Metallocene-catalyzed polymerization of methacrylates to highly syndiotactic polymers at high temperatures

Article abstract of DOI:10.1021/ja710822g

C_s-Ligated ansa-zirconocene bis- and mono(ester enolate) complexes incorporating the diphenylmethylidene-(cyclopentadienyl)(9-fluorenyl) ligand, upon activation with appropriate activators, are highly active catalysts for the syndiospecific pol

Full text of DOI:10.1021/ja710822g

Published on Web 02/02/2008
Metallocene-Catalyzed Polymerization of Methacrylates to Highly Syndiotactic
Polymers at High Temperatures
Yalan Ning and Eugene Y.-X. Chen*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872
Received December 4, 2007; E-mail: eychen@lamar.colostate.edu
Table 1. Polymerization Systems Producing Highly Syndiotactic
PMMA
Technologically important, readily accessible, and remarkably
tunable chiral metallocene catalysts, especially those of cationic
group 4 complexes,1 have been widely employed to precisely control
the stereomicrostructure (tacticity) of polyolefins through their
catalyzed homogeneous, single-site, stereospecific polymerization
of nonpolar R-olefins.2 In comparison, the polymerization of polar
functionalized alkenes with such highly electron-deficient group 4
metallocene and related complexes has been investigated to a much
less extent.3 Nonetheless, there is increasing interest in the latter
area,4 already achieving the synthesis of highly isotactic poly-
(methacrylate)s (g95% mm)5 and poly(acrylamide)s (>99% mm)6
using chiral C2-ligated zirconocenium complexes at ambient tem-
perature. This initial success seemed to indicate that the catalyst
symmetry-polymer stereomicrostructure relationship already es-
tablished for the polymerization of nonpolar R-olefins may be
readily applied to the polymerization of polar functionalized alkenes,
despite their differences in the chain-growth mechanism (i.e.,
migratory insertion2 vs conjugate addition3). To further test this
hypothesis, over the past decade, at least five research groups7 have
attempted the synthesis of syndiotactic poly(methyl methacrylate),
PMMA, using a Cs-ligated cationic zirconocene methyl complex,
[Me2C(Cp)(Flu)ZrMe]+ (Cp ) η5-cyclopentadienyl; Flu ) η5-
fluorenyl)swhich is known for its ability to catalyze syndiospecific
polymerization of propylene8sbut none observed any activity for
polymerization of methyl methacrylate (MMA). Although we9
recently solved this inactivity issue using an ester enolate derivative,
{Me2C(Cp)(Flu)Zr(THF)[OC(OiPr)dCMe2]}+, the resulting PMMA
is essentially a syndio-biased atactic polymer (64% rr, 32% mr,
4.0% mm) via a chain-end control mechanism. Another Cs-ligated
zirconium ester enolate complex CGCZr(L)[OC(OtBu)dCMe2]+
[L ) neutral donor ligand, CGC ) Me2Si(η5-Me4C5)(tBuN)] affords
highly isotactic PMMA via a site-control mechanism at low
temperatures,10 whereas the isostructural titanium CGC complex
produces syndiotactic PMMA (up to 82% rr) by a chain-end control
mechanism.11 Isoelectronic neutral lanthanocenes supported by Cs-
symmetric, Me2C< and Ph2C< bridged (Cp)(Flu) ligands also
produce syndio-rich atactic (rr ∼ 60%) PMMA.12 In short, there
exhibited no strict analogy between olefin insertion and MMA
addition polymerizations catalyzed by Cs-ligated complexes because
of their significant differences revealed in stereoselection4d and
fundamental chain-growth events10,11b between those two processes,
and the synthesis of highly syndiotactic PMMA by a site-control
mechanism remained a challenge.
catalyst or
initiator
Tp
C)
rr
Tg
C)
ref
(
°
(%)
(
°
no.
m-(CH2dCH)C6H4CH2MgCl/THF
[Cp*2SmH]2
-98
-95
-93
-93
-78
-40
-30
94
95
96
95
94
93
92
14
15
16
17
7a
18
19
tBuLi/3Al(nOct)3
Ph3P/AlEt3
135
138
Me2CdC(OMe)OLi/2Al(C6F5)3
Cp*2YbAlH3‚NEt3
[HC(C(Me)dN-2,6-iPr2C6H3)2Mg
(µ-OC(dCH2)-2,4,6-Me3C6H2)]2
1/Ph3CB(C6F5)4 (i.e., 3)
2/THF‚B(C6F5)3 (i.e., 4)
1/Ph3CB(C6F5)4 (i.e., 3)
135
25
25
50
94
95
93
139
139
136
this work
this work
this work
Scheme 1
(46% mm, 46% rr), syndio-biased atactic (60% rr), syndiotactic
(81% rr), and highly syndiotactic (95% rr) PMMA, respectively.
