Page 218 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Ionic Chain-Reaction and Complex Coordination Polymerization 181

prepared and polymerization is carried out under pressure and at low temperatures, generally less
o
than 100 C. The polymer forms viscous slurries. Care is taken so that the polymer does not cake up
on the sides and on the stirrer.

5.15 SUMMARY
1. Chain reactions, including ionic-chain polymerization reactions, consist of at least three steps:
initiation, propagation, and termination. Termination generally occurs through chain transfer,
producing a new ion and the “dead” polymer.
2. Cationic polymerizations occur with vinyl compounds that contain electron-donating groups
using Lewis acids along with a cocatalyst as the initiators. Polymerizations generally occur
at low temperatures in solvents with high-dielectric constants. The DP is proportional to the
concentration of monomer, and overall rate of polymerization is proportional to the square of
the monomer concentration. In general, the rate of polymerization is dependent on the dielec-
tric constant of the solvent, resonance stability of the carbocation, the degree of solvation of
the gegenion, and electropositivity of the initiator.
3. Monomers with electron-withdrawing groups can undergo anionic polymerization in the pres-
ence of anionic initiators. The rate of polymerization is dependent on the dielectric constant
of the solvent, stability of the carbanion, electronegativity of the initiator, degree of solvation
of the gegenion, and strength of the electron-withdrawing substituents.
4. Stereoregular polymers can be formed. These polymers can be divided into three general
stereoregular unit combinations. When the pendent groups, such as methyl for PP, are all on
one side of the polymer chain, the polymer combination is isotactic; when the methyl groups
alternate from one side to the other, the polymer combination is syndiotactic; and when the
position of the methyl group is somewhat random, it is atactic. The tacticity of polymers
infl uences the physical properties of the products. In general, polymers with greater tacticity
(order) have higher glass transition and melting temperatures, have a greater tendency to form
crystalline products, and are stronger and denser. Stereoregular polymers are produced at low
temperatures in solvents that favor formation of ion pairs between the carbocation and the
gegenion. One of the most widely used stereoregulating systems is called the Ziegler-Natta
catalyst system that generally employs a transition-metal salt such as titanium chloride and a
cocatalyst such as alkylaluminum. A proposed mechanism involves a reaction on the surface
of TiCl , activated by the addition of an alkyl group from the cocatalyst. The monomer adds
3
to this active site producing a pi complex, which forms a new active center by insertion of the
monomer between the titanium and carbon atoms. This step is repeated in the propagation
reactions in which the alkyl group from the cocatalyst is the terminal group. Stereospecifi c
polymers are also produced using the alfin and the chromia on silica initiators. The alfi n sys-

tem consists of allyl sodium, sodium isopropoxide, and sodium chloride.
5. Soluble stereoregulating systems have been developed using an organometallic transition
complex such as Cp TiCl and a cocatalyst often methylalumoxane. This system has advan-
2 2
tages over the Ziegler-Natta and similar systems in that the polymers produced are more ste-
reoregular, a wider range of monomers can be used, and little or no catalyst incorporated into
the polymers, allowing the polymer to be directly used without having to undergo a procedure
to remove the catalyst.
6. Two of the highest-volume polymers are made using ionic polymerization—HDPE and iPP.
There exist a number of commercially available PEs that vary in extent and kind of branching,
chain length, and amount of crystallinity.
7. A number of polymers have been made using ROMs. Nylon-6, similar to structure and prop-
erties to nylon-6,6, is made from the ring opening of the lactam caprolactam. Poly(ethylene
oxide) is made from the ring opening of ethylene oxide and is made more stable by capping
the ends preventing ready depolymerization.

9/14/2010 3:39:13 PM
K10478.indb 181 9/14/2010 3:39:13 PM
K10478.indb 181