Abstract
High resolution n.m.r. spectra are readily obtained at room temperature on ten per cent solutions of polymethyl methacrylate in chloroform. For polymers prepared by free radical polymerization, the tacticity parameter β (the probability that neighbouring asymmetric carbon atoms in the chain are in syndiotactic placement) computed accurately (±2 per cent) from the relative areas of the α-methyl magnetic resonance peaks, is observed to obey Bernoulli trial statistics (as reported by others), to increase from 0·64 to 0·86 as the temperature chosen for polymerization ranges from 250° to −40°, and to be unaffected by other polymerization variables such as the initiator, the conversion, the molecular weight of the product, and the presence of any of a variety of solvents. Accordingly, the reaction producing an isotactic placement occurs with an Arrhenius factor 1·65-fold greater, and an activation energy 1·07 kcal larger, than the corresponding variables for the competing syndiotactic addition, a result somewhat different from that found in Bovey's pioneering investigation. Speculations on correlations between sequence length distributions and the diffraction pattern and solubility of partially crystalline polymers are included.
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