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.

The polymerization of methyl methacrylate in toluene solution at temperatures in the range of −50 to −78°C. has been studied using 9‐fluorenyllithium as an anionic initiator. The strong ultraviolet absorption of the fluorenyl group has been utilized to calculate number‐average molecular weights after comparison with osmotic pressure measurements had shown that one fluorenyl group becomes chemically attached to each chain. Under the conditions of these polymerizations, the chains are not rapidly self‐terminating. Termination of the reaction with C14‐labelled carbon dioxide or tritium‐labelled acetic acid produces radioactive polymers. Comparison of the activity of a polymer with its fluorenyl content determined from ultraviolet measurements gives the percentage of chains which were active when the terminator was added. For example, polymerizations reaching 70% conversion after one hour still had from 80 to 100% of the chains active. The magnitude of a possible tritium‐hydrogen kinetic isotope effect in the acetic acid termination has been estimated by comparing the specific activity of fluorene prepared under the polymerization conditions by reacting 9‐fluorenyllithium with tritium‐labelled acetic acid at −78°C. in toluene with that of the acetic acid. Appropriate correction factors were therefore applied in calculating the specific activities of the polymers. Isotope dilution studies which detect unreacted fluorenyllithium indicate that the initiation reaction is very rapid.