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178 Carraher’s Polymer Chemistry

belts, and gaskets. It is also used in construction for highway joint seals, bridge mounts and expansion
joints, and for soil-pipe gaskets. Further, it is used for wet laminating and contact-bond adhesives, in
coatings and dipped goods, as modifiers in elasticized bitumens and cements, and in fi ber binders.

As noted above, the presence of the chlorine atom in polychloroprene makes it less apt to burn in
comparison to materials, such as hydrocarbon-only elastomers. Thus, it is used in fire doors, some

combat-related attire, such as gloves and face masks and other similar applications. It is also used
in the construction of objects that come into contact with water, such as diving suits and fi shing
wader boots.

5.10 POLYISOBUTYLENE
Polyisobutylene (PIB); Equation 6.46, was initially synthesized in the 1920s but was developed by
William Sparks and Robert Thomas at Standard Oil’s (to become Exxon) Linden, NJ laboratory. It
is one of the few examples of the use of cationic catalysis to produce commercial-scale polymers.
PIB and various copolymers are also called butyl rubber (IIR). Low molecular weight (about 5,000
Da) PIB can be produced at room temperature but large chains (over 1,000,000 Da) are made at low
temperatures where transfer reactions are suppressed.
R

CH 3
123
H C H 3 C CH 3
2
CH 3
H C CH 3 (5.57)
3
CH
C 3
C H 3
H 3
CH 3
R
As noted above, PIB and various copolymers are called butyl rubber. Butyl rubbers have lower
permeability and higher damping than other elastomers, making them ideal materials for tire inner-
liners and engine mounts.
Because of the symmetry of the monomer, it might be expected that the materials would be
quite crystalline like linear PE. PIB does crystallize under stress but not under nonstressed condi-
tions. This is because the geminal dimethyl groups on alternating carbons in the backbone cause
the bond angles to be distorted from about the usual tetrahedral bond angle of 109.5 degrees to 123
degrees forcing the chain to straighten out. As a consequence of this the chains pack effi ciently
giving a relatively high-density material (density of 0.917 g/cc compared to densities of about
0.85 g/cc for many amorphous polymers) even when amorphous. This close packing reduces the
incentive for crystallization, accounts for its low permeability, and produces an unusually low T
g
o
of −60 C for such a dense material.
Because PIB is fully saturated, it is cured as a thermoset elastomer through inclusion of about
1%–2% isoprene that supplies the needed double bonds used in the curing process. Other materials,
including brominated paramethyl styrene, are replacing isoprene for this use. PIB is also used in
sealing applications and medical closures and sealants.
Polyisobutylene is often produced as a copolymer containing a small amount (1%–10%) of iso-
prene. Thus, the random copolymer chain contains a low concentration of widely spaced isolated
double bonds that are later cross-linked when the butyl rubber is cured.
Butyl rubbers have lower permeability (including being essentially impermeable to air) and higher
damping than other elastomers, making them ideal materials for tire innerliners, basketball inside
coatings, and engine mounts. It is used as an adhesive, caulking compound, chewing gum base, and an
oil additive. Its use as an oil additive is related to its change in shape with increasing temperature. Since

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