amount. Cellulose phosphonate modified with N,N-dimethylacrylamide and 4-vinylpyridine was also investigated showing powerful flame-retardant properties [14]. Dialkylphosphate reagents could be introduced onto epoxidized natural rubber (ENR) via a ring opening substitution mechanism on oxiranes [15]. However, the grafting of monomers bearing phosphonate groups onto natural rubber has not been previously reported.

In order to improve NR properties (i.e., flame-retardancy and oil resistance), but also to obtain new NR derivatives being able to be used as blend compatibilizers to promote the compatibility between NR and some polar thermoplastics, the grafting of polymers bearing phosphonate groups have been envisaged on NR by using a “grafting from” methodology we have recently developed [16], [17], [18]. The procedure is based on the pioneering works of Otsu et al. in 1982, which were the first to report “living” radical photopolymerization of vinyl monomers initiated by “iniferter” [19], [20], [21], [22], [23] (Scheme 1). It consists to introduce N,N-diethyldithiocarbamate iniferter groups in side position of the rubber chains [16], and then to use these “iniferter” groups to initiate the photopolymerization of acrylate or methacrylate phosphorus-containing monomers [17], [18]. Under UV irradiation the rubbery macroiniferters dissociate to form stable N,N-diethyldithiocarbamate radicals and active radicals along the NR chains capable to initiate monomer polymerization. During propagation, there are two major processes based on equilibrium between “living” polymer grafts and “non-living” ones. The reversibility between the growing radical grafts and stable N,N-diethyldithiocarbamate radicals cause reversible terminations to form non-propagating polymer grafts. The grafting chain growth can also proceed after chain transfer. Moreover, irreversible terminations such as combination or transfer can occur to stop definitively the chain growth.

The present paper deals with the syntheses of graft copolymers of NR and poly(dimethyl(acryloyloxymethyl)phosphonate) (NR-g-PDMAMP), and NR and poly(dimethyl(methacryloyloxyethyl)phosphonate) (NR-g-PDMMEP), respectively. In a first part, the syntheses of the phosphorus-containing monomers, i.e., dimethyl(acryloyloxymethyl)phosphonate (DMAMP) and dimethyl(methacryloyloxyethyl)phosphonate (DMMEP), will be described. Then, the results of the study of their graft copolymerization onto NR chains carried out in latex medium will be given.