Isotactic polyethylidene

The contrast between formulas 20 and 21, both pertaining to isotactic polyethylidene, should be noted This contrast occurs because the polymer repeating unit has only one carbon atom in the chain and thus there is no correspondence between such periodicity and that of the zigzag representation. The classical definition of an isotactic polymer (as one in which all substituents are on the same side of the chain) holds true, in general terms, only if the polymer is represented in the Fischer projection. Analogous considerations pertain to syndiotactic polyethylidene 22 and 23. 

The first extensive lUPAC publications on stereochemistry in high polymers were published in the early 1960s, and subsequently in 1966 as a single article (39). In addition to more conventional polymer names, eg, polyethylidene and polypropylene, the -amer nomenclature was introduced in 1952 (10). lUPAC basic definitions relating to stereochemistry, eg, tacticity and isotactic polymer, were published in 1974 (1). 

The first definition of an isotactic polymer was made with reference to a zig-zag planar conformation. Take, however, the case of polyethylidene the isotactic polymer will be represented, in the modified projection, as foilows ... 

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