Mechanism of network formation by radical copolymerization

Abstract

Radical copolymerization of two monomers, one of them being bi— functional, has been widely used for the synthesis of polymer networks. However, the actual structure of such networks is still controversial, and it has never been related accurately with the copolymerization parameters. It has been assumed that large fluctuations of the length of the elastic chain originate, at least in part, from the fact that the consumption of the two monomers follow different rate laws. It occured to us that the decisive factor in that respect is not the consumption of the bifunctional monomer, but it is the ability of the pendant double bonds to undergo reaction with growing radicals and to form the actual branch points. This is clearly evidenced by recent works carried out on the system styrene—divinylbenzene in which it was shown that, once the gel point has been reached, cross— linking goes on; more links between individual chains are formed, whereby the crosslink density increases and the average length of the elastically effective network chain decreases. A kinetic investigation of the radical copolymerization was carried out on several systems of this type: styrene—divinylbenzene, styrene—diisopropenyl— benzene, styrene—ethylene dimethacrylate, methylmethacrylate—ethylene di— methacrylate. In some cases, a chain transfer agent was added to the system to delay the occurence of network formation. From the conversion curves of each individual monomer — that were obtained from vapor phase chromatography taken at regular time intervals — the instantaneous composition of the co— polymers formed were determined. Thus, the values of the radical reactivity ratios of the two monomers involved could be calculated. It was confirmed that the bifunctional monomer is more reactive and therefore more readily consumed than the monofunctional monomer. In addition, it was shown that the rate of polymerization of styrene is enhanced noticeably in the presence of a small amount of divinylbenzene. Also it was established that the rate of crosslinking does not parallel the rate of consumption of the divinyl monomer. To give account of all results it has to be assumed that in the early stages of the reaction most of the bifunctional monomer reacted gives yield to pendant double bonds (and, perhaps, to a much lesser extent, to cyclization). The reactivity of these pendant unsaturations is far lower than that of the monomers involved, consequently the pendant double bonds are still numerous, once the gel point has been reached. As these double bonds are slowly consumed in the later stages of the reaction (when not much monomer is left over), the network becomes tighter, its swelling ratio decreases and its modulus increases. These findings throw new light into the problem of the structural homogeneity of network synthesized by radical copolymerization. It cannot be stated anymore that the crosslink density is bound to be inhomogeneous in such networks, nor that once the bifunctional monomer is consumed pendant chains are formed solely. In fact it appears that these species are far more homogeneous in structure that it had been assumed previously, and the crosslink density can be related with the composition of the monomer mixture, provided the reaction was pursued until all double bonds have been consumed. INTRODUCTION Polymer networks are commonly obtained by radical copolymerization of two monomers, one of them being bifunctional (Ref. 1—3). It is a method of great industrial interest, which was applied to a large number of systems. However, even in the absence of syneresis (solvent expulsion during the