Chemistry Reference
In-Depth Information
Maron and coworkers [
180
] reported that theoretical methods were used to investigate the
syndiospecificity of the styrene polymerization catalyzed by single-site, single-component allyl
ansa-lanthanidocenes:
H
(S)
Ln
H
Pol
(R)
Si
Two limiting chain end stereocontrol mechanisms were studied by them, namely, migratory
insertion through a site epimerization and site stereoconfiguration independent of backside insertion
on a “stationary” polymer chain. Four consecutive insertions of styrene were computed to reveal that
(i) backside insertions are more favorable than, or at least as favorable as, frontside insertions.
The formation of a syndiotactic polymer is controlled by the thermodynamics. Moreover, the odd
(first and third) insertions are of 2,1-down-
-ones.
This control is the conjunction of two effects: minimization of styrene-styrene and styrene (phenyl
ring)-fluorenyl repulsions. The steric hindrance of the polymer chain induces a fourth insertion by an
exocyclic coordination of the fluorenyl ligand that is compensated by the
si
-type and are kinetically favored over the 2,1-up-
re
6
coordination of one of
Z
the phenyl ring in the growing chain.
Syndiotactic polystyrene is available commercially under the trade name of Questra. This material
is produced with the aid of a metallocene catalyst and is sold in several grades [
181
].
There is a small interest in forming isotactic polystyrenes with vary narrow molecular weight
distributions, because of some very limited practical applications, and from purely academic
interests. Several preparations of virtually monodisperse polystyrenes of
1.06 by anionic
polymerizations were developed. The materials are available commercially [
181
-
186
], small
quantities for use as standards for GPC.
M
w
/
M
n
¼
6.12.3 Polymers from Substituted Styrenes
Many derivatives of styrene can be readily synthesized. Some are commercially available. One of
them is
-methyl styrene. It is formed from propylene and benzene by a process that is very similar to
styrene preparation:
a
AlCl
3
+
