Crystalline melting point monomer/polymer systems

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

If the value of this probability parameter is a <= 1, an essentially iso tactic structure is obtained. If, on the other hand a 0, almost all the monomer units in the chain are in syndiotactic placements. If the polymer is capable of crystallization and the crystallization takes place under equilibrium conditions, then the limitation of this model is that a small melting point depression implies also a high degree of percent crystallinity. Although there are a number of systems, for example stereoregular methyl methacrylate (2, 8), in which this is true and this model is valid, this is not the case for polymers of propylene oxide from different catalysts that we discuss in another chapter (1). 

This method cannot be used for determination of the impurity concentrations in macromolecular systems,because, as previously mentioned, the melting of polymeric crystals is a process far from equilibrium. Therefore, the purity of crystalline polymers cannot be evaluated on the basis of melting point lowering. But even in polymer chemistry and physics, there is often a need to deal with low-molecular-mass crystalline compounds, such as certain monomers, initiators, and additives. In those cases, the method described below provides a valuable way for quick and simple determination of purity. 

The polymer so-obtained was a waxy material exhibiting a smectic fan-type texture, shown in Fig. 3, from room temperature up to 155 °C, at which it completely disappears. The polymer, therefore, exhibits a clearing point about 90 °C higher than that of the monomer. On cooling from the isotropic melt the liquid crystalline phase only partially appears this is indicative probably of some decomposition of the polymer. The complexity of the system is also demonstrated in the polymer DSC traces in Fig. 4. 

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