Several initiator systems have been developed to produce highly
syndiotactic PMMA, but all require low to extremely low polym-
erization temperatures (Tp; Table 1). The syndioselectivity of such
non-site-control systems typically erodes to a moderate to low level
of rr e 80% when Tp reaches 25 °C. Our work communicated
herein describes a highly active polymerization system based on
Cs-ligated ansa-zirconocene bis(ester enolate) 1 and mono(ester
enolate) 2swhich, upon activation with appropriate activators,
generate the corresponding chiral cationic catalysts 3 and 4 (Scheme
1)sthat produces highly syndiotactic PMMA at ambient or higher
temperatures (Table 1).
Intense research efforts have been directed at development of a
polymerization system that can lead to the efficient production of
highly syndiotactic PMMA, due to the fact that the glass transition
temperature (Tg)san important materials property parametersof
PMMA rises as an increase in the polymer syndiotacticity. It has
been shown that PMMA exhibits a wide range of the Tg values as
a function of its stereomicrostructure: ca. 55,13 87, 13 110, 13 130,11b
and 140 °C7a for highly isotactic (96% mm), isotactic-b-syndiotactic
Zirconocene bis(ester enolate) 1 was conveniently synthesized
by the reaction of the corresponding dichloride precursor20 with
Me2CdC(OiPr)OLi in 92% yield.21 The synthesis of mono(ester
enolate) 2 involved three steps: methylation of the dichloride
precursor with MeMgBr to yield the dimethyl derivative (96%
yield), treatment of the dimethyl with Me3SiOTf to give the methyl
triflate complex (>99% yield), and reaction of the triflate with
Me2CdC(OiPr)OLi to afford 2 in 75% yield.21 Activation of the
9
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J. AM. CHEM. SOC. 2008, 130, 2463-2465
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C O M M U N I C A T I O N S
methyl ester enolate complex 2 with THF•B(C6F5)3 in CH2Cl2 at
room temperature generated cleanly the corresponding chiral
cationic species 4; the same activation approach has been well-
established for C2- and Cs-ligated zirconocene methyl ester enolate
complexes.5a,b,9 For the activation of the bis(ester enolate) complex
1, we employed a unique activation process that we recently
developed for oxidative activation of the silyl ketene acetal initiator
Me2CdC(OMe)OSiMe3 with a catalytic amount of Ph3CB(C6F5)4
to a highly active propagating species structure containing both
nucleophilic [R(Me)CdC(OMe)OSiMe3] and electrophilic (Me3-
Si+) catalyst sites.22 Accordingly, we reasoned that the reaction of
1 with 1 equiv of Ph3CB(C6F5)4 should occur in the similar
fashion: H- abstraction from the methyl group within the enolate
[OC(OiPr)dCMe2] moiety23 by Ph3C+ forms Ph3CH and the
resulting isopropyl methacrylate coordinated to Zr, and subsequent
nucleophilic addition of another enolate ligand to this activated
methacrylate monomer gives the cationic eight-membered ring
chelate 3 (a catalyst resting state).24 Indeed, the formation of Ph3-
CH was observed at temperature as low as -60 °C (5.55 ppm,
CH, CD2Cl2); however, unlike 4, complex 3 is unstable in the
absence of monomer.
Figure 1. First-order kinetic plot of ln{[MMA]0/[MMA]t} versus time for
the MMA polymerization in CH2Cl2 at 25 °C: [MMA]0 ) 0.468 M, [2]0
) [B(C6F5)3]0 ) 4.68 mM.
