Organolanthanide-catalyzed synthesis of phosphine-terminated polyethylenes

Article abstract of DOI:10.1021/ja045965n

Organolanthanide-mediated hydrophosphination and ethylene polymerization are coupled in a catalytic cycle to produce diphenylphosphine-terminated polyethylenes. The resulting polymers were characterized by ¹H, ¹³C, and ³¹P

Full text of DOI:10.1021/ja045965n

Published on Web 09/14/2004
Organolanthanide-Catalyzed Synthesis of Phosphine-Terminated
Polyethylenes
Amber M. Kawaoka and Tobin J. Marks*
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113
Received July 6, 2004; E-mail: t-marks@northwestern.edu
Functionalized polyolefins have highly desirable physical char-
acteristics vs nonfunctionalized polyolefins, such as paintability,
adhesion, and compatibility with other materials. However, selective
and catalytic incorporation of polar functionalities into such
chemically unreactive polymers, produced by highly electrophilic
catalysts, remains a significant challenge. One approach to func-
tional polyolefins1-3 is by employing a chain transfer agent4-7 to
introduce heteroatom groups at chain termini. To date, this
transformation has been limited to electron-deficient or electron-
neutral reagents, such as boranes,4 alanes,5 and silanes,7 with little
success achieved in incorporating potentially important electron-
Figure 1. 1H NMR spectra of (a) eicosyldiphenylphosphine oxide and (b)
diphenylphosphine oxide-terminated polyethylene synthesized by in situ
generated Cp′2YPPh2.
rich species (e.g., groups 15, 16).6
Organolanthanide complexes are efficient olefin hydrophosphi-
nation8 and ethylene polymerization catalysts,9 raising the intriguing
question of whether the two transformations can be coupled using
electron-rich phosphines as chain transfer agents for single-site
olefin polymerization. This would represent a new, efficient way
of delivering an electron-rich and chemically versatile fragment to
the terminus of a polyolefin chain. With electron-deficient or
electron-neutral chain transfer agents, such as boranes4 and si-
lanes,7,10 the heteroatom is chain-transferred to the polymer chain
at the end of a hydride-based catalytic propagation cycle, with the
final C-heteroatom bond-forming step (Scheme 1a, step iii) proposed
to occur via four-centered σ-bond metathesis transition state I. In
contrast, analogy drawn from hydrophosphination8 and hydroami-
nation11 mechanistic observations argues that a cycle to incorporate
electron-rich fragments would require C-heteroatom bond formation
at the initiation of the catalytic cycle (Scheme 1b, step iii), with
the Brønsted acidic E-H functionality (E ) P, N) effecting
protonolytic chain transfer (transition state II). We report here the
first efficient embodiment of such a process.
Scheme 1. Contrasting Catalytic Cycles for
Organolanthanide-Catalyzed Synthesis of (a) Silyl-Terminated and
(b) Phosphine-Terminated Polyethylenes.
The proposed catalytic cycle for lanthanide-mediated synthesis
of phosphine-capped polyethylenes (Scheme 1b) involves: (i)
insertion of CdC unsaturation into a lanthanide-phosphido bond,
(ii) subsequent insertions of ethylene into the resulting Ln-C bond,
and (iii) protonolysis of the propagating polyolefin chain by
incoming phosphine to close the cycle and regenerate the lanthanide-
phosphido species. Polymerizations were carried out at 21°C/1.0
atm ethylene under rigorously anaerobic/anhydrous conditions with
Cp′2LnCH(SiMe3)2 and [Cp′2LnH]2 precatalysts (Cp′ ) η5-Me5C5)12
using procedures minimizing mass transport effects and with [olefin]
held constant and [diphenylphosphine] in pseudo-zero-order ex-
cess.13 Since polymers would be produced via ethylene insertion
into the Ln-P bond, Cp′2LnPPh2 complexes were generated prior
to polymerization either in situ or as isolated complexes. For ease
of handling, polymer samples were oxidized to the corresponding
1
phosphine oxide-capped derivatives (eq 1) prior to H, 13C, and
31P NMR, DSC, and GPC characterization.13 1H NMR spectra of
the phosphine-terminated polyethylenes produced by in situ gener-
ated Cp′2YPPh2 (1) exhibit characteristic phenyl, polyethylene
backbone, and -CH3 chain end resonances (Figure 1b). Vinylic
9
12764
J. AM. CHEM. SOC. 2004, 126, 12764-12765
10.1021/ja045965n CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Organolanthanide-Catalyzed Ethylene Polymerization in
the Presence of HPPh2
synthesis is similar to that observed for intramolecular phosphi-
noalkene hydrophosphination/cyclization:8c Y > Sm, Lu > La.15
In addition, the characteristic orange color of the Y-based polym-
erization solutions during turnover suggests that the catalyst resting
state is a lanthanide-phosphido species (rather than an alkyl).8,12
In summary, diphenylphosphine is shown to be an efficient chain
transfer agent in organolanthanide-catalyzed ethylene polymeriza-
tion, yielding phosphine-terminated polyethylenes. This reaction
is a versatile, efficient way of incorporating an electron-rich
functional group into an otherwise inert polymer. Further investiga-
tions of scope and mechanism are currently in progress.
