Question

The number-average molecular weight of a poly(styrene-butadiene) alternating copolymer is 1346000 g/mol; determine the average number of (a) styrene and (b)butadiene repeat units per molecule.

Answer 1

Since it is an alternating copolymer, the number of both types of mer units will be the same. Therefore, consider them as a single mer unit, and determine the number-average degree of polymerization. For the styrene mer, there are eight carbon atoms and eight hydrogen atoms, while the butadiene mer consists of four carbon atoms and six hydrogen atoms. Therefore, the styrene-butadiene combined mer weight is just m = 12(AC) + 14(AH)

m = (12)(12.01 g/mol) + (14)(1.008 g/mol) = 158.23 g/mol

the number-average degree of polymerization is just

= 1346000 (g/mol) / 158.23 (g/mol)

= 8506

Thus, there is an average of 8506 of both mer types per molecule.