11194 J. Am. Chem. Soc., Vol. 123, No. 45, 2001
Liu et al.
Scheme 1
in which two monomers are coordinated to the metal as the
8,11
complexes passes through the insertion transition states, and
6b,d,7,12
(2) various two-state models
involving a fast propagating
state and a slow propagating state for which the steady-state
distribution of the catalyst between the two states depends on
[alkene].
Chain termination processes in 1-alkene polymerization play
a critical role in controlling polymer molecular weight and vary
1
3
dramatically with the structure of the catalyst and the nature
1
4
of the 1-alkene. Here we focus only on chain termination
processes which yield unsaturated termini: unimolecular and
bimolecular â-H transfer reactions. Following a 1,2 insertion
â-H transfer yields a vinylidene end group, whereas a vinylene
end group results from â-H transfer following a 2,1 insertion
a distribution of activating species, further complicate the
analysis because they may further speciate the catalytic centers.
Scheme 1, taken from the work of Siedle, Richardson, and
(or regioerror). Empirical data for propene polymerization
catalyzed by rac-C2H4(1-indenyl)ZrCl2/MAO demonstrate that
5
the ratio of vinylene end groups to vinylidene end groups is
co-workers, illustrates a common, simple paradigm for met-
1
3a,b
proportional to [alkene].
Chain termination processes that
allocene-catalyzed polymerization of alkenes. Critical attributes
of this model include the following: (1) solvent separation of
ion-pairs precedes alkene insertion; (2) initiation and propagation
proceed by simple bimolecular processes; and (3) â-hydride
elimination is the primary chain termination process.
Many observations are difficult to reconcile with this simple
model. For example, numerous reports of propene polymeri-
zation kinetics as catalyzed by Zr ansa-metallocenes with MAO
depend on alkene concentration yield saturation-like behavior
in terms of the increase of polymer molecular weight with
increased alkene concentration. Such data invariably are inter-
preted as indicating that vinylidene end groups arise from
unimolecular â-hydride elimination (â-H transfer to metal) after
a 1,2 insertion, whereas vinylene end groups arise from
bimolecular â-H elimination (chain transfer to monomer)
1
3a
following a 2,1 insertion of propene. It is not obvious why
the bulkier secondary alkyl must associate another monomer
to terminate whereas the primary alkyl does not. For 1-hexene
polymerization as catalyzed by rac-(C2H4(1-indenyl)2)ZrCl2/
6
cocatalysts indicate an apparent order in [alkene] that is >1.
Similar observations have been made for ethene and propene
7
polymerization using unbridged bis-cyclopentadienyl and mono-
8
9
cyclopentadienyl complexes as well as heterogeneous catalysts.
Varying orders in [monomer] have been reported for the rates
10a
MAO, Deffieux et alia report that polymer molecular weights
are independent of [1-hexene], whereas Odian et al.1 find that
the molecular weights vary with [1-hexene] in a complex way.
Furthermore, they report that the frequencies of vinylidene and
vinylene end groups both depend on [1-hexene] but that the
orders change with temperature, suggesting that both unimo-
lecular and bimolecular pathways for â-H transfer following
either 1,2 or 2,1 insertions may occur.
0c
10
of 1-hexene polymerization. Deffieux and co-workers reported
a first-order dependence with rac-(C2H4(1-indenyl)2)ZrCl2/
1
0b
MAO, similar to the order found by Chien and Gong for the
same system. In contrast, Odian and co-workers1 found the
order to vary from 1 to 1.4 for the rac-(dimethylsilyl)bis(4,5,6,7-
tetrahydro-1-indenyl)ZrCl2/MAO catalyst. Siedle and co-
0c
10d
workers report a complex dependence of polymerization rate
on [1-hexene] for the Cp2ZrCl2/MAO catalyst with an average
order close to 2. Schrock’s non-metallocene living polymeri-
zation catalysts for 1-hexene polymerization demonstrate first-
To resolve some of these outstanding issues, we have under-
taken detailed kinetic studies of metallocene-catalyzed alkene
polymerization. Recently, we reported a method for determining
active site concentrations in metallocene-catalyzed alkene
10c
order dependence of the rate on [1-hexene].
3a
polymerizations (Scheme 2). This method consists of quench-
ing the polymerization reaction with the concomitant introduc-
tion of a D label into the polymer. Only propagating species,
such as 1*, are monitored because the deuteriomethane produced
by reaction of 1 with MeOD is lost in workup. The time
dependence of the appearance of label in the polymer reveals
To accommodate the rate laws with orders in monomer
greater than 1, several mechanistic refinements have been pro-
posed. Prominent hypotheses include (1) the “trigger” model,
(
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2
1
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