Dramatic enhancement of activities for living ziegler-natta polymerizations mediated by "exposed" zirconium acetamidinate initiators: The isospecific living polymerization of vinylcyclohexane [1]

Full text of DOI:10.1021/ja0057326

J. Am. Chem. Soc. 2001, 123, 6197-6198
Dramatic Enhancement of Activities for Living
6197
Ziegler-Natta Polymerizations Mediated by
“Exposed” Zirconium Acetamidinate Initiators: The
Isospecific Living Polymerization of Vinylcyclohexane
Richard J. Keaton, Kumudini C. Jayaratne,
David A. Henningsen, Lisa A. Koterwas, and
Lawrence R. Sita*
Department of Chemistry and Biochemistry
UniVersity of Maryland, College Park, Maryland 20742
ReceiVed October 25, 2000
ReVised Manuscript ReceiVed April 18, 2001
In addition to high activities and stereoselectivities, it is
desirable to have homogeneous Ziegler-Natta catalysts that can
extend the range of polyolefin materials that are obtainable from
readily available monomers. Commercially, it is of further interest
if these catalysts can be procured in as few steps as possible, and
from “nonexotic” starting materials. Recently, we reported that
initiators derived from mono-demethylation of the half-sandwich
zirconium acetamidinate precatalysts, (η5-C5R5)ZrMe2[N(R1)C-
(Me)N(R2)] (R ) Me) (1), can mediate the stereospecific living
polymerization of linear R-olefins and the living cyclopolymer-
ization of nonconjugated dienes at -10 °C in chlorobenzene.1
Herein, we now report extension of these results with the family
of cyclopentadienyl derivatives 2 (R ) H in 1) which can be
used to carry out the living polymerization of “difficult”
monomers, such as vinylcyclohexane (VCH), including the
synthesis of well-defined block copolymers containing high Tg
poly(vinylcyclohexane) (PVCH) segments.2,3 We further show that
these precatalysts can be prepared in one step from inexpensive,
commercially available precursors. As homopolymers and block
copolymers that are derived, or formally derived, from the
Ziegler-Natta polymerization of vinylcycloalkanes may prove
of technological value,4,5 the present results serve to open the
door to an important new area for polymer engineering.
Figure 1. Monomer conversion as a function of time for 1-hexene using
(4) 2a, (]) 2b and (O) 2c and for (0) VCH using 2b. Solid lines are
guides for the eye.
Scheme 1
the methylation reaction prior to the addition of a carbodiimide
in a one-pot procedure conducted according to Scheme 1.7
Importantly, this route to 2 occurs in high yield to provide a crude
material that contains few byproducts that must be separated.
Compounds 2a-c were found to be indefinitely stable at room
temperature, and of low configurational stability in solution in
the case of chiral C1-symmetric 2c.8 Finally, the solid-state
structures of 2a and 2c, as obtained by X-ray crystallography,
revealed no surprises in terms of any unusual geometrical
parameters being observed.7
As can be seen by Figure 1, catalysts derived from 2a-c and
the borate cocatalyst, [PhNMe2H][B(C6F5)4], were found to be
extremely active for the polymerization of 1-hexene at -10 °C.
Further, in each case, polymerizations were found to go to
completion, and Mn and Mw/Mn values similar to those obtained
for the living system derived from 1a (R1 ) Et, R2 ) tBu)1a were
observed (see Table 1). Regarding kinetic data, due to the very
rapid consumption of monomer observed for 2a and 2b, reliable
numbers could not be obtained for polymerizations employing
these precatalysts (i.e., 79% monomer conversion within 2 min
in the case of 2a). However, for the 2c system, the rate of
propagation was now amenable for kinetic analysis and a plot of
ln([M0]/[Mt]) versus time (not shown) was found to be linear (R
) 0.993), which is consistent with a living system in which the
concentration of propagating species remains constant.9 Quanti-
tatively, from these data, a value of 0.051 min-1 could be extracted
for the apparent rate constant for propagation, kobs, and this is
similar in magnitude to the kobs of 0.057 min-1 recorded for the
The synthesis of 2 by carbodiimide insertion1a into a Zr-CMe
bond of CpZrMe3 (Cp ) η5-C5H5) is complicated by the chemical
and thermal sensitivity of this compound which appears to have
never been isolated in pure form. Indeed, in our hands, meth-
ylation of CpZrCl3 with a slight excess of methyllithium6
invariably leads to decomposition of the initially formed meth-
ylated product. Fortunately, a critical solution to this problem was
found by coupling strict control of the amount of MeLi employed
with an excess of trimethylsilyl chloride that serves to “quench”
(1) (a) Jayaratne, K. C.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 958-
959. (b) Jayaratne, K. C.; Keaton, R. J.; Henningsen, D. A.; Sita, L. R. J. Am.
