Common Chain-Growth Polymers - Principles of Polymer Chemistry - page 352

substantially atactic polypropylene. 1,3-Butadiene polymerizations produced cis -1,4 polybutadiene.

Use of zirconium complexes produced polymers with a content of cis -1,4 units higher than 99.9%

were claimed.

6.6.2 Polyisoprene

Polyisoprenes occur in nature. They are also prepared synthetically. Most commercial processes try to
duplicate the naturally occurring material.

6.6.2.1 Natural Polyisoprenes

Rubber hydrocarbon is the principle component of raw rubber. The subject is discussed in greater
detail in Chap. 7 . Natural rubber is 97% cis -1,4 polyisoprene. It is obtained by tapping the bark of

rubber trees ( ) and collecting the exudates, a latex consisting of about 32-35%

rubber. A similar material can also be found in the sap of many other plants and shrubs. The structure
of natural rubber has been investigated over 100 years, but it was only after 1920, however, that the
chemical structure was elucidated. It was shown to be a linear polymer consisting of head to tail links
of isoprene units, 98% bonded 1,4.

Hevea brasiliensis

6.6.2.2 Synthetic Polyisoprenes

In following natural rubber, the synthetic efforts are devoted to obtaining very high -1,4 polyiso-

prene and to forming a synthetic “natural” rubber. Two types of polymerizations yield products that
approach this. One is through use of Ziegler-Natta type catalysts and the other through anionic
polymerization with alkyllithium compounds in hydrocarbon solvents. One commercial process, for
instance, uses reaction products of TiCl 4 with triisobutylaluminum at an Al/Ti ratio of 0.9-1.1 as the
catalyst. Diphenyl ether or other Lewis bases are sometimes added as catalyst modifiers [ 113 - 116 ].
The process results in an approximately 95%

cis

cis-

1,4 polyisoprene product. Typically, such reactions
are carried out on continuous basis, usually in hexane and take 2-4 h. Polymerizations are often done in
two reaction lines, each consisting of four kettles arranged in series. The heat of the reaction is partially
absorbed by precooling the feed streams. The remaining heat is absorbed on cooled surfaces. When the
stream exits, the conversion is about 80%. Addition of a shortstop solution stabilizes the product.
Alkyllithium-initiated polymerizations of isoprene yield polymers with 92-93% 1,4 content.

One industrial process uses butyllithium in a continuous reaction in two lines each consisting of four
reaction kettles. The heat of the reaction is removed by vaporization of the solvent and the monomer.
The catalyst solution is added to the solvent stream just before it is intensively mixed with the
isoprene monomer stream and fed to the first reactor. After the stream leaves each reactor, small
quantities of methanol are injected between stages into the reaction mixture. This limits the molecular
weight by stopping the reaction. Fresh butyllithium catalyst is added again at the next stage in the next
reactor to initiate new polymer growth [ 117 - 119 ].
As is described in Chaps. 3 and 4 , the monomer placement into the polyisoprene chain can occur
potentially in nine different ways. These are the three tactic forms of the 1,2 adducts, two 1,4 adducts,
cis

cis-

, and three tactic forms of 3,4-adducts. In addition, there is some possibility of head to
head and tail to tail insertion, though the common addition is head to tail. Table 6.8 presents the
various microstructures that can be obtained in polymerizations of isoprene with different catalysts.

and trans