5 Gel-Effect i n Emulsion Polymerization of V i n y l Monomers N. FRIIS and A. E . HAMIELEC
Downloaded by TUFTS UNIV on December 13, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch005
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada
In the bulk polymerization of vinyl monomers at ordinary polymerization temperatures (50-100°C), the termination reactions involving macroradicals become diffusion controlled and the termination rate constant decreases by three or in some cases even by four orders of magnitude in the conversion interval 0-100%. This decrease in termination rate constant, which will be referred to as gel-effect, always causes a significant increase in rate of polymerization and can also shift the molecular weight distribu tion to higher molecular weights, but the magnitude of the shift depends upon which reactions control molecular weight development. By increase in polymerization rate we mean the increase over the rate which would have been observed had gel-effect been absent. In the absence of gel-effect the rate of bulk polymerization would be first order with respect to monomer concentration and hence decrease with reaction time. Therefore, i f for instance a con stant polymerization rate is observed experimentally, this means that gel-effect has increased the polymerization rate and is therefore significant. In bulk p o l y m e r i z a t i o n at a given temperature the i n c r e a s e i n r a t e due to g e l - e f f e c t i s almost independent o f experimental cond i t i o n s , as f o r instance i n i t i a t o r c o n c e n t r a t i o n . In emulsion polymerization the s i t u a t i o n i s e n t i r e l y d i f f e r e n t i n that the increase i n r a t e due t o g e l - e f f e c t depends on experimental c o n d i t i o n s such as i n i t i a t o r concentration, p a r t i c l e s i z e and p a r t i c l e number. Therefore, accounting f o r g e l - e f f e c t i n emulsion p o l y m e r i z a t i o n i s c o n s i d e r a b l y more comp l e x than i n bulk p o l y m e r i z a t i o n . However, we have r e c e n t l y shown that the increase i n r a t e due t o g e l - e f f e c t i n emulsion polymeri z a t i o n o f v a r i o u s monomers can be accounted f o r q u a n t i t a t i v e l y by means o f data from bulk p o l y m e r i z a t i o n (1, 2_ _3, 4 ) . I t i s the purpose o f t h i s paper t o o u t l i n e a general technique o f t r e a t i n g g e l - e f f e c t i n emulsion p o l y m e r i z a t i o n and t o d i s c u s s the r o l e o f g e l - e f f e c t i n emulsion p o l y m e r i z a t i o n o f v a r i o u s monomers. 9
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Piirma and Gardon; Emulsion Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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FRIIS AND H A M I E L E C
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We w i l l furthermore present data showing the e f f e c t o f p o l y m e r i z a t i o n temperature on l i m i t i n g conversion. With polymeri z a t i o n s below the g l a s s t r a n s i t i o n p o i n t o f the polymer, the monomer-polymer s o l u t i o n reaches i t s g l a s s t r a n s i t i o n p o i n t at a conversion
