The 13C NMR spectra of ring‐opened copolymers of cyclopentene and norbornene; reactivity ratios

Ring‐opened copolymers of cyclopentene (M1) and norbornene (M2) were prepared by WCl6/(C6H5)4Sn initiation in chlorobenzene and analysed by 13C NMR spectroscopy. At low conversion and for feed composition in the range f1 = 0–0,7 the copolymerization behaviour is ideal, with r1 = 1/r2 = 0,44; at high conversion the copolymer tends to contain more than the ideal proportion of M2 units. For copolymers in the composition range F1 = 0–0,5 the dyad distribution is random but for F1 > 0,6 there is a tendency to blockiness. The fraction of double bonds having cis structure is 0,5–0,6 for M2M2, 0,4–0,6 for M1M2, and 0,4–0,7 for M1M1 dyads. The pair sequence of cis and trans double bonds is somewhat blocky for both M2M2M and M1M2M triads (M = M1 or M2). The chemical shifts of the central non‐olefinic carbons in both M1 and M2 units (C5 and C3) show sensitivity to triad structure provided at least one adjacent double bond is cis. This unusual ϵ‐substitution effect is interpreted in terms of conformational restrictions about certain of the main‐chain bonds when an adjacent double bond is cis. WCl6/(CH2CHCH2)4Sn initiation gives a copolymer containing practically no M1 units.