Majority Rules in the Copolymerization of Mirror Image Isomers

摘要

We had an interesting surprise when a polyisocyanate prepared from a sample of 2,6-dimethylheptyl isocyanate derived from citronellic acid of low enantiomeric excess exhibited a D-line optical activity indistinguishable from that of a nearly enantiomerically pure sample. This led to the preparation of a series of copolymers prepared from mixtures of the monomer enantiomers of 2,6-dimethylheptyl isocyanate. The results show an extreme nonlinearity between the enantiomeric excess and the optical activity measured at the sodium D-line (Figure 1). The polyisocyanates form a stiff helical conformation, and the enantiomeric pendant groups derived from citronellic acid cause an equal and opposite excess of one of the helical senses, i.e., left-handed for (R). The circular dichroism (CD) data in Figure 2, which are a measure of the helical conformation, demonstrate that the copolymer constructed of 56% (R) and 44% (S) is indistinguishable from that of the homopolymer of the (R) enantiomer. Figure 2 shows also the spectrum of a copolymer with only a 2% (S) enantiomeric excess. Even here the CD spectrum is characteristic of the (S) homopolymer, and the extrema are still about one-third as intense. It is surprising that such a helix sense bias can be produced by the tiny excess of the (S) enantiomer.