Polymerization and degradation of methacrylate systems

Abstract

The polymerization of methyl methacrylate in bulk initiated by azoisobutyronitrile has been studied to high conversions at a range of initiator concentrations ([AIBN] = 0.0125 - 0.2 mol dm-3) and polymerization temperatures (45 - 80°C), with the aim of evaluating the rate parameters for polymerization across the entire conversion range.Conversion-time data were obtained using modern spectroscopic techniques such as Raman, nuclear magnetic resonance and Fourier-transform near infrared spectroscopy to follow the decrease in monomer concentration during polymerization in situ. A novel technique for estimating conversion, by indirectly measuring the change in dielectric constant during in situ polymerization in an electron spin resonance (ESR) spectrometer, was also developed.The propagating radical concentration was measured continuously throughout polymerization by ESR spectroscopy. A cryogenic, quenching technique was used to measure the low radical concentrations in the steady-state region of polymerization below the gel transition.Combination of the radical concentration-time and conversion-time dependences enabled the calculation of the rate constants for propagation, kp, and termination, kt. It has been possible to obtain these rate parameters at high conversions for the first time, as well as at low conversions. For the polymerization of methyl methacrylate at 60°C, kp = 500 ± 100 dm3mol-1s-1, and kt = 2.1 ± 0.2 x 107 dm3mol-1s-1 in the steady-state region. At higher conversions, kp and kt decrease; the decline in kp occurs after the glass transition point, while kt decreases by several orders of magnitude during the gel effect. In the glass region, for polymerizations at 60°C, kp ≃ 10 ± 5 dm3mol-1s-1, while kt ≃ 1 - 10 dm3mol-1s-1. The decrease in initiation efficiency, f, has also been estimated; at high conversions f ≃ 10-4 - 10-3.The study was extended to a brief examination of the homopolymerizations at 45°C of methacrylic acid and acrylic acid, which are heterogeneous. The concentrations of methacrylic acid propagating radical observed by ESR spectroscopy were an order of magnitude larger than those measured for the polymerization of methyl methacrylate under similar conditions. It was possible to estimate termination rate constants and initiation efficiencies for the homopolymerization of methacrylic acid at high conversion; kt ≃ 0.1 dm3mol-1s-1, and f ≃ 0.1 .The γ-radiolysis of two water-soluble polymers, poly(methacrylic acid) and poly(acrylic acid), has also been investigated. Radical intermediates were identified by ESR spectroscopy. The major reaction is decarboxylation; volatile products detected by gas chromatography comprised mainly carbon dioxide and carbon monoxide, with G(CO2) + G(CO) ≃ 12 for poly(acrylic acid). The yields of these gases from the polymers were considerably larger than yields measured for aliphatic and dicarboxylic acids.It has been demonstrated that scission and crosslinking yields (G(S) and G(X)) can be determined from the dose-dependences of weight- and z-average molecular weights. An advantage of the proposed procedure is the sensitivity of the z-average molecular weight to crosslinking, which allows G(S) and G(X) to be obtained from studies of samples exposed to low radiation doses. The procedure was shown to be applicable to a wide range of initial molecular weight distributions and G(S)/G(X) ratios.Scission and crosslinking yields for poly(acrylic acid) and poly(methacrylic acid) were determined from the dose-dependences of weight- and z-average molecular weights obtained by sedimentation equilibrium measurements using an analytical ultracentrifuge. Poly(acrylic acid) was shown only to crosslink, whereas poly(methacrylic acid) undergoes scission.