Vinyl monomers ususally have a strong preference for polymerization in head-to-tail sequences. Thus polymerization of propylene yields head-to-tail polypropylene. A form of polypropylene with head-to-head, tail-to-tail sequences is available,
-CH2-CH(CH3)-CH(CH3)-CH2-
but it is not prepared from propylene. Instead it is prepared by polymerization of 2,3-dimethylbutadiene, followed by hydrogenation. The diene polymerizes primarily to give the 1,4-product, but there is a few per cent of 1,2-addition, so the head-to-head, tail-to-tail polypropylene contains a few defects in the form of isopropyl branches.
Polymerization of vinylidene fluoride gives primarily (but not exclusively) head-to-tail addition. The content of the defect in poly(vinylidene fluoride) arises from head-to-head monomers can be deduced by fluorine nmr, as illustrated in PC Fig. 6.35, reproduced from Wilson, C. W., III; Santee, E. R., Jr. "Polymer Analysis by High Resolution NMR, with Application to Poly(vinylidene fluoride) and Poly(vinyl fluoride)" J. Polym. Sci. C 1965, 8, 97-112. The fluorines are all in a similar environment if the polymer is exclusively head-to-tail. The defect produces three new environments, and there are equal numbers of fluorines in these three environments. The real polymer has a dominant peak (from the head-to-tail regions), and three lesser peaks of comparable intensity (from the defects), permitting quantization of the defects, typically a few per cent in number.
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