Synthesis, characterization, and heterobimetallic cooperation in a titanium-chromium catalyst for highly branched polyethylenes

Article abstract of DOI:10.1021/ja4039505

A heterobimetallic catalyst, {Ti - Cr}, consisting of a constrained-geometry titanium olefin polymerization center (CGC^EtTi) covalently linked to a chromium bis(thioether)amine ethylene trimerization center (SNSCr) was synthesized and fully characterized. In ethylene homopolymerizations it affords linear low-density polyethylene with molecular weights as high as 460 kg·mol^-1 and exclusively n-butyl branches in conversion-insensitive densities of ～18 branches/1000 carbon atoms, which are ～17 and ～3 times (conversion-dependent), respectively, those achieved by tandem mononuclear CGC^EtTi and SNSCr catalysts under identical reaction conditions.

Full text of DOI:10.1021/ja4039505

Communication
pubs.acs.org/JACS
Synthesis, Characterization, and Heterobimetallic Cooperation in a
Titanium−Chromium Catalyst for Highly Branched Polyethylenes
Shaofeng Liu,† Alessandro Motta,†,‡ Massimiliano Delferro,*,† and Tobin J. Marks*,†
†Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States
‡Dipartimento di Scienze Chimiche, Universita
̀
di Catania and INSTM, UdR Catania, 95125 Catania, Italy
S
* Supporting Information
synthesis since they utilize ethylene as the only feed.6 However,
coupling of intermolecular elimination with re-enchainment
sequences is typically challenged by the low probability that the
product of one catalytic center can be efficiently captured by the
other in dilute solutions. In principle, multimetallic catalysts3,7
could provide such products if centers having significantly
different catalytic characteristics were held proximate. To explore
modes of delivering significant quantities of precisely defined
short-chain oligomers to nearby CGCTi polymerization centers
via mechanisms different from group 4,8 we envisioned
heterobimetallic catalyst D that combines the aforementioned
properties of CGCTi-type catalysts (E) with a selective ethylene
trimerization catalyst6a,9 such as the Cr(III) catalyst SNSCr
(F).10 Here we report on the cooperative properties of the first
group 4−group 6 heterobimetallic olefin polymerization catalyst,
{Ti--Cr} (D, n = 1), which covalently joins CGCEtTi and SNSCr
centers. It is shown that {Ti--Cr} affords high-Mw PEs with
selective, conversion-insensitive, enhanced intramolecular n-
butyl (>99% C4) branch introduction, in contrast to the E + F
tandem system, which under identical conditions yields lower-
Mw PEs with conversion-sensitive introduction of significantly
less dense intermolecular branching.
ABSTRACT: A heterobimetallic catalyst, {Ti--Cr}, con-
sisting of a constrained-geometry titanium olefin polymer-
ization center (CGCEtTi) covalently linked to a chromium
bis(thioether)amine ethylene trimerization center
(SNSCr) was synthesized and fully characterized. In
ethylene homopolymerizations it affords linear low-density
polyethylene with molecular weights as high as 460
kg·mol−1 and exclusively n-butyl branches in conversion-
insensitive densities of ∼18 branches/1000 carbon atoms,
which are ∼17 and ∼3 times (conversion-dependent),
respectively, those achieved by tandem mononuclear
CGCEtTi and SNSCr catalysts under identical reaction
conditions.
