Sequencing in Homopolymers

Inoue and co-workers [1], Zambelli and co-workers [2] and Randall [3] have shown that C-NMR is an informative technique for measuring stereochemical sequence distributions in polypropylene. These workers reported chemical shift sensitivities to confignrational tetrad, pentad and hexad placements for this polymer. 

The seqnence lengths of sterochemical additions in amorphous and semi-crystalline polypropylene were accurately measured using C-NMR [4]. This method has some limitations for addition polymers having predominantly isotactic sequences. 

As representative examples, Busico and co-workers [9] selected two fractions (fraction A, diethyl ether-soluble fraction B, hexane-soluble/pentane-insoluble) of a polypropene sample prepared in the presence of the catalyst system MgCl2/TiCl4-TMP/A1(C2H5)3 (TMP = 2,2,6,6-tetramethylpiperidine). 

When not assigned in the literature [10, 11], resonances were attributed on the basis of chemical shift calculations according to the y-gauche effect [12] and by comparative analysis with the 150 MHz C-NMR spectra of samples of isotactic 

Complete resolution was achieved for the resonances arising from the mmmm-centred and rr-centred heptads, the latter showing a fine structure reaching the nonad or even the undecad level. 

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Poly(vinylidene fluoride) (PVF2) is a simple example of regioirregular sequences in homopolymers, because PVF2 does not have the stereoconfigurational irregularity... 

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