Organo-fn,d0-mediated synthesis of amine-capped polyethylenes. Scope and mechanism

Article abstract of DOI:10.1021/om700831t

Amines of varying Bronsted acidity and steric encumberance are investigated as chain-transfer agents to functionalize polyolefins via organolanthanide-mediated olefin polymerization processes. Ethylene homopolymerizations are carried out with activated Cp′ ₂LnCH(Si(CH₃)₃)₂ (Cp′ = η⁵-Me₅C₅; Ln = La, Sm, Y, Lu) precatalysts in the presence of aniline, n-propylamine, N,N-bis(trimethylsilyl)amine, di-sec-butylamine, N-tert-butyl(trimethylsilyl)amine, di-isopropylamine, and dicyclohexylamine. In the presence of these amines, polymerization activities up to 10⁴ g polymer/(mol of Ln · atm ethylene · h) and narrow product polymer polydispersities are observed, consistent with single-site polymerization processes. Amine chain-transfer efficiency follows the trend C₆H₅NH₂ ≈ ⁿC ₃H₇NH₂ << [Si(CH₃) ₃]₂NH ≈ ^secBu₂NH < N- ^tBu[Si(CH₃)₃]NH ≈ ⁱPr ₂NH < Cy₂NH to yield polyethylenes of the structure H(CH₂CH₂)_nNRR′, where an efficient chain-transfer agent is defined as a reagent that both terminates polymer chain growth and facilitates reinitiation of polymer chain growth. Under the conditions investigated, primary amines are found to be the most inert toward Cp′₂La-mediated polymerizations, affording no detectable insertion products, while di-sec-butylamine and N,N-bis(trimethylsilyl)amine are marginally efficient and produce monoethylene insertion products. In contrast, N-tert-butyl(trimethylsilyl)amine and di-isopropylamine afford amine-capped oligoethylenes, while dicyclohexylamine is the most efficient chain-transfer agent investigated, producing high molecular weight amine-terminated polyethylenes. For these Ln catalysts, dicyclohexylamine chain transfer exhibits a linear relationship between product M_n and [dicyclohexyl-amine] ^-1, consistent with a well-behaved aminolysis chain termination pathway. In all of the above systems, protonolysis appears to be the dominant chain-transfer pathway. Organotitanium-mediated ethylene and propylene polymerizations in the presence of secondary amines result in modest polymerization rates with activities of 10⁴ g polymer/(mol of Ti · atm ethylene · h).

Full text of DOI:10.1021/om700831t

Organometallics 2008, 27, 2411–2420
2411
Articles
Organo-fn,d0-Mediated Synthesis of Amine-Capped Polyethylenes.
Scope and Mechanism
Smruti B. Amin, SungYong Seo, and Tobin J. Marks*
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208
ReceiVed August 15, 2007
Amines of varying Brønsted acidity and steric encumberance are investigated as chain-transfer agents
to functionalize polyolefins via organolanthanide-mediated olefin polymerization processes. Ethylene
homopolymerizations are carried out with activated Cp′2LnCH(Si(CH3)3)2 (Cp′ ) η5-Me5C5; Ln ) La,
Sm, Y, Lu) precatalysts in the presence of aniline, n-propylamine, N,N-bis(trimethylsilyl)amine, di-sec-
butylamine, N-tert-butyl(trimethylsilyl)amine, di-isopropylamine, and dicyclohexylamine. In the presence
of these amines, polymerization activities up to 104 g polymer/(mol of Ln · atm ethylene · h) and narrow
product polymer polydispersities are observed, consistent with single-site polymerization processes. Amine
n
chain-transfer efficiency follows the trend C6H5NH2 ≈ C3H7NH2 << [Si(CH3)3]2NH ≈ secBu2NH <
i
N-tBu[Si(CH3)3]NH ≈ Pr2NH < Cy2NH to yield polyethylenes of the structure H(CH2CH2)nNRR′, where
an efficient chain-transfer agent is defined as a reagent that both terminates polymer chain growth and
facilitates reinitiation of polymer chain growth. Under the conditions investigated, primary amines are
found to be the most inert toward Cp′2La-mediated polymerizations, affording no detectable insertion
products, while di-sec-butylamine and N,N-bis(trimethylsilyl)amine are marginally efficient and produce
monoethylene insertion products. In contrast, N-tert-butyl(trimethylsilyl)amine and di-isopropylamine
afford amine-capped oligoethylenes, while dicyclohexylamine is the most efficient chain-transfer agent
investigated, producing high molecular weight amine-terminated polyethylenes. For these Ln catalysts,
dicyclohexylamine chain transfer exhibits a linear relationship between product Mn and [dicyclohexyl-
amine]-1, consistent with a well-behaved aminolysis chain termination pathway. In all of the above
systems, protonolysis appears to be the dominant chain-transfer pathway. Organotitanium-mediated ethylene
and propylene polymerizations in the presence of secondary amines result in modest polymerization
rates with activities of 104 g polymer/(mol of Ti · atm ethylene · h).
polymeric materials due to their enhanced physical properties
such as adhesion, paintability, and compatibility with diverse
other materials.2 There are several synthetic routes to function-
alized polyolefins including postpolymerization modification,3
copolymerization with polar monomers,4 and catalytic chain
transfer with heteroatom reagents.5–11 Of these approaches,
postpolymerization modification presents challenges due to the
unreactive nature of saturated hydrocarbon polymers as well
as the lack of precise control over functionalization levels and
locations. Copolymerization with polar comonomers is effective,
however is generally restricted to less oxophilic, more polar
Introduction
Polyolefins are attractive commodity materials with an
impressive range of important applications.1 As a consequence
of their microstructural versatility, polyolefins offer a myriad
of useful macromolecular properties, including, but not limited
to, elasticity, melt-fracture resistance, and impressive process-
ability. Functionalized polyolefins are also highly desirable
* Corresponding author. E-mail: t-marks@northwestern.edu.
(1) Recent reviews of single-site olefin polymerization and copoly-
merization: (a) Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15288–
15354, and contributions therein (special feature on “Polymerization”). (b)
Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283. (c) Pedeutour,
J.-N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid
Commun. 2001, 22, 1095. (d) Chen, Y.-X.; Marks, T. J. Chem. ReV. 2000,
100, 1391. (e) Gladysz, J. A. Chem. ReV 2000, 100, special issue on
“Frontiers in METAL-CATALYZED Polymerization”. (f) Marks, T. J.,
Stevens, J. C., Eds. Top. Catal. 1999, 7, andreferences therein. (g) Jordan,
R. F., Ed. J. Mol. Catal. 1998, 128, andreferences therein. (h) Kaminsky,
W.; Arndt, M. AdV. Polym. Sci. 1997, 127, 144–187. (i) Bochmann, M. J.
J. Chem. Soc., Dalton Trans. 1996, 255–270, and references therein. (j)
Brintzinger, H.-H.; Fisher, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M.
Angew. Chem., Int. Ed. Engl. 1995, 34, 1143–1170. (k) Simonazzi, T.;
Nicola, A.; Aglietto, M.; Ruggeri, G. In ComprehensiVe Polymer Science;
Allen, G., Aggarwal, S. L., Russo, S., Eds.; Pergamon Press: Oxford, 1992;
First Supplement, Chapter 7.
(2) Effects of functional groups and branching on polyolefin properties:
(a) Chung, T. C. Prog. Polym. Sci. 2002, 27, 39. (b) Imuta, J.-I.; Kashiwa,
N.; Toda, Y. J. Am. Chem. Soc. 2002, 124, 1176. (c) Dong, J. Y.; Wang,
Z.; Hong, H.; Chung, T. C. Macromolecules 2002, 35, 9352. (d) Chung,
T. C. Polym. Mater. Sci. Eng. 2001, 84, 33. (e) Chum, P. S.; Kruper, W. J.;
Guest, M. J. AdV. Mater. 2000, 12, 1759. (f) Alt, H. G.; Föttinger, K.; Milius,
W. J. Organomet. Chem. 1999, 572, 21. (g) Harrison, D.; Coulter, I. M.;
Wang, S. T.; Nistala, S.; Kuntz, B. A.; Pigeon, M.; Tian, J.; Collins, S. J.
