Question

Compute the number-average molecular weight for a polypropylene for which the degree of polymerization is 25,000.

Answer 1

Given:

Propylene is $C_{3}H_{6}$

Degree of polymerization, $D_{p}$ = 25000

Solution:

no. of carbon atoms, $n_{1}$ = 3

no. of hydrogen atoms, $n_{2}$ = 6

atomic weight of carbon, $A_{c}$ =12.01 g/mol

atomic weight of hydrogen, $A_{h}$ = 1.008 g/mol

molecular mass of propylene, m = $n_{1}A_{c}+n_{2}A_{h}$

m = $3\times 12.01+6\times 1.008 = 42.08$ g/mol

Now,

$D_{p}$ =  $\frac{\bar{M_{n}}}{m}$

${\bar{M_{n}}}$ = m( $D_{p}$)

${\bar{M_{n}}}$ = $(42.08\times 25000)$

${\bar{M_{n}}}$ = 1052000 g/mol

where,

${\bar{M_{n}}}$ =  number-average molecular weight