d
[precat.] [HPPh2]
yield
(g)
activityb
107)
Tm
C)
c
c
entry
precatalysta
(
µM)
(mM)
(
×
Mn
Mw/Mn
(
°
1
2
3
4
5
6
7
8
Cp2′LuR
Cp′2YR
Cp′2SmR
Cp′2LaR
Cp′2YPPh2
Cp′2YPPh2
Cp′2YPPh2
Cp′2YPPh2
Cp′2YPPh2
Cp′2YPPh2
Cp′2YPPh2
79
78
83
85
36
36
39
35
35
35
33
20
20
20
20
22
45
67
89
0.25
0.70
0.27
0.73
2.1
0.76
-
3.1
2.9
2.9
2.7
2.3
1.5
0.46
37500
25500
18900
-
29500
18800
12500
11000
9400
1.6
1.9
2.1
-
1.8
2.2
2.3
2.4
2.3
2.3
2.0
137.9
137.6
137.3
-
137.6
137.2
136.4
135.3
134.9
134.6
130
e
-
0.48
0.45
0.48
0.41
0.35
0.23
0.070
9
10
11
121
154
418
7100
3100
Acknowledgment. Financial support by the NSF (Grant No.
CHE-0078998) is gratefully acknowledged. We thank Dr. R.
Lepointe of Dow Chemical and Dr. T. R. Jensen for helpful
discussions.
a Cp′ ) η5-Me5C5, R ) CH(SiMe3)2; polymerization conditions: 50 mL
of toluene, average temperature ) 21 °C (see Supporting Information for
details), 30 s. b Units ) g/(mol Ln‚atm ethylene‚h). c By GPC in 1,2,4-
trichlorobenzene vs polyethylene standards. d By DSC. e Trace yields of
polymer obtained (<10 mg).
Supporting Information Available: Detailed experimental pro-
cedures are provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) Post-polymerization modification: (a) Sheng, Q.; Sto¨ver, H. D. H.
Macromolecules 1997, 30, 6451. (b) Chung, T. C.; Lu, H. L.; Li, C. L.
Macromolecules 1994, 27, 7533. (c) Shiono, T.; Kurosawa, H.; Ishida,
O.; Soga, K. Macromolecules 1993, 26, 2085.
(2) For recent developments in olefin copolymerization with functional
monomers, see: (a) Jensen, T. R.; Yoon, S. C.; Dash, A. K.; Luo L.;
Marks, T. J. J. Am. Chem. Soc. 2004, 125, 14482. (b) Dong, J. Y.; Manias,
E.; Chung, T. C. Macromolecules 2002, 35, 3439. (c) Imuta, J.-I.; Kashiwa,
N.; Toda, Y. J. Am. Chem. Soc. 2002, 124, 1176. (d) Byun, D.-J.; Choi,
K.-Y.; Kim, S. Y. Macromol. Chem. Phys. 2001, 202, 992. (e) Desurmont,
G.; Tanaka, M.; Li, Y.; Yasuda, H.; Tokimitsu, T.; Tone, S.; Yanagase,
A. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4095.
(3) Graft polymerization: (a) Dong, J. Y.; Hong, H.; Chung, T. C.; Wang,
H. C.; Datta, S. Macromolecules 2003, 36, 6000. (b) Passaglia, E.; Coiai,
S.; Aglietto, M.; Ruggeri, G.; Ruberta`, M.; Ciardelli, F. Macromol. Symp.
2003, 198, 147. (c) Dolatkhani, M.; Cramail, H.; Deffieux, A.; Santos, J.
M.; Ribeiro, M. R.; Bordado, J. M. Macromol. Chem. Phys. 2003, 204,
1889. (d) Ciolino, A. E.; Failla, M. D.; Valle´s, E. M. J. Polym. Sci., Part
A: Polym. Chem. 2002, 40, 3950.
(4) Borane chain transfer: (a) Bowden, N. B.; Dankova, M.; Wiyatno, W.;
Hawker, C. J.; Waymouth, R. M. Macromolecules 2002, 35, 9246. (b)
Chung, T. C.; Xu, G.; Lu, Y.; Hu, Y. Macromolecules 2001, 34, 8040.
(c) Xu, G.; Chung, T. C. J. Am. Chem. Soc. 1999, 121, 6763.
