Polymerization systems linear step-growth

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

FIGURE 2.4 (a) Number fraction distribution curves for linear step-growth polymerizations curve 1, p = 0.9600 curve 2, p = 0.9875 curve 3, p = 0.9950. (b) Corresponding weight fraction distribution for the same system. 

During isothermal polymerization below Tg, the molecular weight and T, increase, and eventually T, will equal Tjure The main purpose of this section is to discuss the calculation of the time to vitrification, where vitrification is defined to occur when Tj, equals T ure- The concepts of vitrification and the TTT cure diagram are extended to linear systems for both step growth and chain reaction mechanisms, although most of the discussion will focus on the nonlinear step growth case, of which the cure of epoxy resins is an example. 

The development of Equations 7.4-7.17 assumed that linear polymer molecules are formed, with one functional group on each end. In step-growth systems that form a substantial number of cyclic oligomers or polymeric molecules, which have no functional end-groups, Equations 7.8a, 7.8b, 7.11, 7.12 and 7.17 will over-predict the true average chain lengths. The propensity to form cyclic oligomers, in either AB or AA + BB systems depends on the size and shape of the monomer molecules and on the thermodynamic stability of the various cyclic molecules that can form. 

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