Mol. Catal. A: Chem. 1998, 128, 65. (h) Nesarikar, A. R.; Carr, S. H.;
Khait, K.; Mirabella, F. M. J. Appl. Polym. Sci. 1997, 63, 1179.
(3) Reviews on post-polymerization modification: (a) Boaen, N. K.;
Hillmyer, M. A. Chem. Soc. ReV 2005, 34, 267. (b) Mülhaupt, R. Macromol.
Chem. Phys. 2003, 204, 289.
10.1021/om700831t CCC: $40.75
2008 American Chemical Society
Publication on Web 05/03/2008

2412 Organometallics, Vol. 27, No. 11, 2008
Amin et al.
Scheme 1. Simplified Catalytic Cycle for Single-Site-Mediated
Olefin Polymerization in the Presence of Monofunctional
Electron-Rich Chain-Transfer Agents
reagent-tolerant late transition metal catalysts, which have
modest polymerization activities.
An alternative, versatile approach to polyolefin functional-
ization involves implementation of chain-transfer processes and
agents, with the latter defined as chemical reagents that both
terminate and facilitate the reinitiation of polyolefin chain growth
and can efficiently control molecular weight while simulta-
neously and selectively introducing heteroatom functionality into
the macromolecular architecture. To date, electron-deficient/
neutral chain-transfer agents such as alanes, boranes, and silanes
have been successfully introduced into catalytic single-site olefin
polymerization processes. In contrast to the pathways outlined
above, functionally analogous processes with electron-rich
chain-transfer agents have been extensively characterized only
for phosphines.8 Here, the catalytic cycle is envisioned to
proceed via sequences of (i) insertion of C-C unsaturation into
the metal-heteroatom bond, (ii) multiple insertions of C-C
unsaturation into the resulting M-C bond(s), and (iii) proto-
nolysis of the metal–polymeryl bond, presumably via a polar
four-center σ-bond metathesis transition state, to release the
functionalized polyolefin and regenerate the active catalyst
(Scheme 1). While the synthesis of phosphine-terminated
polyethylenes can be achieved via this route, the product
polymers are generally of specialized interest. In contrast, amine-
terminated polyolefins, if accessible via chain-transfer routes,
have broad established utility in a variety of applications
including, but not limited to, drug and gene delivery, antibacte-
rial treatments, sensors, adhesives, and ion-exchange resins.12
Organolanthanide complexes13 are versatile catalysts for
homogeneous single-site coordinative R-olefin polymerization.1
15
Thus, [Cp′2LnH]214 (Cp′ ) η5-Me5C5) and [Me2-SiCp′′2LnH]2
(Cp′′ ) η5-Me4C5) catalysts efficiently polymerize ethylene to
high molecular weight polyethylene with turnover frequencies
exceeding 1800 s-1 and with narrow product polydispersities.
Furthermore, both primary and secondary organosilanes function
as efficient chain-transfer agents for organolanthanide-catalyzed
olefin homopolymerizations and ethylene/R-olefin copolymer-
izations (via a mechanism coupling Ln-H insertion and Ln-C/
Si-H transposition)5 to yield silane-functionalized polyolefins.
Cp′2Y(2-pyridyl), synthesized from [Cp′2YH]2 via aryl C-H
activation (eq 1),16 also mediates chain transfer to afford
2-ethylpyridine and traces of polymeric products in the presence
of excess ethylene and pyridine (eq 2).16
(4) Copolymerization with polar comonomers: (a) Zhang, X.; Chen, S.;
Li, H.; Zhang, Z.; Lu, Y.; Wu, C.; Hu, Y. J. Polym. Sci: Part A: Polym
Chem. 2007, 45, 59. (b) Williams, B. S.; Leatherman, M. D.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2005, 127, 5132. (c) Liu, W.; Malinoski,
J. M.; Brookhart, M. Organometallics 2002, 21, 2836. (d) Shultz, C. S.;
DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 16. (e) Tempel,
D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 2000, 122, 6686. (f) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem.
ReV. 2000, 100, 1169. (g) Shultz, C. S.; Ledford, J.; DeSimone, J. M.;
Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6351.
(1)
(2)
(5) Silane chain transfer agents: (a) Amin, S. B.; Marks, T. J. J. Am.
Chem. Soc. 2007, 129, 2938. (b) Amin, S. B.; Marks, T. J. Organometallics
2007, 26, 2960. (c) Amin, S. B.; Marks, T. J. J. Am. Chem. Soc. 2006, 128,
4506. (d) Makio, H.; Koo, K.; Marks, T. J. Macromolecules 2001, 34, 4676.
(e) Koo, K.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 8791. (f) Koo, K.;
Fu, P.-F.; Marks, T. J. Macromolecules 1999, 32, 981. (g) Koo, K.; Marks,
T. J. J. Am. Chem. Soc. 1998, 120, 4019. (h) Fu, P.-F.; Marks, T. J. J. Am.
Chem. Soc. 1995, 117, 10747.
(6) Borane chain transfer agents: (a) Lu, Y.; Hu, Y.; Wang, Z. M.;
Manias, E.; Chung, T. C. J. Polym. Sci.: Part A: Polym. Chem. 2002, 40,
3416. (b) Chung, T. C.; Xu, G.; Lu, Y.; Hu, Y. Macromolecules 2001, 34,
8040. (c) Xu, G.; Chung, T. C. Macromolecules 1999, 32, 8689. (d) Xu,
G.; Chung, T. C. J. Am. Chem. Soc. 1999, 121, 6763.
Recently, organolanthanide complexes have been used to
selectively synthesize phosphine-capped polyethylenes with
activities as high as 107 g polymer/(mol Ln · atm · h) as noted
above. A diverse range of secondary phosphines effects selec-
(7) Metallocene-mediated alane chain transfer in propylene polymeri-
zation systems see: (a) Tynys, A.; Eilertsen, J. L.; Rytter, E. Macromol.
Chem. Phys. 2006, 207, 295. (b) Quintanilla, E.; di Lena, F.; Chen, P. Chem.
Commun. 2006, 4309. (c) Fan, G.; Dong, J.-Y. J. Mol. Catal. A: Chem.
2005, 236, 246. (d) Kukral, J.; Lehmus, P.; Klinga, M.; Leskela, M.; Rieger,
B. Eur. J. Inorg. Chem. 2002, 1349. (e) Gotz, C.; Rau, A.; Luft, G.
Macromol. Mater. Eng. 2002, 287, 16. (f) Liu, J.; Stovneng, J. A.; Rytter,
E. J. Polym. Sci; Part A: Polym. Chem. 2001, 39, 3566.
(8) Phosphine chain-transfer agents: (a) Kawaoka, A. M.; Marks, T. J.
J. Am. Chem. Soc. 2005, 127, 6311. (b) Kawaoka, A. M.; Marks, T. J.
J. Am. Chem. Soc. 2004, 126, 12764.
(9) Amine chain transfer agents: Amin, S. B.; Marks, T. J. J. Am. Chem.
Soc. 2007, 129, 10102.
(10) Thiophene chain transfer: Ringelberg, S. N.; Meetsma, A.; Hessen,
B.; Teuben, J. H. J. Am. Chem. Soc. 1999, 121, 6082.
(11) Organozinc chain transfer: (a) Arriola, D. J.; Carnahan, E. M.;
Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714. (b)
van Meurs, M.; Britovsek, G. J. P.; Gibson, V. C.; Cohen, S. A. J. Am.
Chem. Soc. 2005, 127, 9913, and references therein.
(12) Applications of amine-functionalized polymers: (a) Ji, H.; Sakel-
lariou, G.; Mays, J. W. Macromolecules 2007, 40, 3461. (b) Yang, Y.;
Sheares, V. V. Polymer 2007, 48, 105. (c) Yang, Y.; Lee, J.; Cho, M.;
Sheares, V. V. Macromolecules 2006, 39, 8625. (d) Dizman, B.; Elasri,
M. O.; Mathias, L. J. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 5965.
(13) For recent reviews of organolanthanide-mediated olefin polymer-
ization see: (a) Gromada, J.; Carpentier, J.-F.; Mortreux, A. Coord. Chem.
