Ultrasonic Solution Degradations of Poly(Alkyl Methacrylates)

Abstract

Ultrasonic (70 W, 20 kHz) solution (2%) degradations of poly(alkyl methacrylates) have been carried out in toluene at 27°C and in tetrahydrofuran (THF) at -20°C. M_w and M_n of all polymers (before and after sonification) were computed from GPC. Irrespective of the alkyl substituent, M_w decreased rapidly at first and then slowly approached limiting values. All M_w/M_n ratios were in the vicinity of 1.5 at the limiting chain lengths. For identical M_n, the rate constants k were (4.2 ± 2.0) × 10⁻⁶ min⁻¹ in toluene at 27°C and (5.4 ± 2.0) × 10⁻⁶ min⁻¹ in THF at -20°C. For poly(isopropyl methacrylate) and poly(octadecyl methacrylate) with higher, but identical, M_n,0, k values were higher ((9.0 ± 1.0) × 10⁻⁶ min⁻¹ at 27°C and (18.0 ± 1.5) × 10⁻⁶ min⁻¹ at -20°C). This suggests that M_n,0 and not the bulk size of the alkyl substituents is the factor that determines the rate of degradation. Lowering of the temperature accelerates degradation due primarily to lower chain mobility of poly-(alkyl methacrylates) and enhanced cavitation. The average number of chain scissions ([(M_n)₀/(M_n)_t] - 1) calculated from component degradation data are much higher than those obtained with overall M_n,t values.