| Abstract: | Samples of poly(vinyl chloride) polymerized at different temperatures (+30, 0, ?30, and ?78°C) and a commercial sample were chlorinated by a photoinitiated, free-radical method in o-dichlorobenzene at ambient temperature. The chlorine content was determined by difference by using a carbon-hydrogen analyzer. High-resolution infrared analysis revealed a selective attack on the methylene hydrogens along with a concomitant decrease in intensity of the isotactic methylene deformation band and the syndiotactic carbon-chlorine stretching band. Graphical representation of the relative intensities of the v(CH), δ(CH2), and v(C? CI) modes indicate that the chlorine radical preferentially attacks the middle carbon of the heterotactic triad. The increased “stiffness” of this unit causes the rotational energy of the system to be transferred to the syndiotactic portion of the chain. It is proposed that this additional energy causes the TTTT conformer of the syndiotactic unit to rotate into the less-preferred TTGG conformation, which would be more reactive. Facile chlorination of this latter conformer is predicted. This mechanism is in accord with the reported equilibrium composition of the different conformers. |
