(b)
From the tabulated data, we are asked to compute
M
w
, the weight- average molecular weight.
Molecular wt.
Range
Mean M
i
w
i
w
i
M
i
8,000-16,000
12,000
0.02
240
16,000-24,000
20,000
0.10
2000
24,000-32,000
28,000
0.20
5600
32,000-40,000
36,000
0.30
10,800
40,000-48,000
44,000
0.27
11,880
48,000-56,000
52,000
0.11
5720
___________________________
M
w
=w
i
M
i
∑
= 36,240 g/mol
(c)
Now we are asked to compute
n
n
(the number-average degree of polymerization), using the
Equation (14.4a).
For polypropylene,
m = 3(A
C
) + 6(A
H
)
= (3)(12.01 g/mol) + (6)(1.008 g/mol) = 42.08 g/mol
And
n
n
=
M
n
m
=
33,040 g/mol
42.08 g/mol
= 785
(d) And, finally, we are asked to compute
n
w
, the weight-average degree of polymerization, which,
using Equation (14.4b), is just
n
w
=
M
w
m
=
36,240 g/mol
42.08 g/mol
= 861
14.7
(a)
From the tabulated data, we are asked to compute
M
n
, the number-average molecular weight.