Nuclear magnetic resonance and stereoregularity of PVC

Abstract

SynopsisThe decoupled a-proton spectrum of PVC was examined using a 220 MHz NMR spectrometer. All of the ten possible pentads were detected and an assignment is given based on Bernoullian statistics. This assignment is in general agreement with conclusions reached by previous workers except for some modifications in the heterotactic resonances. Triad tacticity data were obtained from the pentads and were used to estimate the differential enthalpy and entropy changes for bulk PVC polymerized in the temperature range 70°C to -50°C. THEORY Chain Sequence StereoregularityWhen a vinyl chloride monomer unit adds on to a growing chain during freeradical polymerization in a tail-to-head fashion, the carbon-carbon bond of this molecule still rotates even after the molecule tail (CH2-) has been attached. The chlorine atom assumes different spatial positions due to this rotation, and its configuration relative to the rest of the chain is undefined until the next monomer unit adds on. When this happens, the configuration of that particular chlorine atom is fixed. Whether there is or there is not any influence of this configuration on the configuration of the next chlorine atom is a matter of the statistical pattern of addition. Basically, there are two possible configurations for the chlorine atom. If it lies to the same side of the zigzag chain as the preceding chlorine, an isotactic placement has occurred. If it lies opposite to the preceding one, a syndiotactic placement has occurred. Subsequent additions may result in the same configuration or a switch can take place. Figure 1 is a schematic representation of the possible configurations that result from the stepwise addition of four monomer units. Sequences made up of two monomer units are called diads, those of three units are called triads, four monomer units are called tetrads, five are called pentads, and so on. As shown in the figure in triads and higher sequences, we can define a third configuration called heterotactic. This may not be called heterotactic placement. There are only two possible placements, i.e., isotactic and syndiotactic and that is strictly correct for diads. For higher sequences, the word configuration should be used rather than placement. At the bottom of the figure, the possible number of each of the three configurations is shown and also the total number of configurations possible for any sequence length. Thus, there are two diads, three triad configurations, six tetrads, and ten pentads. It'should be noted

Characterisation of poly(vinylchloride)

“…Syndiotactic segments are more rigid in the glassy state (39) or in solution, and ·conversion level between 0.06 and 0.79 was found tobe without influence (37). From the previous results it has been postulated that aggregates originate from molecular entanglements favored by long chains, and by kinks as a secondary factor (40). These two factors could explain the growing tendency of PVC to aggregate with diminishing polymerization temperature.…”

“…(less for low temperature polymerized PVC) is representative of short chain branching; tertiary chlorine atomsarenot detected, their nurober is at most equal to that of long brailches since they From the abundance of syndiotactic, isotactic and heterotactic triads it is possible to check whether the statistics is Bernoullian (no effect by the penultimate unit). Mitani (28) found no deviation from Bernoullian statistics but Pham (27) and Adel-Alim (26) found it non-Bernoullian outside the 30° -50° range of polymerization temperature. Inoue (29) has shown that the spin-lattice relaxation time T 1 of 1 3c is the same irrespective of the tactic configuration and of the molecular weight, so the intensities of the observed 13c resonances are directly related to the real concentration of carbon atoms in each configuration as for instance for-the tetrads measured by Carman (30).…”

Molecular aggregation in poly(vinyl chloride). III. Role of molecular structure

“…Table I shows the variation of the different persistence ratios with polymerization temperature for poly(viny1 chloride) . 20 A t lower polymerization temperatures, there is an increasing tendency to switch more often between isotactic and syndiotactic placements. Accord- ingly, the average tactic sequences will be shorter and the average number of kinks will be higher than the corresponding Bernoullian case.…”

Correlation between the tacticity and content of labile nondefect structures in poly(vinyl chloride)

“…The configuration and conformation of PVC in solution have been studied extensively with 1 H and 13 C NMR 7–14. However, the 13 C NMR spectra have shown some advantage over 1 H NMR spectra for determining triad and tetrad concentrations.…”