roup 4 homobinuclear polymerization catalysts based on
G
bisconstrained-geometry catalyst (CGC)1 or bis-
(phenoxyiminato)2 scaffolds [e.g., CGC2M2 (A), FI2M2 (B),
M = Ti, Zr] have been characterized, and cooperativity effects
between adjacent catalytic centers shown to induce significant
molecular mass and enchainment selectivity enhancements in
ethylene homopolymerizations and ethylene + α-olefin copoly-
merizations versus the analogous mononuclear catalysts.3
The synthesis of the binuclear ligand H2CGC−SNS is shown
in Scheme S1 in the Supporting Information (SI). Condensation
of 3-(2-aminoethyl)indene hydrobromide (1) with 2.0 equiv of
1 - ( e t h y l t h i o ) - 2 - b r o m o e t h a n e y i e l d e d 3 -
[(EtSCH2CH2)2NCH2CH2]indene (2). H2CGC−SNS was
In contrast to this homobimetallic approach, heterobimetallic
polymerization catalysts in principle offer new pathways for
comonomer introduction and copolymer synthesis.4 In the only
example to date, modest but distinctive cooperative effects were
observed in CGC2TiZr-mediated polymerizations (C).5 Where-
as the isolated mononuclear Ti centers afford narrow-
polydispersity index (PDI), high-Mw polyethylenes (PEs) with
moderate comonomer enchainment activity and the isolated Zr
centers produce low-Mw, larger-PDI PEs with vinylene end
groups, CGC2TiZr produces monomodal, significantly higher
Mw PEs than simple mononuclear catalyst mixtures but with only
∼2 branches (≥C6) per 1000 C atoms, reflecting the limited
activity and chain-transfer characteristics of CGCZr centers.3c
Mixtures of homogeneous oligomerization and polymer-
ization catalysts (tandem catalysts) are of interest for copolymer
n
then synthesized by BuLi deprotonation of 2 followed by
sequential addition of Me2SiCl2 and tBuNH2. All of the products
were characterized by conventional spectroscopic/analytical
methodologies (see the SI). {Ti--Cr} was synthesized as
shown in Scheme 1. The monometallic amido complex
CGCTi(NMe2)2−SNS was prepared by protodeamination of
Ti(NMe2)4 with H2CGC−SNS in refluxing toluene with
Received: April 21, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4039505 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 1. Synthesis of the Heterobimetallic Catalyst {Ti--Cr}
Figure 1. 13C{1H} NMR spectra (100 MHz, C2D2Cl4, 120 °C) of PEs
produced by the CGCEtTi + SNSCr and {Ti--Cr} catalysts (Table 1,
entries 4 and 7), scaled to the PE CH2 backbone resonance at 30 ppm.
constant removal of the byproduct HNMe2 (Figures S5 and
S6).11 The reaction of CGCTi(NMe2)2−SNS with excess
Me3SiCl gave CGCTiCl2−SNS (Figures S7 and S8).
CGCTiCl2−SNS crystals were obtained from hexane solution,
and an ORTEP plot is shown in Scheme 1 (also see Figure S1).
Subsequent reaction with CrCl3(THF)3 afforded dark-red
paramagnetic {Ti--Cr}, the constitution of which was confirmed
by elemental analysis (Ti, Cr, C, H, N), 1H NMR spectroscopy
(very broad), and MALDI−TOF mass spectrometry (Figure
S9). {Ti--Cr} exhibits stability in coordinating solvents, as
1
confirmed by H NMR analysis in THF-d8, where the CrCl3
moiety is not displaced by the solvent (Figure S10).
In initial experiments, ethylene polymerizations/oligomeriza-
tions were carried out with a suite of controls to probe
cooperative enchainment effects, using a conventional methyl-
aluminoxane (MAO) cocatalyst/activator under rigorously
anhydrous/anaerobic conditions, and attending to exothermic
and mass-transfer effects.3,12 Catalysts were investigated under
varied reaction conditions, including the Al:M ratio (M = Ti, Cr),
reaction temperature (25, 80 °C; Table S3), ethylene pressure
(1, 3, 5, 8 atm; Table S3), and reaction time (5, 10, 20, 60 min).
The overall optimum catalytic performance (activity, branches
per 1000 C atoms, Mw, PDI, cooperative effects) was achieved
with Al:M = 500 at 80 °C under a constant ethylene pressure of
8.0 atm. The data in Table 1 indicate that ethylene
homopolymerizations mediated by mononuclear CGCEtTi and
Figure 2. (A) PE branch densities (ρbr) introduced by CGCEtTi +
SNSCr and {Ti--Cr} as functions of reaction time at Pethylene = 8.0 atm
(Table 1, entries 4−9). (B) Relationship of ρbr and Mw for CGCEtTi +
SNSCr and {Ti--Cr} in the same reactions.
