Dichlorobutyl Branch Formation and the Question of Diffusion-Controlled Propagation in the Polymerization of Vinyl Chloride

Abstract

Previously published concentrations (pbb's) of the 2,4-dichloro-n-butyl branches in poly-(vinyl chloride) (PVC) have been used to develop a simple theory that predicts pbb values as a function of monomer concentration and temperature during polymerization. The experimental values of pbb are shown to be consistent, quantitatively, with the well-known "back-biting" mechanism for dichlorobutyl branch formation. They also indicate that the activation energy for back-biting is greater than that for normal chain propagation by 4.4 kcal/mol. In monomer-swollen PVC at 45-80 °C, back-biting:propagation rate-constant ratios (ktJkp's) are invariant with monomer conversion up to total conversions of 83-91%, at least, and at 40 °C the value of kt/kp does not change significantly when the polymerization is carried out in 1,2-dichloroethane solution instead of in monomer-swollen polymer. These results are shown to provide strong evidence against diffusion control of the rate of chain propagation.

Structural defects in poly(vinyl chloride)

“…In recent years, we have carried out other studies that relate to defect structures. One of these led to the development of an equation that successfully predicts the BB content of PVC from the monomer concentration and the temperature during polymerization 49. The results of this study and others involving kinetic analysis45, 48 showed that the rate of chain propagation does not become controlled by diffusion in typical industrial polymerizations of VC.…”

“…This approach should be successful because the activation energies for the formation of the defects ought to be greater than that for conventional chain elongation. This was found to be the case, in fact, for the back‐biting step (eq 7) that leads to structure BB, whose activation energy is higher than that for normal chain growth by some 4.4 kcal/mol 49. In recent publications, some PVC samples prepared by living radical polymerization at 25–35 °C were stated to contain no structural defects at detectable levels 55, 56…”

Thermal characterization of poly(vinyl chloride) samples prepared by living radical polymerization: Comparison with poly(vinyl chloride) prepared by free radical polymerization

“…In terms of tertiary chlorides, PVC chains prepared at temperatures between 40 and 828C have between 1.0 and 2,4-dichloro-n-butyl branches per 1000 monomer units. 18 It is known that the thermal stability of the PVC is a direct consequence of the structural defects concentration, which are expected to be residual in the PVC-LRP. This result is obtained by the conjunction of the following effects: low reaction temperatures; a much lower amount of growing radicals when compared with the FRP process, which are in a fast exchange with their iodine encapped dormant species.…”

Microkinetic Modeling of Structural Properties of Poly(vinyl chloride)

“…3,4,9,23,[25][26][27][29][30][31][32][33]42,43 The main goal of these investigations was to determine how and to what extent structural defects in PVC are formed.…”