ReV. 2004, 248, 397. (b) Hou, Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002,
231, 1. (c) Ephritikhine, M. Chem. ReV. 1997, 97, 2193.
(14) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann,
H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.
(15) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks,
T. J. J. Am. Chem. Soc. 1985, 107, 8103.
(16) Deelman, B.-J.; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek,
A. L. Organometallics 1994, 13, 3881.
(17) Hydrophosphination: (a) Kawaoka, A. M.; Douglass, M. R.; Marks,
T. J. Organometallics 2003, 22, 4030. (b) Douglass, M. R.; Stern, C. L.;
Marks, T. J. J. Am. Chem. Soc. 2001, 123, 10221. (c) Douglass, M. R.;
Marks, T. J. J. Am. Chem. Soc. 2000, 122, 1824.

Amine-Capped Polyethylenes
Organometallics, Vol. 27, No. 11, 2008 2413
further purification unless otherwise stated. Dicyclohexylamine was
dried with LiAlH4 for 48 h, transferred onto molecular sieves 3
times, and stored in a vacuum-tight storage flask. All other amine
substrates (N,N-bis(trimethylsilyl)amine, N-tert-butyl(trimethylsi-
lyl)amine, di-isopropylamine, di-sec-butylamine, and aniline) were
dried over CaH2 for a minimum of 5 days, distilled onto molecular
sieves 2 times, and stored in vacuum-tight storage flasks. The
monomer 1-hexene was stirred over CaH2 for 5 days and distilled
immediately prior to use. The organolanthanide precatalysts
Cp′2LnCH(SiMe3)2 (Ln ) La, Sm, Y, Lu) were synthesized
according to a published procedure.15 The organotitanium precata-
tive, catalytic C-P bond formation and chain termination,
including diphenyl-, diethyl-, and di-isobutylphosphine. Primary
phosphines (e.g., cyclohexylphosphine) are also efficient chain-
transfer agents, producing phosphine-capped oligoethylenes.
These results demonstrate the ability of electron-rich chain-
transfer agents to selectively and catalytically functionalize
polyolefin chains. On the basis of analogies between hydro-
phosphination17 and hydroamination,18 amine chain-transfer
processes would seem to be viable in single-site organolan-
thanide-mediated olefin polymerization catalysis. However, since
Ln-C protonolyses by amines are established to be ∼104 times
faster than by the corresponding phosphines,18b careful tuning
of the amine chain-transfer agent’s steric and electronic
characteristics is essential for achieving efficient chain propaga-
tion (Scheme 1, rate (i, ii) . rate (iii)).
–21
lyst Me2Si(Me4C5)(NtBu)TiMe220 and cocatalyst Ph3C+B(C6F5)4
were prepared by published procedures.
Physical and Analytical Measurements. NMR spectra were
1
recorded on either an Inova-400 (400 MHz H; 100 MHz 13C) or
1
Inova-500 (500 MHz H; 125 MHz 13C) instrument. For polymer
A recent preliminary report communicated that organolan-
thanide-catalyzed ethylene polymerization in the presence of
dicyclohexylamine yields dicyclohexylamine-terminated poly-
ethylenes, demonstrating that despite the potential kinetic
disadvantages of amine chain-transfer processes, such catalytic
cycles are viable with judicious choice of chain-transfer agents.9
In the present contribution, we extend this study to include a
wide range of amines as chain-transfer agents having systemati-
cally varied steric and electronic characteristics, so as to more
fully define the scope of this organolanthanide-mediated amine-
terminated polyolefin synthesis. We also present a full discussion
of the kinetics and mechanism of such C-N bond-forming
processes, focusing on amine substitution effects and drawing
on observations from analogous phosphine chain transfer and
other hydrofunctionalization processes to place the present
observations in context. We also extend the scope to d0 organo-
group 4 catalysts.
NMR characterization, 50–75 mg samples were dissolved in 0.5–0.7
mL of C2D2Cl4 in a 5 mL NMR tube by heating the mixture in a
120 °C oil bath. Melting temperatures of polymers were measured
by DSC (DSC 2920, TA Instruments, Inc.) from the second scan
with a heating rate of 20 °C/min. GPC analyses of polymer samples
were performed on a Polymer Laboratories PL-GPC 220 instrument
using three PLgel 10 µm mixed columns; operation temperature,
150 °C; mobile phase, 1,2,4-trichlorobenzene (stabilized with 125
ppm BHT); flow rate, 1 mL/min). GC-MS analyses were performed
on a HP 6890 instrument equipped with a Zebron ZB-5 dimeth-
ylpolysiloxane column (30 m × 250 µm × 0.25 µm) interfaced to
a HP 6890 mass-selective detector using n-propylamine as an
internal standard. MALDI-TOF MS spectra were collected on a
PE Biosystems Voyager System 6050 time-of-flight mass spec-
trometer using a nitrogen laser for MALDI (λ ) 337 nm). The
measurements were performed in the reflector mode. Dithranol was
used as the matrix with a polymer concentration of ∼10 mg/mL
and a polymer:matrix ratio of ∼1:1 by mass. Melting points were
measured on a Büchi B-450 melting point apparatus and are
uncorrected. Elemental analyses were performed by Midwest
Microlabs, Indianapolis, IN.
Experimental Section
Materials and Methods. All manipulations of air-sensitive
materials were carried out with rigorous exclusion of oxygen
and moisture in flame- or oven-dried Schlenk-type glassware on a
dual-manifold Schlenk line, or interfaced to a high-vacuum line
(10-6 Torr), or in a N2-filled MBraun glovebox with a high-capacity
recirculator (<1 ppm of O2; <1 ppm of H2O). Argon, hydrogen,
and ethylene (Airgas, prepurified) were purified by passage through
MnO oxygen-removal and Davison 4 A molecular sieve columns.
Hydrocarbon solvents (n-pentane and toluene) were dried using
activated alumina columns according to the method described by
Grubbs19 and were additionally vacuum-transferred from Na/K alloy
immediately prior to vacuum line manipulations. All NMR solvents
were purchased from Cambridge Isotope Laboratories, stored under
argon and over Na/K alloy in vacuum-tight storage flasks, and
distilled immediately prior to use. All organic starting materials
were purchased from Aldrich Chemical Co. and were used without
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of Dicyclohexylamine. Representative Experi-
ment. In the glovebox, a three-necked Morton flask, which had
been dried overnight at 160 °C, equipped with a large stir bar and
thermocouple (Omega type K stainless steel sheathed), was charged
with dry toluene (30 mL). The flask was next attached to a high-
vacuum line and the toluene was freeze–thaw degassed, followed
by introduction of ethylene (1.0 atm) with rapid stirring. In the
glovebox, Cp′2LaCH(SiMe3)2 (0.010 mmol) was placed in a dry
storage tube (dried overnight at 160 °C) equipped with a stir bar
and 4 mL of dry toluene. The catalyst storage tube was attached to
the high-vacuum line, and next, dicyclohexylamine (0.20 mL) was
injected into the storage tube with rapid stirring. The solution was
stirred for several min, after which time the contents were taken
up in an N2-purged syringe. The catalyst solution was next injected
through the septum-sealed polymerization flask sidearm into the
rapidly stirring reaction flask. After 90 min, methanol (10 mL) was
injected to quench the reaction. Excess methanol (∼500 mL) was
then used to precipitate the polymer. The product polymer (0.50
g) was collected by filtration, washed with methanol (200 mL),
and dried in Vacuo at 80 °C for 48 h. Tm ) 138 °C. 1H NMR (400
MHz, C2D2Cl4): δ 0.98 (-CH3), 1.2–1.6 (-CH2-), 2.4 (-CH2N-
and -CHN-). 13C NMR (100 MHz, C2D2Cl4): δ 15.7, 24.1, 27.8,
29.1, 30.8, 51.7, 61.
(18) For recent studies on organolanthanide-catalyzed hydroamination
see: Yu, X.; Marks, T. J. Organometallics 2007, 26, 365. (b) Rastaetter,
M.; Zulys, A.; Roesky, P. W. Chem.-Eur. J. 2007, 13, 3606. (c) Motta,
A.; Fragala, I. L.; Marks, T. J. Organometallics 2006, 25, 4763. (d) Zhao,
J.; Marks, T. J. Organometallics 2006, 25, 4763. (e) Tobisch, S. Chem.-
Eur. J. 2006, 12, 2520. (f) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F.
