Incomplete Conversion of Reactive Groups due to Vitrification

When densely crosslinked networks are prepared by low-temperature polymerization, incomplete conversion of the reactive groups is often observed 2-24) 

Horie et al. have correlated the maximum extent of reaction with Tg of the mono-mer/polymer system. They concluded that the reaction stops when Tg equals The influence of polymerization shrinkage on free volume and there by on the rate of polymerization was determined by Balke and Hamielec at moderate conversions and more recendy Panke, Stickler, Wunderlich and Hamielec studied the influence of shrinkage and free volume on the rates of propagation and termination at very high conversions So, with linear systems a fairly complete picture has 

With multifunctional monomers, however, the situation is rather different. There are several reasons for the lack of understanding of rates and maximum extents of reaction. In the case of monovinyl compounds, there is no question about the chemical structure of the polymer, but with multifunctional monomers there exists ambiguity with respect to the status of unreacted groups. These may be present either in free monomer or as pendent groups This ambiguity together with the poor accessibility of networks to chemical and structural analysis has caused that much less progress has been made with the study of the polymerization of multifunctional monomers. 

Still another reason is that the proportionality between shrinkage and conversion, which is commonly observed with linear systems, breaks down with densely cross-linked networks (see Sect. 7.2) and finally it must be mentioned that even the chemistry may change during vitrification (Sect. 7.4.2). At this stage we will only present some qualitative considerations. 

Multifunctional monomers may be reacted to high conversion by increasing the molecular mobility during the reaction in one of four ways  

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