Question

1. A sample of polystyrene is found to have a number-average molar mass of 89,440 g mol−1 . Neglecting contributions from end groups, calculate the number-average degree of polymerization of this sample. Assuming that the sample has a molar mass dispersity of 1.5, calculate its weight-average molar mass.

Answer 1

Answer:

The weight-average molar mass of polystyrene is 134,160 g/mol.

Explanation:

Molar mass of the monomer styrene , $C_8H_8$, M=104 g/mol

Given , number average molar mass of the polymer , M'= 89,440 g/mol

Degree of polymerization = n

$n=\frac{M'}{M}=\frac{89,440 g/mol}{104 g/mol}=860$

The weight-average molar mass = $M_{avg}=?$

Molar mass dispersity is ratio of weight-average molar mass to the number average molar mass of the polymer.

$\text{Molar mass dispersity}=\frac{M_{avg}}{M'}$

$1.5=\frac{M_{avg}}{89,440 g/mol}$

$M_{avg}=89,440 g/mol\times 1.5 = 134,160 g/mol$

The weight-average molar mass of polystyrene is 134,160 g/mol.

Answer 2

The weight average molar mass should be 134160 g/mol.

• The calculation is as follows:

Polydisprsity index = Weight average molecular weight ÷  number averare molecular weight

1.5 = weight average molecular weight ÷ 89440

So,

Weight average molecular weight is

$= 1.5\times 89440$

=134160 g/mol

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