J. Am. Chem. Soc. 2006, 128, 3748. (g) Reigert, D.; Collin, J.; Meddour,
A.; Schulz, E.; Trifonov, A. J. Org. Chem. 2006, 71, 2514. (h) Panda, T. K.;
Zulys, A.; Gamer, M. T.; Roesky, P. W. J. Organomet. Chem. 2005, 690,
5078. (i) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (j) Ryu,
J.-S.; Marks, T. J. J. Org. Chem. 2004, 69, 1038. (k) Seyam, A. M.; Stubbert,
B. D.; Jensen, T. R.; O’Donnell, J. J., III; Stern, C. L.; Marks, T. J. Inorg.
Chim. Acta 2004, 357, 4029. (l) Hong, S.; Kawaoka, A. M.; Marks, T. J.
J. Am. Chem. Soc. 2003, 125, 15878. (m) Molander, G. A.; Pack, S. K.
Tetrahedron 2003, 59, 10581.
(20) (a) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.;
Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. European Patent
Applications EP416815A2, 1991. (b) Devore, D. European Patent Applica-
tion EP514828A1, 1992.
(21) Ihara, E.; Young, V. G.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 8277–8278.
(19) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.

2414 Organometallics, Vol. 27, No. 11, 2008
Amin et al.
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of Di-isopropylamine. Representative Experi-
ment. The same procedure as for the above reaction was
employed except that di-isopropylamine was used as the chain-
transfer agent, and a 5.0 mL aliquot of the reaction mixture was
reserved for analysis. Under high-vacuum conditions (10-6 Torr),
the volatile portion of the 5.0 mL aliquot was vacuum-transferred
away from the nonvolatile portion (0.06 g). The volatile solution
was analyzed by GC-MS, and the nonvolatile portion was analyzed
by MALDI-TOF MS. 1H NMR (400 MHz, C2D2Cl4): δ 0.88
(-CH3), 1.25 (-CH3), 1.4–1.8 (-CH2-), 2.0 (-CH2N-), 2.35
(-CH-); the products H(CH2CH2)nN(iPr)2, n ) 10–17 and n ) 1,
were detected by MALDI-TOF and GC-MS, respectively.
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of Di-sec-butylamine. Representative Experi-
ment. The same procedure as for the above reaction was employed,
except that di-sec-butylamine was used as the chain-transfer agent.
The products H(CH2CH2)nN[C2H5CH(CH3)]2, n ) 1, 2, were
detected by GC-MS in a ∼40% yield.
NaOH, water, and brine and then dried over Na2SO4. This crude
product was used without further purification for the next step. To
a stirred suspension of LiAlH4 (60 mg, 1.6 mmol) in Et2O at 0 °C
under N2 was slowly added a solution of the above amide (470
mg, 1.05 mmol) in Et2O. The reaction mixture was allowed to warm
to room temperature and stirred under reflux for 2 h. Next, the
mixture was diluted with Et2O and then carefully quenched by
sequential additions of water, 15% aqueous NaOH, and water.
Stirring at room temperature for a further 30 min yielded a colorless
solution with a white precipitate, which was then removed by
filtration through a Celite pad. The filtrate was then dried over
Na2SO4, filtered, and concentrated under reduced pressure. The
crude amine was purified by column chromatography (silica gel,
hexane/EtOAc ) 20:1) to afford 330 mg (0.76 mmol, 72% isolated
yield) of pure amine as a white solid; mp 33.7–34.4 °C. 1H NMR
(400 MHz, CDCl3): δ 2.49 (m, 2 H), 2.41 (t, 2 H), 1.70 (m, 10 H),
1.55 (d, 3 H), 1.35–1.13 (m, 36 H), 1.04 (m, 3 H), 0.85 (t, 3 H).
1H NMR (400 MHz, C2D2Cl4): δ 2.50 (br s, 4 H), 1.75 (m, 10 H),
1.60 (d, 3 H), 1.39–1.07 (m, 39 H), 0.89 (t, 3 H). 13C NMR (100
MHz, CDCl3): δ 58.3, 46.90, 32.14, 31.78, 29.91, 29.88, 29.58,
27.66, 26.65, 26.57, 22.91, 14.34. 13C NMR (100 MHz, C2D2Cl4):
δ 60.7, 47.71, 31.80, 29.60, 29.25, 29.02, 27.17, 25.40, 22.60, 14.12.
Anal. Calcd for C30H59N: C, 83.06; H, 13.71; N, 3.23. Found: C,
82.96; H, 13.73; N, 3.31.
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of N,N-Bis(trimethylsilyl)amine. Representative
Experiment. The same procedure as for the above reaction was
employed, except for that N,N-bis(trimethylsilyl)amine was used
as the chain-transfer agent. 1H NMR (400 MHz, C2D2Cl4): δ 0.16
(-SiMe3), 0.94 (-CH3), 1.2–1.6 (-CH2-), 2.1 (-CH2N-), 2.4
(-CHN); the products H(CH2CH2)nN(SiMe3)2, n ) 1, 2, were
detected by GC-MS in a ∼30% yield.
Organotitanium-Mediated Polymerization of Ethylene in
the Presence of Dicyclohexylamine. Representative Experi-
ment. In the glovebox, a three-necked Morton flask, which had
been dried overnight at 160 °C, equipped with a large stir bar and
thermocouple, was charged with dry toluene (30 mL). The flask
was next attached to a high-vacuum line and the toluene was
freeze–thaw-degassed, followed by introduction of ethylene (1.0
atm) with rapid stirring. Next, dicyclohexylamine (0.25 mL) was
rapidly injected into the flask under argon flush and with rapid
stirring. In the glovebox, a 5.0 mL sample vial equipped with a
septum cap was charged with 3.20 mg (0.010 mmol) of
Me2Si(Me4C5)(NtBu)TiMe2 (CGCTiMe2) and 9.22 mg (0.010
mmol) of Ph3C+B(C6F5)4-. A measured amount of toluene (4.0
mL) was then syringed into the vial with a dry, N2-purged gas-
tight syringe. The vial was shaken for several minutes, and the
contents were taken up in the syringe and then removed from
the glovebox immediately prior to the polymerization experiment.
The catalyst solution was next rapidly syringed through the septum-
sealed polymerization flask sidearm into the rapidly stirring reaction
flask. After 120 min, methanol (10 mL) was injected to quench
the reaction. Excess methanol (∼500 mL) was then used to
precipitate the polymer. The product polymer (0.040 g) was
collected by filtration, washed with methanol (200 mL), and dried
in Vacuo at 80 °C for 48 h. 1H NMR (400 MHz, C2D2Cl4): δ 0.98
(-CH2-, -CH3), 1.2–1.6 (cyclohexyl -CH2-), 2.4 (-CH2N- and
-CHN-). 13C NMR (100 MHz, C2D2Cl4): δ 24.6 (cyclohexyl
-CH2-), 29.3 (-CH2-), 54.3 (-CH2N-).
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of N-tert-Butyl(trimethylsilyl)amine. Represen-
tative Experiment. The same procedure as for the above reaction
was employed, except that N-tert-butyl(trimethylsilyl)amine was used
as the chain-transfer agent. The products H(CH2CH2)nNtBu(SiMe3)
(0.04 g), n ) 6–8 and n ) 1, were detected by MALDI-TOF and
GC-MS, respectively.
Organolanthanide-Mediated Polymerization of Ethylene in
the Presence of Aniline. Representative Experiment. The same
procedure as for the above reaction was employed, except that
aniline was used as the chain-transfer agent.
Organolanthanide-Mediated Polymerization of Propylene
in the Presence of Dicyclohexylamine. Representative Experi-
ment. The same procedure as for the above reaction was used
except that dicyclohexylamine was used as the chain-transfer agent
and 1.0 atm of propylene pressure was used as the monomer.
