Page 262 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

Polymers, Synthesis 771

The electron enrichment may occur by either inductive
or resonance routes. Moreover, numerous heteroatom-
containing cyclic monomers, such as

n ) X X O, S, N, etc.
(CH 2

can also be polymerized via positively charged species
(e.g., oxonium ions).
2
The reactive growing chain end will have much sp -
type character in the case of carbocationic polymeriza-
tions. Thus, it should not be surprising that these chain
reactions have much shorter lifetimes than the previously
discussed anionic systems. The carbocationic chain end
has traditionally been referred to as a carbonium ion. How- SCHEME 20 Mechanism of carbenium ion polymeriza-
ever, may workers in the polymerization field now prefer tion. Often low temperature is required to achieve high
the term carbenium ion as a better choice for a trigonal, molecular weights. Controlled end groups can be provided.
2
trivalent sp -hybridized species.
Carbocationic polymerizations of 1-alkenes can be ini-
tiated by a variety of homogeneous or heterogeneous butyl rubber (a copolymer with a small amount of iso-
◦
Lewis acids, prene) is probably produced commercially at −40 C or

R (AlCl 4 ) lower:
RCl AlCl 3
CH 3
CH 3
HOH BF 3 H 2 O BF 3
CH 2 C CH 2 C CH CH 2
CH 3
H 2 C
H 2 O BF 3 C(CH 3 ) 2 H 3 C C (BF 3 OH) Butyl rubber
(CH 3 ) 2
Carbocationic polymerizations may also be used to poly-
such as BF 3 , TiCl 4 , AlCl 3 , and Al(C 2 H 5 ) 2 Cl. The process merize other alkenes such as propylene or styrene to mod-
issomewhatanalogoustoreactionssuchasalkylationsthat erately high molecular weight. However, these polymer-
are very familiar to organic chemists. The reactions may izations are not stereoregular and the commercial utility
require that either a proton donor (e.g., water) or cation has been quite limited.
donor (e.g., a tertiary halide) be present. Thus, isobutene The advent of Ziegler–Natta catalysis in the mid-1950s
is inert in the presence of BF 3 if the system is rigorously amplified and greatly expanded the idea that ionic poly-
dry. Reaction will begin immediately if catalytic quantities merizations can be stereochemically controlled via coor-
of, say tert-butyl halide or water are added. The initiation dination of the growing chain end with its monomer and
mechanismmaybequitecomplex.However,J.P.Kennedy counterion. The industrial importance of these reactions
and colleagues demonstrated that the chain end (“HEAD” is apparent from the fact that coordination polymeriza-
group) formed in the presence of cation donors is, in fact, tion is used to produce linear (high-density) polyethy-
derived from the cation. Thus, they consider the proton or lene (HDPE), linear low-density polyethylene (LLDPE),
cation source to be the initiator and the Lewis acid to serve isotactic polypropylene, isotactic poly(1-butene), various
as the coinitiator. ethylene–propylene co- and terpolymers, cis-1,4-poly-
An overview of the carbocationic polymerization is butadiene, and cis-1,4-polyisoprene. The volume of these
given in Scheme 20. One major difficulty with many important rubbers and plastics probably exceeds that of
cationic polymerizations is that many other chain trans- polymeric materials produce by all other chain (or step-
fer or chain-breaking reactions can effectively compete growth) processes. K. Ziegler and G. Natta received the
with the chain growth step. Carbonium (carbenium) ions Nobel Prize in 1963 for their contributions.
can undergo many reactions such as alkylation, isomeriza- The components of a Ziegler–Natta catalyst are listed in
tion, and so on. In a carbocationic polymerization these Table XIV. The utilization of these usually heterogeneous
reactions have the net result of lowering the molecular catalysis is well developed. However, because of the enor-
weight, often to lower than desired values. Fortunately, mous complexities, mechanistic understanding has been
one may “freeze out” many of these events by lowering relatively slow to evolve. One should also note that the cat-
the polymerization temperature. Thus, poly(isobutene) or alysts may themselves also be deposited on supports such