ISSN:
0449-2951
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
The previous papers in this series contain the experimental results which demonstrated the complexity of the homogeneous, nonterminated, anionic polymerization of methyl methacrylate. In order to explain the observed broad molecular weight distributions, it was necessary to postulate reversible “pseudo-termination” reactions, by which a large fraction of the total anions present in the system became inactive toward further polymerization, yet had a finite probability of recovery of polymerization activity by some process, such as ligand exchange, ring opening of a cyclic complex, etc. In this paper, we have examined the quantitative consequences of a mechanism in which this pseudotermination reaction occurs by a bimolecular exchange of activity between one active and one inactive chain. It has been possible to calculate Mw/Mn, the ratio of weight-average to number-average molecular weights, as a function of the fractional conversion α, the initial monomer to initiator ratio, M0/I0, the fraction of total initiator as active chains, C1/I0, and the ratio of rate of exchange of activity to rate of propagation, kt/kp. The major results of these calculations are: (1) any value of Mw/Mn between 1 and infinity is possible at α = 1; (2) Mw/Mn at α = 1 decreases as kt/kp increases; (3) Mw/Mn at α = 1 increases as M0/I0 increases when kt/kp and C1/I0 are small - under some conditions Mw/Mn at α = 1 increases and then decreases slightly as M0/I0 increases; (4) Mw/Mn goes through a maximum during polymerization if kt ≠ 0; (5) Mw/Mn at any α decreases as C1/I0 increases, for constant M0/I0 and kt/kp. Concentrations of individual chains of any given length as a function of time may be readily calculated for the two special cases kt/kp → 0 or ∞.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1965.100030429
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