Organolanthanide-Mediated Polymerization of 1-Hexene
in the Presence of Dicyclohexylamine. Representative Experi-
ment. The same procedure as for the above reaction was used
except that 4.0 mL of 1-hexene was injected into the flask under
argon flush with rapid stirring prior to polymerization.
NMR-Scale Organolanthanide-Mediated Polymerization of
Ethylene in the Presence of n-Propylamine. Representative
Experiment. In the glovebox, an NMR tube equipped with a Teflon
valve was loaded with Cp′2LaCH(SiMe3)2 (4.8 mg, 8.5 µmol) and
C6D6 (0.6 mL). On the high-vacuum line, the tube was evacuated
while frozen at –78 °C, and n-propylamine (0.06 mL, 0.73 mmol)
and C6D6 (0.2 mL) were added via syringe under an argon flush.
The tube was evacuated and back-filled with Ar while frozen at
–78 °C, and then the tube was sealed. The sample tube was warmed
quickly, and 1.0 atm of ethylene was bubbled through the solution
for 30 min. The polymerization reaction was monitored by 1H NMR
spectroscopy.
Synthesis of N-Cyclohexyl-N-octadecylcyclohexanamine. A
solution of dicyclohexylamine (657 µl, 3.3 mmol) in dry CH2Cl2
was added to a stirred solution of stearoyl chloride (1 g, 3.3 mmol)
and triethylamine (1.38 mL, 9.9 mmol). The reaction mixture was
stirred at room temperature overnight under N2 and then poured
into 1 N HCl solution. The mixture was extracted with CH2Cl2 (3
times), and the combined organic layers were washed with 1 N
Organotitanium-Mediated Polymerization of Propylene in
the Presence of Dicyclohexylamine. Representative Experi-
ment. The same procedure as for the above reaction was employed,
except that 1.0 atm of propylene pressure was used as the monomer.
The product polymer (0.20 g) was collected as described above.
1H NMR (400 MHz, C2D2Cl4): δ 0.95 (-CH3), 1.1–1.3 (-CH2-),
1.3 (cyclohexyl -CH2-), 1.7 (-CH-), 2.4 (-CH2N-), 3.5
(-CHN-). 13C NMR (100 MHz, C2D2Cl4): δ 21 (-CH3-), 29
(-CH2-), 48 (-CH-), 56 (-CH2N-).
Organotitanium-Mediated Polymerization of Ethylene in
the Presence of Di-isopropylamine. Representative Experi-
ment. The same procedure as for the above reaction was employed,
except that di-isopropylamine was used as the chain-transfer agent.
GC-MS of the volatile solutions revealed no detectable insertion
products.

Amine-Capped Polyethylenes
Organometallics, Vol. 27, No. 11, 2008 2415
Table 1. Organolanthanide-Mediated Ethylene Polymerization in the Presence of Dicyclohexylamine
c
e
[precat.]
(µM)
[(C6H11)2NH]
(mM)
activityb
Mn
Tm
c
entry
precatalysta
(×104)
(×103)
Mw/Mn
Td
(°C)
1
2
3
4
5
6
7
8
9
Cp′2LuR
Cp′2YR
Cp′2SmR
Cp′2LaR
Cp′2LaNCy2
Cp′2LaNCy2
Cp′2LaNCy2
Cp′2LaNCy2
Cp′2LaNCy2
370
330
330
360
330
290
350
360
360
34
34
34
34
8.4
20
42
84
126
<0.01f
0.01f
0.70
1.00
2.50
2.03
0.91
0.20
0.10
24
24
24
24
24
24
25
25
24
260
200
1100
270
130
91
2.7
2.5
2.0
2.1
1.7
1.6
2.1
138
138
139
138
139
137
138
53
a Cp′ ) Me5C5; polymerization conditions: 30 mL of toluene, 90 min. b Units ) g/(mol Ln · atm ethylene · h. c By GPC in 1,2,4-trichlorobenzene vs
polyethylene standards. d Reaction temperature. e By DSC. f Trace yields of polymer obtained (<10 mg). Cy ) cyclohexyl; R ) CH(TMS)2 or
secondary amine.
Organotitanium-Mediated Polymerization of 1-Hexene in
the Presence of Dicyclohexylamine. Representative Experi-
ment. The same procedure as for the above reaction was employed,
except that 4.0 mL of the monomer 1-hexene was injected into the
flask under an argon flush and rapid stirring, immediately prior to
polymerization, and dicyclohexylamine was used as the chain-
transfer agent. GC-MS of the volatile solutions revealed no
detectable insertion products.
Results
The goal of this research was to investigate the applicability,
scope, and reaction mechanism of amines as electron-rich, C-N
bond-forming chain-transfer agents in single-site olefin poly-
merization. Here we extend our earlier, preliminary study9 to
include other amines in organolanthanide-mediated polymeri-
zation systems. After a brief discussion of the catalyst activation
process, we discuss the efficacy of dicyclohexyl-, di-isopropyl-,
di-sec-butyl-, N-tert-butyl(trimethylsilyl)-, N,N-bis(trimethylsi-
lyl)amine, aniline, and n-propylamine as chain-transfer agents.
Next, the effect of amine and lanthanide ion on polymerization
characteristics will be discussed from a mechanistic standpoint.
Small molecule hydroamination phenomenology will be used
to understand and place in context the present observations and
trends.
Figure 1. 1H NMR spectra of (a) a dicyclohexylamine-capped
polyethylene (Mn ) 131 600) produced by Cp′2La-mediated po-
lymerization (400 MHz, C2D2Cl4, 130 °C) and (b) N-cyclohexyl-
N-octadecylcyclohexanamine model compound (400 MHz, C2D2Cl4,
21 °C).
Organolanthanide Catalyst Activation. The chain termina-
tion step of the proposed catalytic cycle (Scheme 1, step iii)
involves Ln-C σ-bond protonolysis with concomitant formation
of a lanthanide amido complex (or amine-amido complex,
depending on the amine).18 This step is thermodynamically
favorable, as evidenced by calorimetrically characterized eq 3.22a
Ln-C (σ) aminolysis is very rapid for Cp′2LnCH(SiMe3)2
complexes at room temperature and proceeds at rates up to 104
times those of the analogous phosphine reagents.18b Since
Cp′2Ln-amido precatalyst formation is essentially instantaneous
at room temperature, they are conveniently generated in situ
from the corresponding hydrocarbyls under typical catalytic
conditions.
Chain-Transfer Efficiency of Dicyclohexylamine in Or-
ganolanthanide-Mediated Ethylene Polymerizations. Dicyclo-
hexylamine was investigated as a chain-transfer agent for
organolanthanide-mediated ethylene polymerizations (Table 1).
All polymerizations were carried out under 1.0 atm of ethylene
pressure using rigorously anaerobic/anhydrous conditions and
procedures minimizing mass transport effects with Cp′2-
LnCH(SiMe3)2 precatalysts,15 and with olefin concentrations
maintained in pseudo-zero-order excess. Since polymers would
be produced via ethylene insertion into the Ln-N bond, and
the amine moiety is transferred to the polymer chain at the
beginning of chain growth, Cp′2LnNCy2 (Cp′ ) η5-Me5C5; Cy
) cyclohexyl) complexes were first generated prior to poly-
merization. The colorless precatalyst solution remains colorless
upon addition of amine chain-transfer agent and throughout the
1
course of the polymerization reaction. H NMR spectra of the
product amine-terminated polyethylenes produced using in situ-
generated Cp′2LaNCy2 (Cy ) cyclohexyl) exhibit merged
-CH2N, -CHN amine (δ 2.41), -CH2- polyethylene back-
bone (δ 1.2–1.5), and –CH3 chain end (δ 0.98) resonances
(Figure 1a), while 13C NMR spectra likewise exhibit charac-
teristic amine (δ 51) and polyethylene backbone (δ 29 and 24)
resonances (Figure 2a). Furthermore, the absence of vinylic
(3)
(22) (a) Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989,
111, 7844. (b) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110,
7701.
1
resonances in the H and 13C NMR spectra argues that chain

2416 Organometallics, Vol. 27, No. 11, 2008
Amin et al.
Figure 4. MALDI-TOF mass spectrum of a di-isopropylamine-
capped oligoethylene produced by Cp′2La-mediated ethylene po-
lymerization in the presence of di-isopropylamine.