Chem. Soc. 2000, 122, 10490-10491. (c) Keaton, R. J.; Jayaratne, K. C.;
Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 10490-10491.
(2) To the best of our knowledge, only one report of the homopolymeri-
zation of VCH using a homogeneous catalyst has appeared (activity ) 0.017
kg‚molcat-1‚h-1), see: Longo, P.; Grassi, A.; Grisi, F.; Milione, S. Macromol.
Rapid Commun. 1998, 19, 229-233 (footnote a).
(3) For the properties of homopolymers and block copolymers of PVCH
as derived from the hydrogenation of polystryene precursor polymers, see:
(a) Gehlsen, M. D.; Bates, F. S. Macromolecules 1993, 26, 4122-4127. (b)
Gehlsen, M. D.; Bates, F. S. Macromolecules 1994, 37, 3611-3618. (c)
Hamley, I. W.; Fairclough, J. P. A.; Bates, F. S.; Ryan, A. J. Polymer 1998,
39, 1429-1437.
(4) For potential technological applications of PVCH, see: (a) Nishikawa,
Y.; Murakami, S.; Kohjiya, S.; Kawaguchi, A. Macromolecules 1996, 29,
5558-5566. (b) Tullo, A. Chem. Eng. News 1999, 77(51), 14-15.
(5) For the structures and properties of crystalline isotactic poly(vinyl-
cycloalkanes) prepared via heterogeneous Ziegler-Natta polymerization,
see: (a) Natta, G.; Corradini, P.; Bassi, I. W. Makromol. Chem. 1959, 247-
248. (b) Noether, H. D. J. Polym. Sci., Part C: Polym. Lett. 1967, 16, 725-
753. (c) Ammendola, P.; Tancredi, T.; Zambelli, A. Macromolecules 1986,
19, 307-310. (d) Endo, K.; Otsu, T. J. Polym. Sci., Part A: Polym. Chem.
1992, 30, 679-683.
(7) Details are provided in the Supporting Information.
(8) For R1 * R2, racemization in these complexes proceeds via a facile
amidinate “ring-flipping” process, see: Koterwas, L. A.; Fettinger, J. C.; Sita,
L. R. Organometallics 1999, 18, 4183-4190.
(6) Giannini, U.; Cesca, S. Tetrahedron Lett. 1960, 14, 19-20.
(9) Matyjaszewski, K. J. Phys. Org. Chem. 1995, 8, 197-207.
10.1021/ja0057326 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/01/2001

6198 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Communications to the Editor
Table 1. GPC Data for Polymers Prepared from 1a and 2a-ca
entry precatalyst
monomer
Mn
Mw/Mn
1
2
3
4
5
6
1a
2a
2b
2c
2b
2b
Hex
Hex
Hex
Hex
VCH
24 600 1.05
20 800 1.03
23 500 1.06
21 700 1.09
20 400 1.10
VCH(39),Hex(154),VCH(39)b 24 400 1.08
a GPC data were collected at 40 °C in THF using a 1.1 mL/min
flow rate and polystyrene standards. Polymerizations were conducted
using equimolar (25 µmol) amounts of precatalyst and either
[PhNMe2H][B(C6F5)4] or [Ph3C][B(C6F5)4] and 200 equiv of monomer
at -10 °C in chlorobenzene: total volume, 10 mL. b Numbers in
parentheses are monomer equivalents.