a
Table 1. Ethylene Polymerization Data for Catalysts CGCEtTi, CGCTiCl2−SNS, SNSCr, 1:1 CGCEtTi + SNSCr, and {Ti--Cr}
b
c
d
e
f
f
g
entry
catalyst
CGCEtTi
t (min)
PE (g)
activity (PE)
oligomers (g)
activity (oligomers)
ρbr
Mw (kg·mol−1
)
PDI
Tm (°C)
1
2
5
5
6.500
0.054
0.045
3.200
5.950
10.80
0.184
0.359
0.672
1.440
975.0
8.1
−
−
−
−
0
0
0
42.0
76.9
2.5
3.2
2.2
2.3
2.5
2.3
2.5
2.5
3.2
1.9
128.4
136.8
133.6
125.9
123.2
121.9
123.5
119.2
117.5
121.6
CGCTiCl2−SNS
SNSCr
CGCEtTi + SNSCr
CGCEtTi + SNSCr
CGCEtTi + SNSCr
{Ti--Cr}
3
5
6.7
0.490
0.204
0.382
0.720
0.075
0.138
0.272
0.474
73.5
30.6
28.6
27.0
11.3
10.4
10.2
5.9
143.7
26.2
4
5
480.0
446.0
405.0
27.6
26.9
25.2
18.0
6.4
8.2
5
10
20
5
15.3
6
11.8
18.2
18.6
19.1
18.9
12.5
7
461.3
312.1
276.4
365.5
8
{Ti--Cr}
10
20
60
9
{Ti--Cr}
10
{Ti--Cr}
a
Conditions: 10 μmol of catalyst (10 μmol of each component for CGCEtTi + SNSCr) with MAO/catalyst = 500 in 50 mL of toluene at 80 °C with
b
c
Pethylene = 8 atm. Each entry was performed in duplicate. In units of (kg of PE)·(mol of catalyst)−1·h−1·atm−1. As determined by GC−TOF with
d
mesitylene added as an internal standard. The selectivity for 1-hexene ranged from 53% (entry 3) to 98% (entries 7−9). In units of (kg of
oligomer)·(mol of catalyst)−1·h−1·atm−1. Branch density (number of branches per 1000 C atoms) as determined by 13C NMR analysis.19 As
e
f
g
determined by triple-detection GPC. Melting temperature as determined by differential scanning calorimetry.
B
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Journal of the American Chemical Society
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Scheme 2. Scenario for Altered Ethylene Polymerization Propagation and Chain-Transfer Processes at the Heterobimetallic {Ti--
Cr} Catalyst
CGCTiCl2−SNS afforded relatively high-Mw, high-melting PEs
with negligible chain branching (Table 1, entries 1 and 2). For all
of the polymerizations, the monomodal gel-permeation
chromatography (GPC) traces and product PDIs are consistent
with single-site processes (Figure S11)3a,13 and catalyst
deactivation processes that are minor under the conditions
examined, even at longer polymerization times (Table 1, entry 10
and Figures S12 and S13). Furthermore, the 1H NMR spectra of
the PEs produced by both the CGCEtTi + SNSCr tandem
catalyst and the {Ti--Cr} heterobimetallic catalyst exhibit
vinylene (−CH2CHCH2) and vinylidene [−CH2C(R)
CH2] end-group distributions, with the former predominating
(Figure S14). This indicates that β-hydride elimination is the
dominant chain-transfer pathway14 and that chain transfer to
alkyl-Al is negligible.15
Under the present polymerization conditions, the Cr catalysts
in either the tandem or bimetallic systems display good
selectivity for ethylene trimerization (72−98% 1-hexene as
determined by NMR spectroscopy and GC−TOF; Table S4).
Under these conditions, monometallic SNSCr is known to be
somewhat less selective for 1-hexene (53% in our hands) and also
to produce small amounts (∼9%) of high-Mw PE.6,16,17 More
importantly, comparison of the tandem and heterobimetallic
polymerization data (Table 1 entries 4−6 vs 7−9) shows that
{Ti--Cr} consistently produces higher-Mw products under all
conditions by a factor of ∼20 but with ∼18-fold lower activity;
the latter is likely due to steric constraints and competition by 1-
hexene (vide infra), both of which should retard the polymer-
ization rate.1,18 Furthermore, the very different polymerization
characteristics of the {Ti--Cr} and CGCTiCl2−SNS catalysts
(e.g., entry 7 vs 2) argue that the SNSCr center remains intact
during the polymerization. In regard to the copolymerization
selectivity, Table 1 (entries 4−6 vs 7−9) and the PE 13C NMR
spectra (Figure 1) show that {Ti--Cr} enchained ∼18.6 n-butyl
branches/1000 C atoms, with a <1% yield of branches of any
other length. Also, the branch density (ρbr) is essentially
independent of reaction time and conversion despite increasing
concentrations of available “free” oligomer (Tables 1 and S5 and
Figure S16). In contrast, the tandem CGCEtTi + SNSCr system
introduces far lower ρbr under identical reaction conditions (e.g.,
6.4 n-butyl branches/1000 C atoms; Table 1, entry 4 and Figure
1) despite the far higher “free” oligomer concentrations. Also, the
ρbr values obtained with the tandem system are far more
conversion-sensitive (Table 1, entries 4−6; Table S5; and Figure
S16), and differences in product microstructure are also evident
in the product melting points (Table 1 and Figure S15).
