Propylene oxide percent crystallinity

In poly(propylene oxide), both crystalline and amorphous polymer can occur. Amorphous polymer, or amorphous segments in the polymer chain, can arise either from atactic sequences along the chain (stereochemistry) or from head-to-head isomerism in an otherwise tactic sequence. Both Vandenberg (76) and Price (77) found up to 30 percent head-to-head structures in some poly(propylene oxide)s. Polymer from mixed abnormal and normal ring opening of propylene oxide is most likely to occur when there is considerable cationic character to a coordination polymerization initiator. 

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). 

Table I. Temperature of Melting and Percent Crystallinity of Crystalline Fractions of Propylene Oxide Polymers from Different Catalysts...

This polymerization system, a complex of ferric chloride and propylene oxide, produced high-molecular-weight polymer rapidly. Further, a significant part of the poly (propylene oxide) formed, about 30 percent of the product, was found to be crystalline (i.e., acetone insoluble), with a melting point of about 75°C. 

Furukawa and coworkers demonstrated that the effectiveness of diethylzinc increased with controlled addition of water, methanol, or oxygen (81, 82) (Tables 6 and 7). These polymerization systems with propylene oxide produced a substantial fraction of crystalline polymer (16 percent in the case of water/diethylzinc). In some cases, poly(propylene oxide) was prepared with molecular weights estimated to be tens of millions (84). Efforts to understand these polymerization systems centered on identifying the coordination sites available as initiation sites on different metal alkoxides and on differentiating cationic initiation systems from coordinate initiation systems. In some cases, very similar systems led to quite different polymerization reactions and sometimes very different polymer products. 

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