Regioselectivity in growth reactions

If head-to-head and tail-to-tail additions occur during the polymerization of VAC, then a chain grown from left to right can be represented thus when it has been converted to poly(vinyl alcohol) 

Ovenall [62] used a different approach he showed that the adjacent methylene carbons (y and z in the formula), resulting from the tail-to-tail addition, gave clear signals in a 100 MHz spectrum, with chemical shifts 28.0 and 33.2 ppm. The chemical shift for methylene carbon x was 38.8 ppm. The number of reversed growth reactions was found to be 1.9 for 100 monomeric units, in good agreement with the results obtained by the physicochemical method for similar polymers. 

Head-to-head growth is comparatively frequent during the polymerizations of vinyl fluoride and vinylidene fluoride. The polymers have been studied by 470 MHz FNMR with proton-decoupling [63, 64]. The spectrum for poly (vinylidene fluoride) is simpler than that for poly(vinyl fluoride) because the former polymer contains no asymmetric carbons. 

A section of a poly (vinylidene fluoride) molecule is shown in the formula 

The assignment of resonances was originally based on comparison of the chemical shifts with those for appropriate fluorocarbons. Confirmation has been provided by a procedure involving the preparation of polymers by a method leading to materials with various proportions of inverted monomeric units. Copolymers of CH2. CF2 with low contents of CF2 CHC1 were prepared. The presence of the large chlorine atom in the molecule of the comonomer ensures that it enters the copolymer in such a way that the structure -CH2 CF2 CF2 CHCI CH2 CF2- is produced. The copolymers were treated with tri-n-butyltin hydride so that the comonomer units were converted to those that would be formed by head-to-head addition of a molecule of vinylidene fluoride. 

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