Carbon-13 nuclear magnetic resonance spectroscopy of polymers. Part V. Monomer sequence in styrene–butadiene copolymers
The complete characterisation of styrene-butadiene copolymer in terms of the 16 diads is achieved from the intensities of the 17 high-field peaks. Percentage compositions in terms of the four monomer units [styrene and 1,2-(v), cis-(c), and trans-(t) 1,4-linked butadiene] derived in this manner correlate well figures obtained from low-field peaks.
Average block lenghts for each unit as well as block lenghts for mixed units are derived: cis- and trans-1,4-linked butadiene units show a marked preference to block together and block or unit styrene and 1,2-linked butadiene units occur preferentially between trans-1,4-linked butadiene units. Styrene possesses the greatest tendency to block.
The relative content of butadiene units (t,c,v) was not systematically affected by the percentage of styrene. The distribution of the butadiene diads tt, tc, ct, and cc was virtually constant below 35 mole % styrene. The proportions of diads containing vinyl (v) and/or styrene units showed much greater variation.