Chemistry Reference
In-Depth Information
Table 6.2
Properties of commercial polyethylene
Metal oxides on
support
Properties
Free-radical polymerization
Ziegler-Natta type catalysts
0.92-0.93 g/cm
3
0.94 g/cm
3
0.95-0.96 g/cm
3
Density
108-110.7
C
129-131
C
136
C
Melting point
% Amorphous
43.1
25.8
25.8
Structure
20-30 ethyl and butyl branches/1,000 carbons,
a few long branches
Mainly linear 7 ethyl branches/
1,000 carbons
Almost linear
Double bonds
0.6-2/1,000 carbons
0.1-1/1,000 carbons
Up to 3/1,000
carbons
Types of bonds
15% terminal vinyl
43% terminal
94% terminal
68% vinylidene
32% vinylidene
1% vinylidene
17% internal
trans
olefin
25% internal
trans
olefins
5% internal
trans
olefins
a
From various sources in the literature
The weight average molecular weights of most commercial low- and high-density polyethylenes
range between 5,000 and 300,000. Very low molecular weight polyethylene waxes and very high
molecular weight materials are also available. The molecular weight distributions for high-density
polyethylene vary between 4 and 15. The product generally has fewer than three branches per
thousand carbon atoms [
9
].
Table
6.2
summarizes the properties of various polyethylenes.
6.1.4 Materials Similar to Polyethylene
Materials that are quite similar to polyethylene can be obtained from other starting materials.
The most prominent is formation of polymethylene and similar high molecular weight paraffin
hydrocarbons from diazoalkanes. The reaction was originally carried out by Pechmann [
27
] when
small quantities of a white flocculent powder formed in an ether solution of diazomethane.
Bamberger and Tschirner [
28
] showed that this white powder is polymethylene -(-CH
2
-)
n
- that
melts at 128
C. The synthesis was improved since by introduction of various catalysts. The reaction
can yield highly crystalline polymers that melt at 136.5
C[
29
] with the molecular weight in millions
[
30
]. Among the catalysts, boron compounds are very efficient [
30
]. Bawn et al. [
31
] postulated the
mechanism of catalytic action. It consists of initial coordination of a monomer with the initiator, BF
3
.
This is followed by a loss of nitrogen and a shift of a fluorine atom from boron to carbon.
The successive additions of molecules of diazoalkane follow a similar path with a shift of the chain
fragment to the electron-deficient carbon:
N
BF
3
N
CH
2
N
F
3
B
CH
2
N
N
N
CH
2
N
2
F
3
B
N
+
F
3
B
F
F
3
B
CH
2
N
2
N
F
F
3
B
CH
2
n
+
CH
2
N
2
F
3
B
N
2
F
3
B
F
F
F
n