(5) Aluminum chain transfer: (a) Go¨tz, C.; Rau, A.; Luft, G. Macromol.
Mater. Eng. 2002, 287, 16. (b) Kukral, J.; Lehmus, P.; Klinga, M.; Leskela¨,
M.; Rieger, B. Eur. J. Inorg. Chem. 2002, 1349. (c) Han, C. J.; Lee, M.
S.; Byun, D.-J.; Kim, S. Y. Macromolecules 2002, 35, 8923. (d) Liu, J.;
Støvneng, J. A.; Rytter, E. J. Polym. Sci., Part A: Polym. Chem. 2001,
39, 3566. (e) Po’, R.; Cardi, N.; Abis, L. Polymer 1998, 39, 959. (f) Kang,
K. K.; Shiono, T.; Ikeda, T. Macromolecules 1997, 30, 1231.
(6) Thiophene C-H chain transfer: Ringelberg, S. N.; Meetsma, A.; Hessen,
B.; Teuben, J. H. J. Am. Chem. Soc. 1999, 121, 6082.
(7) Silane chain transfer: (a) Koo, K.; Marks, T. J. J. Am. Chem. Soc. 1999,
121, 8791. (b) Koo, K.; Fu, P.-F.; Marks, T. J. Macromolecules 1999,
32, 981.
(8) (a) Kawaoka, A. M.; Douglass, M. R.; Marks, T. J. Organometallics 2003,
22, 4630. (b) Douglass, M. R.; Ogasawara, M.; Hong, S.; Metz, M. V.;
Marks, T. J. Organometallics 2002, 21, 283. (c) Douglass, M. R.; Stern,
C. L.; Marks, T. J. J. Am. Chem. Soc. 2001, 123, 10221. (d) Douglass,
M. R.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 1824.
(9) Recent reviews of organolanthanide-catalyzed polymerization: (a) Gro-
mada, J.; Carpentier, J.-F.; Mortreux, A. Coord. Chem. ReV. 2004, 248,
397. (b) Hou, Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002, 231, 1.
(10) Fu, P.-F.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 10747.
(11) (a) Ryu, J.-S.; Marks, T. J.; McDonald, F. E. J. Org. Chem. 2004, 69,
1038. (b) Hong, S.; Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc.
2003, 125, 15878. (c) Ryu. J.-S.; Li, G. Y.; Marks, T. J. J. Am. Chem.
Soc. 2003, 125, 12584. (d) Gagne´, M. R.; Stern, C. L.; Marks, T. J. J.
Am. Chem. Soc. 1992, 114, 275.
(12) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.;
Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091-8103.
(13) See Supporting Information for details.
(14) Davies, J. A.; Mierzwiak, J. G.; Syed, R. J. Coord. Chem. 1988, 17, 25.
(15) Only trace amounts of polymer are obtained using Cp′2LaCH(SiMe3)2 as
precatalyst.
Figure 2. Relationship of polyethylene number average molecular weight
(GPC versus polyethylene) to inverse HPPh2 concentration at fixed catalyst
and ethylene concentrations.
chain end resonances are absent in 1H and 13C NMR spectra,
suggesting that chain termination via â-H elimination is not
significant. Furthermore, the ∼1:1 PPh2 and -CH3 chain end
resonance ratio implies that a phosphine moiety terminates each
polymer chain. For comparison, the 1H, 13C, and 31P NMR spectra
of the model product 1-eicosyldiphenylphosphine oxide (2; Figure
1a)14 are in good agreement with the polymer spectral assignments.
Polymerization and product characterization data (Table 1) reveal
surprisingly high polymerization activities, which are not ap-
preciably depressed over a ∼20-fold increase in phosphine con-
centration. In comparison, the highest activity observed for
organolanthanide-mediated ethylene polymerization in the presence
of phenylsilane (0.02 M) is 8.97 × 105 g of polymer/(mol Ln‚atm
ethylene‚h).7 Narrow monomodal polydispersities are also observed
in the present case, consistent with a single-site process. At constant
[catalyst] and [ethylene], molecular weight is inversely proportional
to phosphine concentration (Table 1, entries 6-11; Figure 2),
supporting a chain transfer mechanism as in Scheme 1b. With
respect to metal ion size effects, it can be seen that product Mn
increases with decreasing lanthanide ionic radius, most likely
reflecting the steric constraints of the growth-limiting chain transfer
process (Scheme 1b, step iii), involving a crowded transition state
(II). Consistent with these results is the previous observation, for
lanthanide-mediated hydrophosphination, that Ln-C protonolysis
is more rapid for larger metal radii.8c Furthermore the metal ionic
radius-activity trend for the present phosphine-capped polyethylene
JA045965N
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J. AM. CHEM. SOC. VOL. 126, NO. 40, 2004 12765