Page 263 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/GRI P2: GTV Final pages
Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

772 Polymers, Synthesis

TABLE XIV Typical Ziegler–Natta Catalyst Components “homogeneous” conditions. This enormously important
mechanism remains an active area of research.
Transition metal halide [Me(I)]
The process of transforming a heteroatom-containing
TiX 4 , TiX 3 , VX 4 , VX 3 , VOX 3
ring to a linear chain is often described as a ring-opening
Co, Ni complexes
polymerization:
Organometallic compound [Me(II)]
AIR 3 , AIR 2 X, ZnR 2 , LiR, etc.
Generally agreed X (
M (CH 2 n ) CH 2 ( X n (
Mi(II) alkylates, reduces Me(I) m
Alkylated Me(I) responsible for chain growth
AIEt 3 + TiCl 4 → TiCl 3 Et + AlEt 2 Cl This process almost always involves anionic, cationic, or
TiCl 3 Et → TiCl3 + Et coordination initiation (catalysis). Only in very special
AlEt 3 + TiCl 3 → TiCl 2 R + AlEt 2 Cl situations have free-radical initiators been successful.
Cl 2 TiR + x(monomer) → Cl 2 Ti(monomer) x R Some major types of cyclic monomers that undergo
ring-opening reactions are shown in Table XV. The
thermodynamic feasibility of such a process for cyclic
ethers as a function of ring size is shown in Table XVI. For
as silica or alumina. The most generally accepted mecha-
small rings such as ethylene oxide, the reaction is highly
nism is shown in Scheme 21. Although free radicals can
exothermic, and polymerization proceeds with either
be produced, they are not involved in these types of poly-
anionic or cationic initiators. Dioxane has not been poly-
merizations. The olefin is believed to coordinate first via
merized. Larger cyclic ethers such as tetrahydrofuran are
π bonding with vacant d orbitals in the transition metal
−
initiatedonlybyoxoniumsaltssuchas(C 2 H 5 ) 3 O (BF 4 ) .
+
complex. The availability and stability of these coordi-
Presumably, the oxonium salt initiates by coordination
nation sites can be influenced significantly by the metal
with the electron pair of the cyclic ether. Propagation then
alkyl. Nevertheless, it is usually now considered to be a
monometallic mechanism. The coordination species must
undergo a cis rearrangement both to yield the stereoregu-
TABLE XV Some Important Examples of Ring-Opening
lar placement and to produce a new vacancy in the transi-
Polymerization
tion metal structure, which may coordinate with the next
monomer unit (the propagation step). Cyclic monomer Ring-opening reaction
Most of the industrial interest has centered on 1-
Ethers O
alkenes, dienes, or ethylene polymerizations. Many other CH 2 CH 2 O
C C n
monomers, such as methyl methacrylate and vinyl alkyl
ethers, have also been studied. Heteroatom-containing Acetals
C
monomers have further sites for coordination and hence
O O
can yield stereospecific polymerizations under even CH 2 O
C C n
O
Esters (lactones) O
O
C
O O (CH 2 ) 5 C
n
(CH 2 ) 5
Amides (lactams) O
H O
C
HN N (CH 2 ) 5 C
n
(CH 2 ) 5
Siloxanes
Si
CH 3
O O
Si O
Si Si n
SCHEME 21 Evolution of ideas on the mechanism of olefin co-
O CH 3
ordination polymerization. The essential steps are as follows. An
octahedral titanium complex has a chlorine vacancy. Alkylation oc- Phosphazines Cl Cl
curs, giving an alkylated titanium species that still has a chlorine
vacancy. Monomer is adsorbed on the vacancy and is π-bonded P N P N n
to the titanium atom. A “cis migration” occurs, leading to a new Cl Cl
titanium-carbon bond.