Synthesis of metal-centered star-shaped polyoxazolines using Fe(II) and Ru(II) tris-bipyridine derivatives as multifunctional initiators

Full text of DOI:10.1021/ja962834g

J. Am. Chem. Soc. 1997, 119, 1801-1802
1801
Synthesis of Metal-Centered Star-Shaped
Polyoxazolines Using Fe(II) and Ru(II)
Tris-bipyridine Derivatives as Multifunctional
Initiators
strategy. It will also be demonstrated that the reactivity of [Fe-
2
+
(bpy)3] may be exploited to produce dramatic changes in the
properties of Fe(II) core materials. Since living oxazoline
polymerizations are initiated by electrophilic reagents (e.g.,
1
0
benzyl bromide) and are run in polar CH3CN solutions, they
were expected to be compatible with cationic metal complex
Jaydeep J. S. Lamba and Cassandra L. Fraser*
cores. Thus, the ligand (4,4′-bromomethyl)-2,2′-bipyridine
1,12
(
bpyBr2, 1a) and some of its complexes (2a and 3a)1
were
Department of Chemistry, UniVersity of Virginia
McCormick Road, CharlottesVille, Virginia 22901
selected as model systems for testing the metalloinitiator
approach (eq 1). Complexes 2 and 3 probe the compatibility
ReceiVed August 13, 1996
Due to their varied structures, physical properties, and
reactivities, metal complexes often play key roles in macro-
molecules.1 They serve as templates for self-assembly, as
cross-links, or as part of the polymer backbone. Since metal
ions may be labile or inert, may be electron donors or acceptors,
and are often chromophoric, magnetic, or conducting centers,
they allow for the introduction of a variety of features into
polymers. Thus, metal-containing materials are of interest as
2
3
sensors, as storage and switching devices, as supported
4
4a,5
catalysts, and as models of structural proteins and enzymes,
among other uses in nanotechnology. Although it is well
documented that living polymerizations allow for exquisite
control over molecular weight (MW) and architecture in organic
of the polymerizations with both labile and inert metal centers
and with different numbers of functionalities at the cores. Also,
materials of these metal complexes are of great interest for their
6
polymers, there have been few reports of their extension to
transition metal-containing materials. In a few cases, metal-
7
containing monomers have been used for living polymerizations
5d,e,13
12
reactivities and photophysical properties.
was used to prepare the labile hexafunctional compound [Fe-
The ligand 1a
and monodisperse polymers have been functionalized with
8
metals subsequent to polymerization. However, the use of
(
[
bpyBr2)3](PF6)2 (2a); however, the inert, difunctional complex
Ru(bpy)2(bpyBr2)](PF6)2 (3a) proved difficult to access in
metalloinitiators or terminators as structural templates in living
7
,9
polymerization reactions remains largely unexplored.
sufficiently high purity for use as an initiator. Instead, the less-
1
2
reactive dichloride analog 1b was used to prepare a difunc-
tional Ru(II) complex (3b), which was converted to the diiodide
1
0a
initiator 3c (X ) I) in situ using NaI.
b was also prepared and treated in an analogous manner for
comparison.
The Fe(II) preinitiator
2
(7) Here, “metalloinitiator” refers to cases were the metal complex
remains behind as part of the dead or nonliving end of the polymer chain.
This may be contrasted with coordination polymerizations wherein metals
are essential components of the living chain end. For examples of both
concepts, as well as the use of metal-containing monomers, see: Schrock,
R. R. In Ring Opening Polymerization; Brunelle, D. J., Ed.; Hanser: Munich,
1993; Chapter 4.
Herein is described the synthesis of metal-centered star-shaped
polymers (MCSPs), namely, Fe(II) and Ru(II) tris-bipyridine-
(
bpy)-centered polyoxazolines by a multifunctional initiator
(8) (a) Peters, M. A.; Belu, A. M.; Linton, R. W.; Dupray, L.; Meyer, T.
(
1) (a) Pomogailo, A. D.; Savost’yanov, V. S. Synthesis and Polymer-
ization of Metal-Containing Monomers; CRC Press: Boca Raton, FL, 1994.
b) Ciardelli, F., Tsuchida, E., W o¨ hrle, D., Eds. Macromolecule-Metal
J.; DeSimone, J. M. J. Am. Chem. Soc. 1995, 117, 3380-8. (b) Chujo, Y.;
