Molecular weight distributions in free-radical polymerizations. 1. Model development and implications for data interpretation

Clay, P.A. and Gilbert, R.G. (1995) Molecular weight distributions in free-radical polymerizations. 1. Model development and implications for data interpretation. Macromolecules, 28 2: 552-569. doi:10.1021/ma00106a021


Author Clay, P.A.
Gilbert, R.G.
Title Molecular weight distributions in free-radical polymerizations. 1. Model development and implications for data interpretation
Journal name Macromolecules   Check publisher's open access policy
ISSN 0024-9297
1520-5835
Publication date 1995-01
Sub-type Article (original research)
DOI 10.1021/ma00106a021
Volume 28
Issue 2
Start page 552
End page 569
Total pages 18
Place of publication Washington, DC, United States
Publisher American Chemical Society
Language eng
Abstract Many properties (e.g., film formation) of a polymer depend on the full molecular weight distribution (MWD) as well as on the averages of this distribution. Moreover, the MWD contains the complete kinetic history of a polymerization system, and hence an experimental MWD can yield considerable mechanistic information. Complete equations are developed which enable the MWD to be calculated for a wide range of straight-chain free-radical polymerizations. These equations also give insight into the qualitative form of the MWD, thus providing a method for extracting kinetic and mechanistic information from experimental MWDs. Methods are also described which enable numerical solutions to be obtained for the resulting nonlinear integrodifferential equations. The scheme takes into account initiation, transfer, propagation, and termination, allowing all of these to be dependent upon the degrees of polymerization of the chains involved. The model is applicable to bulk and solution polymerizations and to compartmentalized systems (conventional and micro- and miniemulsion polymerizations); in the latter case, phase-transfer events (radical entry into and exit from latex particles, and the aqueous phase kinetics of the various radical species) are specifically included. The dependence of the termination rate coefficient upon the lengths of both participating chains is especially important in systems where termination is kinetically significant. Approximate analytical solutions are also developed for a number of cases of interest. These show that the instantaneous number MWD at high molecular weights is a single exponential - even when termination occurs to a significant extent - which is a direct consequence of the chain-length dependence of termination rate coefficients, i.e., most termination events involve at least one very short chain. When the concentration of radicals in the system is low, the "decay constant" of the exponential part of the MWD is given by the ratio of transfer to propagation rate coefficients. In addition, the cumulative MWD frequently has the same behavior as the instantaneous MWD. This has the important implication that considerable mechanistic information can be obtained from size exclusion chromatography data by plotting ln(number MWD) against molecular weight. Such plots can reveal, for example, the termination mechanism that controls the MWD, values of transfer constants, and nucleation mechanisms in emulsion polymerization. Illustrative calculations are carried out for a number of systems. These verify the conclusions from the approximate analytic solutions and show also that the MWD in bulk or solution polymerizations at low conversions and high initiator concentrations is sensitive to the mode of termination (combination or disproportionation).
Keyword Diffusion-controlled Processes
Termination Rate-processes
Dense Polymer Systems
Emulsion-polymerization
Methyl-methacrylate
Rate Coefficients
Kinetics
Chain
Styrene
Propagation
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Chemistry and Molecular Biosciences
Centre for Nutrition and Food Sciences Publications
 
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Created: Mon, 07 Mar 2011, 15:39:32 EST