Significantly, the activated species promote rapid and controlled
MMA polymerizations with high syndiospecificity at ambient
temperature (ca. 25 °C). The polymerization by the bis(ester enolate)
1 was carried out in a in-reactor activation mode, that is, addition
of a premixed solution of 100 equiv of MMA with 1 equiv of Ph3-
CB(C6F5)4 to a CH2Cl2 solution of 1. A 92% monomer conversion
(F) was reached within 14 min of the reaction, and a linear increase
of Mn (number-average molecular weight) versus F was observed
with PDI (polydispersity index) ranging from 1.10 at low F (Mn )
4.51 × 103) to 1.37 at high F (Mn ) 2.09 × 104).21 Most remarkably,
the resulting polymer shows rr ) 94.4% and Tg ) 139 °C! The
triad-level stereomicrostructure of the PMMA (mr ) 4.0%, mm )
1.6%) is more characteristic of site control than chain-end control,
but the test result of 2[mm]/[mr] ) 0.8 somewhat deviates from
unity, which may suggest that it does not conform to a pure site-
control mechanism. Furthermore, this high level of syndiotacticity
is not markedly altered by changing the [MMA]/[1] ratio (400, rr
) 93.5%; 800, rr ) 93.2%), solvent (400 MMA, toluene, rr )
93.7%), temperature (400 MMA, Tp ) 50 °C in toluene, rr )
92.8%, Tg ) 136 °C; 400 MMA, Tp ) 0 °C in CH2Cl2, rr ) 96.0%,
Tg ) 140 °C), and addition sequence (preactivation mode, i.e.,
premixing 1 with Ph3CB(C6F5)4 for 10 min before addition of 400
MMA, rr ) 94.5%, mr ) 3.7%, mm ) 1.8%, Tg ) 139 °C).
However, the use of the borane B(C6F5)3 as activator for bis(ester
enolate) 1 resulted in the formation of PMMA with substantially
lower syndiotacticity (rr ) 76.4%, mr ) 20.6%, mm ) 3.0%);
this observation is attributable to a combination of a different
activation pathway involving B(C6F5)3 (which undergoes electro-
philic addition to the ester enolate R-carbon forming an adduct
analogous to the structure previously characterized for the C2-
symmetric metallocene derivative24) with a competing bimetallic
pathway as a result of the slow and reversible activation process
using B(C6F5)3 (i.e., coexistence of both neutral and cationic species
under this activation condition).
Figure 2. Plots of Mn and PDI of PMMA versus MMA conversion in
CH2Cl2 at 25 °C: [MMA]0 ) 0.468 M, [2]0 ) [B(C6F5)3]0 ) 4.68 mM.
Figure 3. 13C NMR (125 MHz) spectrum showing the CdO pentad region
of the PMMA produced by 4 in CH2Cl2 at 25 °C: rrrr ) 93.1%.
The mono(ester enolate) 2 is instantaneously activated using
THF•B(C6F5)3 (in situ mixing or isolation of 4), B(C6F5)3 (in-reactor
activation), or Ph3CB(C6F5)4, all leading to highly active (a 95%
monomer conversion is typically achieved in 7 min) and highly
syndiospecific (94% rr) polymerization of MMA at ambient
temperature. Kinetic profiling of the MMA polymerization in a
[MMA]/[2] ratio of 100 at 25 °C in CH2Cl2 gives a first-order rate
dependence in [MMA] (Figure 1) and a linear increase of Mn versus
F with a nonzero intercept and small PDI values in the range of
1.09-1.23 (Figure 2). The PMMA produced by 4 derived from 2
+ THF•B(C6F5)3 + 400 MMA exhibits rr ) 94.9% (1H NMR,
500 MHz), rrrr ) 93.1% (13C NMR, 125 MHz, Figure 3), and Tg
) 139 °C. Polymerization of 200 equiv of n-butyl methacrylate by
this system also affords highly syndiotactic poly(n-butyl methacry-
late): F ) 90% (1 h), Mn ) 3.47 × 104, PDI ) 1.30, rr ) 94.1%,
mr ) 3.9%, mm ) 2.0%.
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C O M M U N I C A T I O N S
Scheme 2
References
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of 10 or 20 equiv of THF. Consistently, addition of 10 or 20 equiv
of THF decreased the syndiotacticity of the PMMA produced by 3
or 4 in CH2Cl2 by only ∼1.0%, suggesting little to no site
epimerization during polymerization by displacement of the coor-
dinated chain end from the metal prior to monomer coordination.
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between the Me2C< and Ph2C< bridged Zr systems may be
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In summary, we have developed a highly active catalyst system
based on the Cs-ligated ansa-zirconocene bis- and mono(ester
enolate) complexes 1 and 2, leading to the production of highly
syndiotactic poly(methacrylate)s at industrially convenient temper-
atures, thus accomplishing a long-standing scientific goal of the
field. However, a fundamental understanding of the unique features
that the current system possesses and further development of
possibly even better systems based on this understanding require
much further work, which will be a focus of our ongoing studies.
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(21) See Supporting Information for experimental details and further results.
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Acknowledgment. This work was supported by the National
Science Foundation (NSF-0718061). We thank Boulder Scientific
Co. for the research gifts of Ph2C(Cp)(Flu)ZrCl2, Ph3CB(C6F5)4,
and B(C6F5)3, Prof. Luigi Cavallo for discussions, as well as the
reviewers for the valuable comments.
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Supporting Information Available: Experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
JA710822G
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