Figure 2. 13C NMR spectra of (a) a dicyclohexylamine-capped
polyethylene (Mn ) 131,600) produced by Cp′2La-mediated eth-
ylene polymerization in the presence of dicyclohexylamine (100
MHz, C2D2Cl4, 130 °C) and (b) N-cyclohexyl-N-octadecylcyclo-
hexanamine model compound (100 MHz, C2D2Cl4, 21 °C).
propylene and 1-hexene homopolymerization systems using
Cp′2LaCH(SiMe3)2 as the precatalyst. MALDI-TOF and GC-
MS analyses of both quenched polymerization mixtures indicate
no significant quantities of R-olefin insertion products. Ad-
ditional mechanistic insights are presented in the Discussion
section.
Chain-Transfer Efficiency of Di-isopropylamine. Di-iso-
propylamine was also investigated as a chain-transfer agent
for ethylene polymerization using a Cp′2LaCH(SiMe3)2
precatalyst. In situ-generated Cp′2LaNiPr2 is colorless, similar
to Cp′2LaCH(SiMe3)2, and during the course of the polymeri-
zation the reaction solution remains colorless. The concentration
of vinyl chain end resonances in the product polymer 1H NMR
spectrum is below the detection limits, again indicating that
chain termination via ꢀ-hydride elimination is negligible and
that di-isopropylamine chain transfer is the dominant chain-
transfer pathway (Figure S1). Furthermore, MALDI-TOF analy-
sis of the product oligoethylenes indicates 10–17 ethylene
insertions per di-isopropylamine chain end (Figure 4). GC-MS
analysis of the volatile portion reveals trace monoethylene
insertion product.
Figure 3. Relationship of polyethylene number average molecular
weight (GPC versus polyethylene) to inverse (C6H11)2NH concen-
tration at fixed catalyst and ethylene concentrations. Several
intermediate Mn samples formed gels and could not be analyzed
by GPC.
Chain-Transfer Efficiency of N-tert-Butyl(trimethylsilyl)-
amine. N-tert-butyl(trimethylsilyl)amine was also investigated
as a chain-transfer agent using Cp′2LaCH(SiMe3)2 as the
precatalyst. In situ-generated Cp′2LaN(tBu)(SiMe3) is colorless,
similar to Cp′2LaCH(SiMe3)2, and the reaction solution remains
colorless during the course of the polymerization. The MALDI-
TOF analyses of the product oligomeric material indicate 6–8
ethylene insertions per N-tert-butyl(trimethylsilyl)amine chain
end (Figure 5). GC-MS analysis of the volatile portion reveals
trace monoethylene insertion product.
termination via ꢀ-hydride elimination is insignificant, while the
∼1: 1 –CH2N:–CH3 1H NMR chain end resonance ratio argues
that one amine functional group is delivered to the terminus of
each polyethylene chain. 1H and 13C NMR spectra of the model
compound N-cyclohexyl-N-octadecylcyclohexanamine (Figures
1b and 2b, respectively) are in good agreement with the polymer
spectral structural assignments. Furthermore, the resulting
amine-capped polyethylenes have narrow, monomodal poly-
dispersities, consistent with a single-site process (Table 1; see
more below).
Chain-Transfer Efficiency of Di-sec-butylamine and N,N-
Bis(trimethylsilyl)amine. Di-sec-butylamine and N,N-bis(trim-
ethylsilyl)amine were also investigated as chain-transfer agents
using Cp′2LaCH(SiMe3)2 as the precatalyst. The Cp′2LaNR2
solutions remain colorless during the course of the polymeriza-
tions. MALDI-TOF analyses of the quenched polymerization
mixtures indicate no significant oligomeric ethylene insertion
products or Si-N alcoholysis products; however, GC-MS
analysis indicates monoethylene insertion products in ∼30–40%
yield (eq 4).
For polymerizations conducted in the presence of constant
[dicyclohexylamine], lanthanide ionic radius and polymer mo-
lecular weight appear to be inversely related. Ethylene poly-
merizations mediated by in situ-generated Cp′2LaNCy2 in the
presence of dicyclohexylamine (∼34 mM) require relatively
long reaction times to produce significant amounts of polymer
with slightly lower molecular weight (Mn ) 200 000; Table 1,
entry 4) versus the polyethylenes produced by Cp′2SmNCy2 (Mn
) 260 000; Table 1, entry 3).
Furthermore, at constant Cp′2LaNCy2 and ethylene concentra-
tions, product polyethylene molecular weight is inversely
proportional to [amine] (Table 1, entries 5–9, Figure 3),
supporting the mechanism shown in Scheme 1. Dicyclohexyl-
amine was also investigated as a chain-transfer agent in
Chain-Transfer Efficiency of Primary Amines. Aniline and
n-propylamine were also investigated as chain-transfer agents
using Cp′2LaCH(SiMe3)2 as the precatalyst. In situ-generated
Cp′2LaNHR complexes are colorless, and reaction solutions

Amine-Capped Polyethylenes
Organometallics, Vol. 27, No. 11, 2008 2417
Figure 5. MALDI-TOF mass spectrum of an N-tert-butyl(trimeth-
ylsilyl)amine-capped oligoethylene produced by Cp′2La-mediated
ethylene polymerization in the presence of N-tert-butyl(trimethyl-
silyl)amine.
Figure 6. Hydrophosphination/cyclization SCF energy profile
compared with that for analogous hydroamination/cyclization. The
labeling is for the hydroamination case. Adapted from ref 24.
presence of dicyclohexylamine also yield no detectable insertion
products by GC-MS.
Discussion
(4)
As a prelude to discussing polymerization kinetics and
mechanism in the presence of amines, it is useful to outline
key constraints on the present catalytic processes: (1) the olefin
activation/insertion barrier must be adequately low to allow
ethylene insertion into the Ln-amido bond at acceptable rates
(step i, Scheme 1); (2) if initial insertion is viable, subsequent
insertions must be faster than chain-terminating protonolysis
for efficient polymer chain growth (step ii, Scheme 1); (3) if
ethylene insertion is rapid, then chain-terminating protonolysis
rates must be sufficient to control the product polymer chain
length (step iii, Scheme 1). We summarize the interplay of these
processes here and discuss their dependence on amine steric
and electronic properties. Additionally, we compare and contrast
analogous polymerization/chain-transfer processes involving
phosphines to better understand the present trends.
remain colorless during the course of the polymerization. The
MALDI-TOF, GC-MS, and 1H NMR analyses of the quenched
polymerization mixtures indicate no detectable ethylene insertion
products.
Secondary Amine Chain-Transfer Efficiency in Organ-
otitanium-Mediated Olefin Polymerizations. Dicyclohexyl-
amine and di-isopropylamine were investigated as chain-transfer
agents for CGCTiMe2/Ph3C+B(C6F5)4--mediated ethylene and
propylene homopolymerizations. All polymerizations were
performed under 1.0 atm monomer pressure with [olefin] held
constant and the chain-transfer agent maintained in pseudo-zero-
order excess. The orange active catalyst solutions remain
brightly colored throughout the course of the polymerization
reactions, suggesting the presence of a stable active species.
