Chemistry Reference
In-Depth Information
constitutional isomers of isoprene or chloroprene (structure of chloroprene is
given in
Fig. 1.4
), to say nothing of the potential for mixed structures.
The constitution of natural rubber is head-to-tail 1,4-polyisoprene. Some meth-
ods for synthesis of such polymers are reviewed in Chapter 11.
Unconjugated dienes can produce an even more complicated range of macro-
molecular structures. Homopolymers of such monomers are not of current commer-
cial importance but small proportions of monomers like 1,5-cyclooctadiene are copo-
lymerized with ethylene and propylene to produce so-called EPDM rubbers. Only
one of the diene double bonds is enchained when this terpolymerization is carried
out with Ziegler
Natta catalysts (Section 11.5). The resulting small amount of unsa-
turation permits the use of sulfur vulcanization, as described in
Section 1.3.3
.
1.11.2
Branching
Linear and branched polymer structures were defined in
Section 1.6
. Branched
polymers differ from their linear counterparts in several important aspects.
Branches in crystallizable polymers limit the size of ordered domains because
branch points cannot usually fit into the crystal lattice. Thus, branched polyethyl-
ene is generally less rigid, dense, brittle, and crystalline than linear polyethylene,
because the former polymer contains a significant number of relatively short
branches. The branched, low-density polyethylenes are preferred for packaging at
present because the smaller crystallized regions which they produce provide trans-
parent, tough films. By contrast, the high-density, linear polyethylenes yield plas-
tic bottles and containers more economically because their greater rigidity enables
production of the required wall strengths with less polymer.
A branched macromolecule forms a more compact coil than a linear polymer
with the same molecular weight, and the flow properties of the two types can dif-
fer significantly in the melt as well as in solution. Controlled introduction of rela-
tively long branches into diene rubbers increases the resistance of such materials
to flow under low loads without impairing processability at commercial rates in
calenders or extruders. The high-speed extrusion of linear polyethylene is simi-
larly improved by the presence of a few long branches per average molecule.
Branching may be produced deliberately by copolymerizing the principal
monomer with a suitable comonomer. Ethylene and 1-butene can be copolymer-
ized with a diethylaluminum chloride/titanium chloride (Section 11.5) and other
catalysts to produce a polyethylene with ethyl branches:
H
H
CH
2
CH
2
+ CH
2
C
CH
2
CH
2
CH
2
C
CH
2
CH
2
(1-18)
CH
2
CH
2
CH
3
CH
3
The extent to which this polymer can crystallize under given conditions is
controlled by the butene concentration.
