Living poly(alpha-methylstyrene) near the polymerization line. VII. Molecular weight distribution in a good solvent

We have measured the molecular weight distribution (MWD) in a case of equil ibrium polymerization. We have studied the time development of the MWD of " living" bifunctional poly(alpha-methylstyrene) in tetrahydrofuran after a q uench to 21 K below the polymerization temperature, T-p. We see an intermed iate Gaussian distribution evolving toward a final exponential distribution , as expected from theoretical considerations. We see a longer equilibratio n time for the number average molecular weight (M-n) as well as for the wei ght average molecular weight (M-w) than for the monomer concentration ([M)] , whereas theories predict that M-n and [M] will relax together and that M- w will take much longer. We attribute the delayed equilibration and a secon d peak at about M-n/4 to the effects of ionic aggregation of the living pol ymers. We have also studied the equilibrium MWD of this system as a functio n of the temperature below T-p, and thus as a function of the number averag e degree of polymerization (L). These measurements and the time study discu ssed above are the first experimental evidence that the equilibrium MWD for an organic polymer in a state of equilibrium polymerization is an exponent ial/Flory-Schulz distribution, and is consistent with scaling predictions. Near T-p and at low L, we observe a deviation from the exponential distribu tion, which may be evidence of the effect of a chain-length dependence of t he equilibrium constant for polymerization, or of the effects of polydisper sity 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. (C) 1999 American Institute of Physics. [S0021-9606(99)50844-1].