Solvent effect, radiation initiation

The principal features of this radiation initiated ionic polymerization as understood at the present time has been recently presented and will not be repeated here. The unequivocal free ion nature of the polymerizations gives us an opportunity to study such reactions and compare the results with those obtained with, for example, chemical initiation with stable carbonium ion salts. This paper will describe the results of such studies with cationic polymerization and be limited to two vinyl ethers, ethyl vinyl ether, EVE, and isopropyl vinyl ether IPVE which behave in somewhat different ways. In particular, methods of estimating the rate constants for propagation will be presented and the results obtained discussed with particular emphasis on the effect of solvents on these values. 

Grafting to cellulose in the absence of solvent (87) is initiated by y-rays by means of a °Co source. The polymer-to-monomer ratio has only a minor effect on the PVF content, which is in the range of 2-5 wt%. In the presence of solvents that cause swelling of cellulose (mercerization), the PVF content increases dramatically. At a radiation of 30 kGy (3 Mrad) in the presence of dimethylformamide, the PVF content of the graft copolymer is 26.6% in its absence it is 4.1%. 

The radiation-initiated polymerization of vinylidene fiuoride is used on a laboratory scale only. The effect of polymerization conditions on the chain defects, content, and changes in the crystalline phases have been studied [528,536,537,544]. Doll and Lando [544] used a °Co source with an average dose rate of 0.33 Mrad/h. The polymerization was carried out between 0 and 400 °C at a pressure equal to the vapor pressure of the solvent-vinylidene fluoride mixture. Esters and ketones (acetone, methyl ethyl ketone, ethyl acetate, acetophenone), DMF, DMSO, and y-butyrolactone were used as solvents. All these solvents are good chain-transfer agents for vinylidene fluoride. The molding characteristics of the resulting polymers were very poor and the intrinsic viscosity of the sample polymerized in acetone solution was low (0.183 dL/g) compared to that of a suspension-polymerized polymer (1.68 dL/g) [544]. 

Lando JB, Semen J, FarmerB. Solvent and temperature effects in the radiation-initiated synthesis of stereoregular poly(methacryhc acid). Macromolecules. 1970 3 524-527. 

Polymerisations of undiluted, bulk monomer are rare except for those initiated by ionising radiations and they require a special treatment which will be given later. The most common situation is to have the propagating ions in a mixture of monomer and solvent, and as the solvation by the solvent is ubiquitous and may dominate over that by other components of the reaction mixture, mainly because of the mass-action effect, it will not be noted by any special symbol, except in a few instances. This means that we adopt the convention that the symbol Pn+ denotes a growing cation solvated mainly by the solvent correspondingly kp+ denotes the propagation constant of this species, subject to the proviso at the end of Section 2.3. Its relative abundance depends upon the abundance of the various other species in which the role of the solvent as the primary solvator has been taken over by any or all of the anion or the monomer or the polymer. The extent to which this happens depends on the ionic strength (essentially the concentration of the ions), and the polarity of the solvent, the monomer and the polymer, and their concentrations. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

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