Taken together, these results argue that covalently linking the
metallic sites in {Ti--Cr} spatially confined the catalytic centers
in such a way that the efficiency of intramolecular comonomer
transfer to the CGCTi center is significantly increased, with high
selectivity for the C6 comonomer. The fact that the PE ρbr
introduced by {Ti--Cr} remains essentially constant with
increasing reaction time (Table 1, entries 7−9; Figure 2A;
Table S5 and Figure S16) argues that the “local concentration” of
α-olefin remains nearly constant. Also, even though increasing
the “free” oligomer concentration/reaction time depresses the
copolymer Mw and activity in the tandem system, as is typical for
CGCTi catalysts,20 this has little effect on the heterobimetallic
catalyst (Figure 2B). Clearly, the presence of the tethered SNSCr
oligomerization center dramatically alters both the propagation
and chain-transfer characteristics of the mononuclear CGCTi
catalyst, producing higher-Mw copolymers with higher C4-only
branching densities. To probe further the integrity of 1-hexene
transfer at {Ti--Cr}, 0.10 M 1-pentene was added to the
polymerization and allowed to compete with the 1-hexene
produced by the Cr catalytic center (see the SI). In these
experiments, the CGCEtTi + SNSCr tandem catalyst yields PEs
with 68.0 branches/1000 C atoms, of which 91% were n-propyl
C
dx.doi.org/10.1021/ja4039505 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(4) Conventionally, LLDPE is produced via copolymerization of
ethylene with α-olefin comonomers. Single-site homogeneous CGCs
are particularly well-suited for this purpose because the open
coordination spheres afford enhanced α-olefin selectivity. See:
Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.; Nickias,
P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. Y. Eur. Pat. Appl.
EP416815A2, 1991.
(5) Wang, J.; Li, H. B.; Guo, N.; Li, L. T.; Stern, C. L.; Marks, T. J.
Organometallics 2004, 23, 5112.
(6) (a) McGuinness, D. S. Chem. Rev. 2010, 111, 2321. (b) Wasilke, J.-
C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Chem. Rev. 2005, 105, 1001.
(c) Schwerdtfeger, E. D.; Price, C. J.; Chai, J. F.; Miller, S. A.
Macromolecules 2010, 43, 4838.
(Figure S17), consistent with intermolecular α-olefin capture. In
contrast, under identical conditions, {Ti--Cr} produces PE with
26.4 branches/1000 C atoms, of which only 35% were n-propyl
(Figure S13), and the 1-hexene enchainment density is nearly
unchanged from the experiments without pentene (17.4 n-butyl/
1000 C atoms; Figure S17 and Table 1, entries 7−9).
Scheme 2 presents a tentative scenario to accommodate the
above observations. C6 fragments are produced by established
sequences8 of reductive ethylene coupling and metallacyclopen-
tane expansion to a metallacycloheptane followed by reductive
elimination (cycle A → B), yielding 1-hexene, which can either
“leak” from the immediate {Ti--Cr} environment or be
captured/enchained at the CGCTi center. The present data do
not distinguish between concerted or stepwise reductive
elimination and 1-hexene capture or even Ti-mediated metal-
lacycloheptane opening. However, preliminary density func-
tional theory (DFT) calculations21 identify an energetic
minimum in which 1-hexene is π-bound to the Cr center while
engaging in a −CH3···M agostic interaction with Ti (Ti···Cr
distance = 6.35 Å, Scheme 2 inset). This transfer process is
efficient enough to limit enchainment of exogenous α-olefin, as
evidenced by the near-constant n-butyl branch content and PE
Mw as conversion progresses and by the 1-pentene competition
results.
In summary, we report a heterobimetallic catalyst linking
single-site Ti constrained-geometry and Cr bis(thioether)amine
centers. This catalyst selectively produces n-butyl-branched
polyethylenes from ethylene as the only feed with conversion-
insensitive Mw’s and branch densities that are ∼17 and ∼3 times,
respectively, those achieved using the analogous tandem catalyst.
The results argue that proximity of the catalytic centers
dramatically alters the propagation and chain-transfer character-
istics of the heterobimetallic catalyst.
(7) (a) Mandal, S. K.; Roesky, H. W. Acc. Chem. Res. 2010, 43, 248.
(b) Mitic, N.; Smith, S. J.; Neves, A.; Guddat, L. W.; Gahan, L. R.;
́
Schenk, G. Chem. Rev. 2006, 106, 3338.
