Two fundamental aspects of polymer chemistry were studied in this work. They were the free radical polymerization of a number of monomers and the high energy radiation chemistry of several polymers.
The bulk copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM) has been studied for a range of compositions to high conversion using azo—bis—iso—butyronitrile as the thermal initiator. The polymerizations were investigated at three temperatures: 45, 52 and 60°C. The presence of the divinyl monomer, EGDM, introduces crosslinks, cyclization and pendant double bonds. The aim of the work was to measure the rate coefficients for propagation and termination across the entire conversion range and to examine the effect that the divinyl monomer has on these coefficients.
The conversion—time data were obtained using a number of different techniques: in situ and solution H NMR spectroscopy, Raman spectroscopy and Fourier transform near infrared spectroscopy. The most suitable technique for the investigation of crosslinking systems to high conversion was FTIR, which could be used to obtain accurate instantaneous monomer concentrations across the entire conversion range.
The propagating radical concentration was measured using ESR spectroscopy. In situ ESR was suitable at conversions beyond the Trommsdorff point when the radical concentration was sufficiently large to give a spectrum with good signal—to—noise ratio from a single scan. In the pre-Trommsdorff region the radical concentration was measured using a cryogenic, quenching technique.
By combining the radical concentration and the double bond concentration, the rate coefficients for propagation and termination could be calculated. For the homopolymerization of MMA at 60°C in the steady state region, kp = 470 100 M-1s-1 and kt = 1.5 x 107 M-1s-1. At higher conversions, kp and kt decrease; kt decreases during the Trommsdorff region while the decrease in kp occurs at the glass point. For the homopolymerization of MMA at 60°C, kp 2.70.2 M-1s-1 and kt≅ 3.6 M-1s-1 at 90% conversion. There has been a decrease in the efficiency of initiation by several orders of magnitude, from 0.6 in the pre—Trommsdorff region to 0.001 in the glass region.
The effect of the divinyl monomer was to decrease the time to Trommsdorff and to lower the conversion of double bonds at the glass point. In the post—glass region, the rate coefficients for propagation and termination were found to decrease with higher proportions of the divinyl monomer. The efficiency of initiation was found to increase in the post—glass region with higher proportions of the divinyl monomer. This reflects the increased double bond concentration in this region. The effect of a longer segment between the double bonds of the divinyl monomers was investigated by studying the homopolymerization of triethylene glycol dimethacrylate. Because of the increased flexibility of the group with the longer chain segment, the conversion at the glass point was found to have increased and the rate coefficients for both propagation and termination were greater than for the homopolymerization of ethylene glycol dimethacrylate.
The radiation chemistry of two polyesters was studied in the second part of this work. The polyesters were poly(methyl methacrylate) (PMMA) where the ester group is in the side chain and copolymers of hydroxy butyrate and hydroxy valerate (PHB/HV) which have the ester group in the backbone. The radiation chemistry was investigated using the following techniques:
(i) gas chromatography — to identify the volatile radiolysis products and measure their yields
(ii) electron spin resonance — to identify the free radical intermediates and measure their yields at ambient temperature and 77K. Annealing studies were helpful in identifying the free radical intermediates.
(iii) gel permeation chromatography to measure the molecular weight changes and therefore the yields of chain scission and crosslinking
(iv) nuclear magnetic resonance to study the changes in the polymer structure.
For PMMA the effect of tacticity on the radiation chemistry was investigated. Three polymers were studied sPMMA, iPMMA and a polymer prepared by free radical initiation. For all three polymers the major volatile products were carbon monoxide and carbon dioxide and their yield was approximately 2.5. The radical yield in all cases was approximately 1.5, though the radicals observed was dependent on the tacticity of the polymer. Chain scission was the final result for all three polymers but the yield was dependent on the tacticity of the polymer. On the basis of this evidence it is suggested that the major degradation pathway leading to main chain scission is dependent on the tacticity of the polymer.
By the use of ESR, the carbonyl anion radical was identified after high energy irradiation of P(HB/HV). Decay of this carbonyl anion radical leads to the formation of carbon monoxide and carbon dioxide, which are the major volatile products. The total yield of these products is 1.7. G(scission) and the radical yield for the homopolymer were found to be 1.3 and 1.6. All these yields have the same order of magnitude suggesting that the chemistry which leads to the formation of the carbon dioxide and carbon monoxide results in scission of the polymer chain. In the homopolymer, where the side chain is a methyl group, no crosslinking was observed. However, for the copolymers which contain an ethyl side chain from the hydroxy valerate unit, crosslinking was found to occur and to increase with the proportion of hydroxyl valerate.