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Direct Monitoring of the Cross‐Over from Diffusion‐Controlled to Decomposition‐Controlled Initiation in Free Radical Polymerization
Author(s) -
Kreft Tomasz,
Reed Wayne F.
Publication year - 2008
Publication title -
macromolecular chemistry and physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.57
H-Index - 112
eISSN - 1521-3935
pISSN - 1022-1352
DOI - 10.1002/macp.200800435
Subject(s) - monomer , polymerization , radical polymerization , decomposition , chemistry , polymer , polymer chemistry , diffusion , kinetics , reagent , molar mass distribution , thermodynamics , organic chemistry , physics , quantum mechanics
High‐resolution free radical polymerization kinetics was obtained using automatic continuous online monitoring of polymerization reactions (ACOMP). A sharp cross‐over from diffusion‐controlled initiation at low [monomer] to initiator decomposition control at higher [monomer] was found, and agrees with the quasi‐steady state approximation (QSSA). The cross‐over was also measurable within individual experiments. The kinetic implications for polymer weight average molecular weight $\overline M _{\rm w}$ and intrinsic viscosity ( η ) w were analyzed, and the QSSA‐predicted trends for the ratios of final to initial $\overline M _{\rm w}$ , and ( η ) w , confirmed. Analytical expressions for conversion are contrasted, and it was found that first‐order fits, while not fully justified theoretically, nonetheless are robust, which simplifies calculations needed for controlling molecular weight distributions in “semi‐batch” reactions, where reagents are fed to the reactor through programmable flow profiles. At the low monomer concentrations used, there was no evidence that propagation or termination rate coefficients changed during the reactions.