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We have measured the molecular weight distribution ͑MWD͒ in a case of equilibrium polymerization. We have studied the time development of the MWD of ''living'' bifunctional poly͑␣-methylstyrene͒ in tetrahydrofuran after a quench to 21 K below the polymerization temperature, T p. We see an intermediate Gaussian distribution evolving toward a final exponential distribution, as expected from theoretical considerations. We see a longer equilibration time for the number average molecular weight (M n) as well as for the weight average molecular weight (M w) than for the monomer concentration ͓͑M͔͒, whereas theories predict that M n and ͓M͔ will relax together and that M w will take much longer. We attribute the delayed equilibration and a second peak at about M n /4 to the effects of ionic aggregation of the living polymers. We have also studied the equilibrium MWD of this system as a function of the temperature below T p , and thus as a function of the number average degree of polymerization ͑L͒. These measurements and the time study discussed above are the first experimental evidence that the equilibrium MWD for an organic polymer in a state of equilibrium polymerization is an exponential/Flory-Schulz distribution, and is consistent with scaling predictions. Near T p and at low L, we observe a deviation from the exponential distribution, which may be evidence of the effect of a chain-length dependence of the equilibrium constant for polymerization, or of the effects of polydispersity on correlations due to excluded volume. In addition, the measured L is about two times less than that expected from the initiator concentration; this could result from ionic aggregation or from chain transfer reactions.

We present here measurements of the heat capacity at constant pressure near the ceiling temperature for the anionic polymerization of an organic monomer. The measurements support the suggestion [S. J. Kennedy and J. C. Wheeler, J. Chem. Phys. 78, 953 (1983); J. C. Wheeler and P. M. Pfeuty, Phys. Rev. Lett. 71, 1653 (1993)] that the onset of equilibrium polymerization in initiated monomer solutions as the temperature is changed can be viewed as a second-order continuous phase transition. We compare our measurements on living poly(α-methylstyrene) initiated by sodium naphthalide in tetrahydrofuran to the theoretical predictions, and find qualitative agreement. We attribute quantitative differences to the dependence of the enthalpy of polymerization on the degree of polymerization and on the tacticity.

We measure the extent of polymerization, Φ, as a function of temperature, T, for living poly(α-methylstyrene) in the solvent tetrahydrofuran, using sodium naphthalide and cesium naphthalide as the initiating species. We compare our measurements of Φ(T) to the theoretical predictions of a mean field model and of a nonmean field model [J. C. Wheeler and P. M. Pfeuty, Phys. Rev. Lett. 71, 1653 (1993)]. We find that both models describe our observations semiquantitatively. The nonmean field model is statistically (but not dramatically) a better description for two of the samples. For the third sample, the mean field description is statistically better, probably because the concentration of monomer/polymer in solvent is the smallest for this sample.