CGCTiMe2/Ph3C+B(C6F5)4--mediated ethylene polymerizations
in the presence of dicyclohexylamine produce small amounts
(∼30–40 mg) of product polymers with activities up to 104 g
polymer/(mol of Ti · atm monomer · h). 1H NMR spectra of the
product dicyclohexylamine-terminated polyethylenes exhibit
characteristic -CH2N, -CHN amine (δ 2.3, 3.1, respectively),
-CH2- polyethylene backbone (δ 1.2–1.5), and –CH3 chain
end (δ 0.97) resonances (Figure S2), while the absence of vinylic
resonances argues that chain termination via ꢀ-hydride elimina-
tion is insignificant. Additionally, 1H NMR spectra of the
product dicyclohexylamine-terminated polypropylenes produced
by CGCTiMe2/Ph3C+B(C6F5)4- exhibit characteristic -CH2N,
-CHN amine (δ 2.4, 3.5, respectively), -CH2- backbone (δ
1.1–1.3), -CH backbone (δ 1.7), and –CH3 backbone and chain
end (δ 0.95) resonances (Figure S3). The absence of olefinic
resonances again argues that ꢀ-hydride elimination is insignifi-
cant. Interestingly, CGCTiMe2/Ph3C+B(C6F5)4--mediated olefin
polymerizations in the presence of di-isopropylamine produce
no detectable insertion products by GC-MS, and CGCTiMe2/
Ph3C+B(C6F5)4--mediated 1-hexene polymerizations in the
Amine Steric and Electronic Effects on Ethylene Poly-
merization. The present results indicate that the efficacy of
protonolytic chain termination in amine chain-transfer processes
during lanthanocene-mediated ethylene polymerizations de-
creases in the order
C6H5NH2 ≈ C3H7NH2 (Si(CH3)3)2NH ≈ secBu2NH >
i
N-tBu(Si(CH3)3)NH ≈ Pr2NH > Cy2NH
This trend doubtless reflects a complex interplay of steric
and electronic characteristics that contribute to chain-transfer
efficiency in these systems.
Step i. Unlike lanthanocene-mediated polymerizations con-
ducted in the presence of secondary alkyl- and alkylsilylamines,
primary amines aniline and n-propylamine produce no detectable
ethylene insertion products (Scheme 1, step i). Since these
amines are the sterically least encumbered of those investigated,
it is reasonable that a coordinatively saturated Ln-amido-amine
complex18 is kinetically inert with respect to ethylene insertion
(Scheme 1, step i rate , step iii rate; representative structures
A and B for the aniline case). Additionally, DFT/B3LYP level

2418 Organometallics, Vol. 27, No. 11, 2008
Amin et al.
theoretical calculations on hydrofunctionalization processes
indicate that Ln-NR2 complexes have higher olefin insertion
barriers than do Ln-PR2 complexes (∆H ∼5 kcal/mol, Figure
6),23,24 while calorimetrically determined thermochemical data
indicate that the Sm-N bond enthalpy in Cp′2Sm-NMe2 (∼48
kcal/mol) is far greater than the Sm-P bond enthalpy in
Cp′2Sm-PEt2(∆H ∼32 kcal/mol).22a
phinopent-4-ene (eq 6) reveals both differences and similarities
between N and P.23 Geometry optimizations of the phosphine
cyclization products reveal two stable conformations (K and
L), similar to the aforementioned hydroamination results.24
Conformation K involves P-coordination to the La center and
is computed to be 13.1 kcal/mol more stable than L, and again
coordination of nonbulky free methylphosphine yields a phos-
phine-phosphido complex with ∼7 kcal/mol additional stabiliza-
tion (M) versus L. As a likely consequence, the present systems
implementing di-sec-butyl- and N,N-bis(trimethylsilyl)amine
most frequently afford monoinsertion products.
Step iii. Theoretical and experimental evidence both indicate
rapid, quantitative Ln-alkyl precatalyst protonolysis with amines
(eq 7, Figure 6) in contrast to more sluggish phosphine
protonolysis (eq 8, Figure 6).8,17 Invoking hard/soft acid–base
concepts, these different protonolysis rates may reflect the
“softer” nature of P versus N,25 despite the greater Brønsted
acidity of R2PH versus R2NH.26 Thus, the experimental and
theoretical results argue that alkene insertion is turnover-limiting
for hydroamination, while Ln-C bond protonolysis is turnover-
limiting for hydrophosphination, in excellent agreement with
the more facile chain growth observed in phosphine polymer-
ization/chain-transfer systems.8
Step ii. Regarding the frequent observation of monoinsertion
products here, note that for thiophene,10 pyridine,17 and phe-
nylphosphine chain-transfer agents,8 monoinsertion products are
found to be particularly stable toward further ethylene insertion,
likely a consequence of intramolecular heteroatom coordination
to the electrophilic Ln center (e.g., C-E). Therefore, it is
possible in the present case that the monoinsertion products
Cp′2LaCH2CH2NsecBu2 and Cp′2LaCH2CH2N(Si(CH3)3)2 are
also less reactive toward ethylene insertion due to stabilizing
intramolecular amine coordination (e.g., F and G, respectively;
step ii rate , step i rate, Scheme 1). In this connection, the
DFT/B3LYP computational studies on the geometries and
stabilities of the intermediates and transition states in organo-
lanthanide-catalyzed hydroamination/cyclization of 1-aminopent-
4-ene (eq 5) find two stable product conformations, H and I,
following initial intramolecular olefin insertion into the La-N
bond.23
(7)
(8)
The protonolytic Ln-C bond cleavage rate is likely governed
by amine reagent steric bulk and Brønsted acidity (Scheme 1,
step iii). The experimental solution-phase Brønsted acidity of
the amine chain-transfer agents investigated here decreases in
the order26
(5)
Chelating conformation H is computed to be ∼16.4 kcal/
mol more stable than I, whereas intermolecular coordination
of free, nonbulky methylamine accrues an additional ∼18 kcal/
mol stabilization for J versus I. These calculations therefore
suggest that the monoinsertion products F and G are likely
stabilized by the intramolecular amine coordination.
C6H5NH2 > C3H7NH2/(Si(CH3)3)2NH > N-tBu(Si(CH3)3)
i
NH > Cy2NH/secBu2NH ≈ Pr2NH
As noted above, the product polymer Mn values reflect the
rates governing chain growth in Scheme 1, steps i and ii
(kethylene) counterbalanced by the rates of competing step iii
protonolysis (kNH). Thus, di-isopropyl- and N-tert-butyl(trim-
ethylsilyl)amine are moderately efficient protonolytic chain-
transfer agents and produce amine-capped oligoethylenes in
reasonable yields (rates i/ii > rate iii). However, only with an
appropriate balance of steric and electronic properties can
(6)
DFT/B3LYP analysis of the functionally analogous organo-
lanthanide-catalyzed hydrophosphination/cyclization of 1-phos-
(23) Motta, A.; Lanza, G.; Fragala, I. L.; Marks, T. J. Organometallics
2004, 23, 4097.
(26) Relative Brønsted acidities: (a) Koppel, I. A.; Burk, P.; Koppel, I.;
Leito, I. J. Am. Chem. Soc. 2002, 124, 5594. (b) Koppel, I. A.; Burk, P.;
Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. J. Am. Chem. Soc. 2000,
122, 5114.
(24) Motta, A.; Fragala, I. L.; Marks, T. J. Organometallics 2005, 24,
4995.
(25) Pearson, R. G. J. Chem. Educ. 1987, 64, 561.

Amine-Capped Polyethylenes
Organometallics, Vol. 27, No. 11, 2008 2419
efficient chain propagation occur before chain-growth termina-
tion (rates i/ii >> rate iii), such as for dicyclohexylamine (Table
1). Lanthanocene-mediated (La, Sm) ethylene polymerizations
in the presence of dicyclohexylamine result in high Mn
polyethylenes, likely reflecting the increased amine steric bulk
and decreased secondary amine Brønsted acidity. This also
suggests that steric repulsions in monoinsertion products N and
O may render intramolecular or intermolecular amine coordina-
tion less favorable versus the other amines investigated. The
Cp′2La- and Cp′2Sm-catalysts produce dicyclohexylamine-
terminated polyethylenes with relatively high productivities,
whereas Cp′2Y- and Cp′2Lu-mediated systems are less efficient,
producing only trace amounts of polymer, paralleling known
trends in Cp′2Ln-NR2 olefin insertion reactivity with falling ionic
radius (see more below).19
chain transfer (ktNH; Table 1, entries 3 and 4). Thus, larger La3+
mediates somewhat more rapid ethylene propagation versus
ethylene
ethylene
smaller Sm3+, kp
(La3+) > kp
(Sm3+), but also
slightly more rapid protonolytic chain transfer, ktNH (La3+) >
ktNH (Sm3+). Regardless of lanthanide ion, propylene homopo-
lymerizations in the presence of dicyclohexylamine afford no
detectable olefin insertion products. One possible explanation
lies in facile documented organolanthanide-mediated propylene
C-H activation processes.10,15,17,27,28 η3-Allyl species (P) are
expected to be less reactive with respect to propylene insertion
and may undergo protonolysis to regenerate the corresponding
amido complex.22 That Cp′2LnNCy2-mediated 1-hexene ho-
mopolymerizations in the presence of amines also afford no
detectable insertion products is presumably due to such unfavor-
able C-H activation processes or to sluggish insertion rates
for the more sterically encumbered monomer.
