Ionic and Coordinated Polymerizations - Polymer Science and Engineering - page 480

that the differences between the three products, while slight in terms of polymer
stereo-regularity, are actually serious to rubber compounders. (This illustrates the
point that polymers are materials of construction and must meet all of a range of
performance requirements.) Lithium alkyl living polymerization produces about
92% cis content, with linear structure. Ziegler

Natta catalysis produces a poly-

mer with 96 98% cis content, broad molecular weight distribution, long chain

branching, and some gel. Natural rubber likewise is 98% cis , with branching,
very broad molecular weight distribution, high mean molecular weight, and con-
siderable gel content, depending on storage time. The viscosities of Li-IR and
many grades of natural rubber are too high to allow good mixing with other ingre-
dients of rubber compounds. Their molecular weights therefore must be degraded,
usually by a combination of mechanical and chemical means. Ziegler Natta IR

does not require such mastication, because molecular weight can be controlled
during polymerization with chain transfer agents. The molecular weight of the Li-
IR could be reduced by operating with higher alkyl lithium concentrations, but
this would also lower the cis -1,4 content ( Section 11.2.7 ).
The effects of stereoregularity are seen in the tensile and tear properties of the
three polyisoprene types, which fall

i

in the decreasing order: natural rubber

Ziegler Li-IR. However, since Li-IR is unbranched and less stereoreg-

ular, it flows well during tire building and in injection molding, where the flow of
higher cis -content polyisoprenes may be hindered by crystallization under shear.
Synthetic polyisoprenes are superior to natural rubber in terms of consistency
of properties. They are also freer of contaminants and are preferred for applica-
tions that require lighter color, for personal care items, and for derivatization to
chlorinated and cyclized rubber products that are used in the adhesives and coat-
ings industries.
Table 11.2 summarizes differences between polybutadienes produced by differ-
ent processes. The low cis -content polybutadienes are branched. They have lower
solution viscosities than their linear counterparts and are preferred for the manu-
facture of high-impact polystyrene (HIPS) in which polymerization takes place in
a solution of the elastomer in styrene. As the reaction proceeds under agitation,
polystyrene becomes the continuous phase, with dispersed droplets of rubber. The

Natta IR i

Table 11.2

Catalysts for Polybutadienes

Catalyst system Polybutadiene structure (%)

cis trans vinyl

Co octoate/Et 2 Al . ClH 2 O 97 2 1

OrganoNi/Et 3 Al/BF 3 .Et 2 O 95 5 2

TiCl 4 /Et 3 Al/I 2 92 4 4

BuLi 38 5 11