Scheme 2
Figure 2. Kinetic analysis of the polymerization of VCH using 2b (Mw/
Mn ) 1.04-1.10 for each data point). The dashed line is a linear curve
fit of the data.
1a system under identical conditions (kobs/[Zr]o ) 0.317 and 0.354
M-1 s-1 for 2c and 1a, respectively).
With regard to living character, the above kinetic data, along
with the GPC data of Table 1, support the view that polymeriza-
tions employing 2a-c are indeed living. These results stand in
sharp contrast to those obtained by Schrock and co-workers10 in
which a reduction in the steric bulk of the coordinated ligands
had a detrimental effect on both the activity and the livingness
of their Ziegler-Natta catalysts. It can also be noted that, for
2a-c, the 13C NMR spectra of the poly(1-hexene) materials
obtained are similar to the spectrum recorded for the poly(1-
hexene) provided by the nonstereospecific system of 1b (R1 )
R2 ) Cy).1b,7 This observation indicates that the steric bulk of
the Cp* group appears to be critical for establishing a stereo-
differentiating environment for olefin complexation such as that
found for the 1a system which is isospecific for 1-hexene
polymerization.1a
Given the extreme activities associated with 2a-c, it was of
interest to determine if these precatalysts could be used to
polymerize sterically encumbered monomers in a living fashion.
To our satisfaction, this proved to be the case for polymerizations
of VCH conducted at -10 °C, employing 2a and 2b which could
be taken to ∼99% completion according to Scheme 2. As Figure
2 reveals, a kinetic analysis confirmed the living character of these
polymerizations (kobs/[Zr]o ) 0.131 M-1 s-1 for 2b) which further
provided narrow polydispersities for each data point (Mw/Mn )
1.04-1.10). Surprisingly, it was determined that both of these
achiral, Cs-symmetric precatalysts produced highly isotactic
PVCH material (mmmm >95%) as evidenced by their 13C NMR
spectra.5d,7,11 These results suggest that for these systems, PVCH
microstructure is under chain-end control. Finally, it is interesting
to note that use of [PhNMe2H][B(C6F5)4] as cocatalyst with 2b
provided identical kobs values for VCH polymerization which
implies that the potential reversible coordination of dimethyl-
aniline does not seriously effect propagation.
Figure 3. GPC trace of PVCH-PH-PVCH triblock (entry 6) (solid
line) and a polystyrene standard (Mn; 11 300, Mw/Mn; 1.02) (dashed line).
this proved to be possible for the synthesis of a triblock material
containing 33% iso-PVCH (entry 6).7 Figure 3 shows the GPC
trace for this material which consists of a monomodal molecular
weight distribution possessing a narrow polydispersity of 1.08.
The ability to obtain such a well-defined material after a total
polymerization time of 5 h attests to the robustness of the living
polymers derived from 2.
In conclusion, the extreme activities expressed by 2 for the
living polymerization of (sterically encumbered) R-olefins at low
temperature should pave the way for the production of a range
of new homo- and block copolymer polyolefin materials.
With a living system in hand for VCH polymerization, we were
interested in determining whether novel di- and triblock copoly-
mers of isotactic PVCH could be prepared. As Table 1 indicates,
Acknowledgment. Funding for this work was provided by the
National Science Foundation (CHE-0092493) for which we are grateful.
Supporting Information Available: Details of precatalyst and
polymer synthesis and characterization and the crystallographic analysis
of 2a and 2b (PDF). This material is free of charge via the Internet at
http://pubs.acs.org.
(10) (a) Baumann, R.; Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc.
1997, 119, 3830-3831. (b) Baumann, R.; Stumpf, R.; Davis, W. M.; Liang,
L. C.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 7822-7836.
(11) The C1-symmetric precatalyst 1a also provides isotactic PVCH at
25 °C; however, the system is not living at this temperature.
JA0057326