Kinetics and Mechanism. The catalytic system Cp′2LaNCy2
+ HNCy2 + ethylene produces dicyclohexylamine-capped
polyethylenes over a wide range of amine concentrations (Table
1, entries 5–9). A series of polymerizations with varying
dicyclohexylamine concentrations (in pseudo-zero-order excess)
was conducted using Cp′2LaNCy2 as the catalyst and with
constant [catalyst] and [ethylene] and rapid mixing. Under these
conditions, a linear relationship between Mn and [dicyclohexyl-
amine]-1 is observed (Figure 3), consistent with amine chain
termination being the dominant pathway. The absence of vinyl
1H NMR resonances in the product polymer and the ∼1:1 –CH3:
-CH2N chain end ratio also supports protonolysis as the
dominant chain-transfer pathway (Figure 1, Scheme 1). Under
steady-state conditions, the number-average degree of poly-
merization, Pn, is equal to the sum of all rates of propagation,
∑Rp, divided by the rates of all competing chain-transfer
pathways, ∑Rt (eq 9).8,9 Assuming a single dominant chain-
transfer process by amine protonolysis and rapid chain reini-
tiation after chain transfer, Pn is given by eq 10, where kp is the
The organotitanium-mediated ethylene and propylene ho-
mopolymerizations conducted here in the presence of secondary
amines result in small quantities of product polymers, likely
reflecting the relative inertness of the Ti+-NR2 bond and/or
the greater stability of structures such as O for cationic d0
centers.23b Addition of dicyclohexylamine to the Ti-mediated
systems allows rapid CdC propagation prior to chain termina-
tion with productivities up to 104 g polymer/(mol Ti · atm
monomer · h), likely a consequence of slow chain transfer for
this sterically encumbered amine. However, less bulky di-
isopropylamine results in no detectable insertion products,
presumably due to slow CdC insertion. The rates of propagation
of higher R-olefins, such as 1-hexene, are too slow to effect
chain growth, resulting in no detectable insertion products. Thus,
organotitanium polymerization systems produce amine-func-
tionalized polyolefins, albeit with currently limited scope and
efficiency.
Lanthanide Ionic Radius and Chain-Transfer Reactivity. The
present lanthanide ionic radius-polymerization activity trend
parallels that of small-molecule hydroamination19 and ethylene
homopolymerization15 (Table 1, entries 1–4), indicating more
facile CdC enchainment
(27) C-H activation mediated by organolanthanide complexes: (a)
Evans, W. J.; DeCoster, D. M.; Greaves, J. Organometallics 1996, 15, 3210.
(b) Deelman, B.-J.; Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.;
Spek, A. L. Organometallics 1995, 14, 2306. (c) Booij, M.; Deelman, B.-
J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H. Organome-
tallics 1993, 12, 3531. (d) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W.
Organometallics 1991, 10, 134. (e) Evans, W. J.; Ulibarri, T. A.; Ziller,
J. W. J. Am. Chem. Soc. 1990, 112, 2314. (f) Thompson, M. E.; Baxter,
S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer,
W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (g) Watson, P. L.
J. Am. Chem. Soc. 1982, 104, 337.
Cp2′La- > Cp2′Sm- > Cp2′Y- ≈Cp2′Lu-
in the coordination spheres of larger lanthanide ions (La3+
,
Sm3+ 15 and also steric constraints around the smaller ions (Y3+
) ,
Lu3+). Interestingly, larger La3+ produces slightly lower mo-
lecular weight polymers versus those produced with Sm3+, likely
reflecting the sensitive steric demands and competition between
ethylene insertion/propagation (kpethylene) versus protonolytic
(28) (a) Dahlmann, M.; Erker, G.; Bergander, K. J. Am. Chem. Soc.
2000, 122, 7986. (b) Karl, J.; Dahlmann, M.; Erker, G.; Bergander, K. J. Am.
Chem. Soc. 1998, 120, 5643. (c) Kissin, Y. V. Isospecific Polymerization
of Olefins; Springer-Verlag: New York, 1985; pp 1–93.

2420 Organometallics, Vol. 27, No. 11, 2008
Amin et al.
rate constant for propagation and ktNH is the rate constant for
intermolecular amine chain transfer.
efficiency of chain transfer in lanthanocene-mediated ethylene
polymerizations increases in the order
R
-
Pn )
∑
∑
p
C6H5NH2 ≈ C3H7NH2 (Si(CH3)3)2NH ≈ secBu2NH <
(9)
i
N-tBu(Si(CH3)3)NH ≈ Pr2NH < Cy2NH
R
t
ethylene
[
]
-
Pn )
kp
ethylene
Primary amines are least efficient and do not yield insertion
products, likely due to strong amine/amide binding to the Ln3+
center and the high resulting olefin insertion barrier. Among
secondary amines, N,N-bis(trimethylsilyl)amine and di-sec-
butylamine are the least efficient chain-transfer agents, producing
only monoethylene insertion products. N-tert-butyl(trimethyl-
silyl)amine and di-isopropylamine are also efficient chain-
transfer agents for Cp′2La-mediated polymerizations, producing
oligoethylene products. Dicyclohexylamine is the most effective
chain-transfer agent found for lanthanocene-mediated ethylene
polymerizations, producing high molecular weight linear, chain-
end-functionalized polymeric products.
(10)
ktNH amine
[
]
With a series of polymerizations carried out at constant
[catalyst] and [monomer] and with [dicyclohexylamine] in
pseudo-zero-order excess, Figure 3 shows that eq 10 is
obeyed over a broad [amine] range. Using eq 10 and the data
in Figure 3 yields the kpethylene/ktNH rate constant ratio. For
the present system kpethylene/ktNH ≈ 1700, approximately 10
ethylene
times the ratios observed for the Cp′2YPPh2- (kp
kphosphine ≈ 200, diphenylphosphine),8b [Cp′2SmH]2- (kp
/
/
ethylene
-
kSi ≈ 190, phenylsilane),5e and CGCTiMe2/PhC+B(C6F5)4
-
mediated(kpethylene/kSi ≈180,5-hexenylsilane)5b,c polymerization
systems. The present large kpethylene/ktNH ratio is consistent
with the exceptionally high product polymer molecular
weights, which are ∼10 times those reported in the afore-
mentioned systems using phosphine and silane chain-transfer
agents at similar concentrations. In the present systems,
product molecular weights rival those produced in the absence
of chain-transfer agent.15 The single-site character of the
present systems is completely consistent with the aforemen-
tioned good activities, high product molecular weights, and
narrow polydispersities (Mw/Mn ≈ 2.0), in accord with the
scenario of Scheme 1.
A series of Cp′2LaCy2-mediated polymerizations with varying
amine concentrations was carried out and reveals good poly-
merization rates, high product polymer molecular weights, Mw/
Mn ≈ 2.0, negligible ꢀ-hydride elimination NMR resonances,
a 1:1 ratio of CH3:CH2N chain end units, and a linear
relationship between Mn and [dicyclohexylamine]-1, all con-
sistent with a predominant protonolytic, amine chain-transfer
mechanism. Due to the complex interplay of insertion, propaga-
tion, and protonolysis rates, careful optimization of amine steric
and electronic properties is necessary to afford high molecular
weight heteroatom-functionalized polymers.
Acknowledgment. Financial support by the NSF (Grant
CHE-04157407) is gratefully acknowledged. We thank Dr.
A. Arzadon for helpful discussions.
Conclusions
Supporting Information Available: Detailed analytical data for
polymeric/oligomeric products (Figures S1–S3). This material is
available free of charge via the Internet at http://pubs.acs.org.
This investigation demonstrates that amines can act as
efficient Ln-C cleaving/C-N bond-forming chain-transfer
agents in Cp′2Ln-mediated ethylene polymerizations. The overall
OM700831T