The mechanism of the anionic polymerization of methyl methacrylate. II. The use of molecular weight distributions to establish a mechanism

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Abstract
Calculations of the molecular weight distribution to be expected from a nonterminated polymerization in which the rate of initiation is rapid with respect to that of propagation predict that the weight‐ to number‐average molecular weight ratio should approach 1.0. The ratios actually obtained are considerably larger than the predicted value; the resolution of this apparent contradiction is the central problem of this paper. Fractionation of isotactic poly(methyl methacrylate) was accomplished by a combination of two techniques: liquid‐liquid counter‐current extraction and elution chromatography. The fractionation indicated that: (1) The molecular weight distribution of these polymers is bimodal. (2) More than 90 mole‐% of the polymer chains has a molecular weight less than 2000. (3) These low molecular weight chains add tritium upon reaction with tritiated acetic acid and do not therefore result from irreversible termination reactions. (4) The molecular weight distribution of the high molecular weight material separated by elution chromatography is sufficiently broad to approximate a log‐normal distribution. (The log‐normal distribution is commonly used to characterize linear polyethylenes, which have very broad molecular weight distributions.) Kinetics were measured in a 90% toluene‐10% diethyl ether by volume mixture at −60°C. These experiments showed that: (1) The rate of polymerization appears to be first order in monomer and remains constant to very high conversion of monomer to polymer. (2) Plots of –In M/M0 versus time (where M and M0 represent monomer concentration at time t and 0. respectively) can be extrapolated to a finite conversion intercept at zero time. (3) This conversion corresponds to that required to form a species containing one fluorenyl group and three monomer units. (4) Plots of number‐average molecular weight versus time converge to the same intercept at zero time, regardless of the ratio of initial concentration of monomer to that of initiator, and that intercept is the molecular weight of the species just mentioned, FlM3. The mechanism which has been derived from these data has the following essential postulates: (1) Chains of different steric configurations may propagate at different rates even in a homogeneous medium. (2) A high percentage of the total number of chains started undergoes a “pseudotermination” whose probability is a function of the configurations of the last three monomer units in the chain. (3) When a chain attains a minimum length of 8–10 monomer units, the probability that further additions will take place in isotactic sequence is greatly increased. (4) The source of this increased probability is thought to lie in the inherent asymmetry of preferred helical conformations which may exist for chains of this minimum length in solution at −60°C.

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