(8) (a) Yang, Y.; Liu, Z.; Zhong, L.; Qiu, P. Y.; Dong, Q.; Cheng, R. H.;
Vanderbilt, J.; Liu, B. P. Organometallics 2011, 30, 5297. (b) Arteaga-
Muller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. J.
Am. Chem. Soc. 2009, 131, 5370. (c) McGuinness, D. S.; Suttil, J. A.;
Gardiner, M. G.; Davies, N. W. Organometallics 2008, 27, 4238.
(d) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007, 129,
14281. (e) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.;
Haasbroek, D.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D.
H. J. Am. Chem. Soc. 2005, 127, 10723. (f) Blok, A. N. J.; Budzelaar, P. H.
M.; Gal, A. W. Organometallics 2003, 22, 2564.
(9) (a) Agapie, T. Coord. Chem. Rev. 2011, 255, 861. (b) Wass, D. F.
Dalton Trans. 2007, 816. (c) Dixon, J. T.; Green, M. J.; Hess, F. M.;
Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641.
(10) (a) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.;
Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238.
(b) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.; Dixon, J.
T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605. (c) McGuinness, D.
S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann, A.;
Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272.
(11) Li, H.; Li, L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am.
Chem. Soc. 2003, 125, 10788.
(12) (a) Weberski, M. P.; Chen, C.; Delferro, M.; Marks, T. J. Chem.
Eur. J. 2012, 18, 10715. (b) Weberski, M. P.; Chen, C.; Delferro, M.;
Zuccaccia, C.; Macchioni, A.; Marks, T. J. Organometallics 2012, 31,
3773.
(13) (a) Kaminsky, W. Macromolecules 2012, 45, 3289. (b) Chen, E. Y.-
X. Chem. Rev. 2009, 109, 5157. (c) Chen, E. Y.-X.; Marks, T. J. Chem.
Rev. 2000, 100, 1391.
(14) Liu, Z.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R. J.
Am. Chem. Soc. 2001, 123, 11193.
(15) Bochmann, M. Organometallics 2010, 29, 4711.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
m-delferro@northwestern.edu; t-marks@nortwestern.edu
(16) The characteristics of the PE produced by the SNSCr catalyst
(Mw, ρbr, Tm) are completely different from those obtained using the
tandem and bimetallic systems.
Notes
The authors declare no competing financial interest.
(17) (a) Bowen, L. E.; Charernsuk, M.; Hey, T. W.; McMullin, C. L.;
Orpen, A. G.; Wass, D. F. Dalton Trans. 2010, 39, 560. (b) Wohl, A.;
Muller, W.; Peitz, S.; Peulecke, N.; Aluri, B. R.; Muller, B. H.; Heller, D.;
Rosenthal, U.; Al-Hazmi, M. H.; Mosa, F. M. Chem.Eur. J. 2010, 16,
7833. (c) Walsh, R.; Morgan, D. H.; Bollmann, A.; Dixon, J. T. Appl.
Catal., A 2006, 306, 184.
ACKNOWLEDGMENTS
■
Financial support by NSF (CHE-1213235) is gratefully
acknowledged. Purchases of the NMR and GC−TOF
instrumentation at IMSERC were supported by NSF (CHE-
1048773 and CHE-0923236, respectively). We acknowledge
CINECA Award HP10CPZK0T 2012 for the availability of high-
performance computing resources and support.
(18) Mohring, P. C.; Coville, N. J. Coord. Chem. Rev. 2006, 250, 18.
̈
(19) Seger, M. R.; Maciel, G. E. Anal. Chem. 2004, 76, 5734.
(20) Copolymerization of ethylene with 1-hexene (∼0.5 M) catalyzed
by CGCEtTi/MAO exhibits behavior similar to the tandem system:
decreased activity [360 (kg of PE)·(mol of catalyst)−1·h−1·atm−1] and
Mw (18.5 kg·mol−1, PDI = 1.94) vs ethylene homopolymerizations,
along with C4 branch introduction (46.1 branches/1000 C atoms).
(21) Motta, A.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 2009, 131,
3974.
REFERENCES
■
(1) (a) Nomura, K.; Liu, J.-Y. Dalton Trans. 2011, 40, 7666.
(b) Braunschweig, H.; Breitling, F. M. Coord. Chem. Rev. 2006, 250,
2691.
(2) (a) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011,
111, 2363. (b) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42, 1532.
(3) (a) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111, 2450. (b) Li,
H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. (c) Li, H.;
Stern, C. L.; Marks, T. J. Macromolecules 2005, 38, 9015.
D
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