The number-average molecular weight of a poly(styrene-butadiene alternating copolymer is 1,350,000 g/mol; determine the average number of styrene and butadiene repeat units per molecule.

Given:

No. average molecular weight, = 1350000 g/mol

Formulae to be used:

1) m =                  (1)

2)                  (2)

where,

m = total average weight of the combination

= Atomic weight of carbon

= Atomic weight of hydrogen

= Degree of polymerization

Solution:

No. of both the types of repeat unit is sameas it is a copolymer.

For styrene, there are 8 carbon and 8 hydrogen atoms and for buta di-ene repeat unit, there are 4 carbon and 6 hydrogen atoms.

Therefore, the average weight of the combined unit is given by eqn (1)

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

Using eqn (2) to calculate average of repeat units per molecule:

= = 8531.77 ≈ 8532

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