Naka, A.; Kramer, M.; Sada, K.; Saegusa, T. J. Macromol. Sci., Pure Appl.
Chem. 1995, 32, 1213-23.
(
Complexes; Springer: Berlin, 1996. (c) Biwas, M.; Mukherjee, A. AdV.
(9) Another class of monodisperse macromolecules are metal-containing
dendrimers. These highly branched molecules are typically prepared by self-
assembly or iterative routes. In contrast, star polymers possess linear arms
and are accessed in a single step from multifunctional reagents. See: ref 2.
(a) Balzani,V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759-833. (b) Bhyrappa, P.; Young, J. K.; Moore, J. S.;
Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708-11. (c) Ardoin, N.;
Astruc, D. Bull. Soc. Chim. Fr. 1995, 132, 875-909.
Polym. Sci. 1994, 115, 89-123.
(2) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1
154-96. (b) Neve, F. AdV. Mater. 1996, 8, 277-89. (c) Metallomesogens.
Synthesis, Properties and Applications; Serrano, J. L., Ed.; VCH: Weinheim,
995.
1
(3) (a) Ward, M. D. Chem. Soc. ReV. 1995, 24, 121-34. (b) Fabbrizzi,
L.; Poggi, A. Chem. Soc. ReV. 1995, 24, 197-202. (c) Diamond, D.;
McKervey, M. A. Chem. Soc. ReV. 1996, 25, 1524. (d) Yu, L.; Chan, W.
K.; Peng, Z.; Gharavi, A. Acc. Chem. Res. 1996, 29, 13-21.
(10) (a) Kobayashi, S.; Uyama, H.; Narita, Y.; Ishiyama, J.-I. Macro-
molecules 1992, 25, 3232-6. (b) Chujo, Y.; Saegusa, T. In Ring Opening
Polymerization; Brunelle, D. J., Ed.; Hanser: Munich, 1993; Chapter 8 and
references cited therein. (c) Cai, G.; Litt, M. H. J. Polym. Sci., Part A:
Polym. Chem. 1989, 27, 3603-18.
(11) Gould, S.; Strouse, G. F.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem.
1991, 30, 2942-9.
(12) Compounds 1a and 1b can be accessed in considerably higher yield
and purity via reaction of 4,4′-[(trimethylsilyl)methyl]-2,2′-bipyridine with
BrCF2CF2Br or CCl3CCl3 in the presence of SiO2/(n-Bu)4NF in THF at
-78 °C. Lamba, J. J. S.; Fraser, C. L. Manuscript in preparation.
(13) For Fe, see: (a) Dictionary of Inorganic Compounds; Chapman &
Hall: New York, 1992; pp 2124-5. For Ru, see: (b) (General) Roundhill,
D. M. Photochemistry and Photophysics of Metal Complexes; Plenum
Press: New York, 1994. (c) (Photopolymer films) Bergstedt, T. S.; Hauser,
B. T.; Schanze, K. S. J. Am. Chem. Soc. 1994, 116, 8380-1. (d)
(Photorefractive polymers) Peng, Z.; Yu, L. J. Am. Chem. Soc. 1996, 118,
3777-8. (e) (Oxygen sensors) Xu, W.; McDonough, R. C., III; Langsdorf,
B.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1994, 66, 4133-41. (f)
(Artificial photosynthesis) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-
70.
(4) (a) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812-32. (b)
Bergbreiter, D. E. Macromol. Symp. 1996, 105, 9-16. (c) Polymeric
Reagents and Catalysts; Ford, W. T., Ed.; ACS Symposium Series 308;
American Chemical Society: Washington, DC, 1986.
(5) (a) Bryson, J. W.; Betz, S. F.; Lu, H. S.; Suich, D. J.; Zhou, H. X.
X.; Oneil, K. T.; Degrado, W. F. Science 1995, 270, 935-41. (b) Rabanal,
F.; Degrado, W. F.; Dutton, P. L. J. Am. Chem. Soc. 1996, 118, 473-4. (c)
Ghadiri, M. R.; Soares, C.; Choi, C. J. Am. Chem. Soc. 1992, 114, 825-
3
1, 4000-2. (d) Langen, R.; Chang, I. J.; Germanas, J. P.; Richards, J. H.;
Winkler, J. R.; Gray, H. B. Science 1995, 268, 1733-5. (e) Sardesai, N.;
Lin, S. C.; Zimmermann, K.; Barton, J. K. Bioconjugate Chem. 1995, 6,
3
02-12.
(6) The term “living” is used to describe chain growth polymerizations
which proceed with no appreciable chain transfer or termination steps. If
initiation is faster than propagation, the MW distribution of the material is
narrow (Mw/Mn ≈ 1). (a) Quirk, R. P.; Lee, B. Polym. Int. 1992, 27, 359-
6
7. (b) Living Polymer Systems. In Encyclopedia of Polymer Science and
Engineering; Supplement Volume; Mark, H., Ed.; Wiley: New York, 1989;
pp 380-445.
S0002-7863(96)02834-X CCC: $14.00 © 1997 American Chemical Society

1802 J. Am. Chem. Soc., Vol. 119, No. 7, 1997
Communications to the Editor
Table 1. Molecular Weight Data for Polyoxazolinesa
calcd M
([mon]/[X])
w
/103 b
c
M
w
/103
PDId
initiator
2
+
16e
14
28
(1)
[Fe(bpyBr
macroligands
2
)
3
]
60 (100)
20 (100)
76 (125)
25 (125)
40 (200)
25 (125)
25 (125)
1.06
1.27
1.07
1.08
1.04
1.03
1.09
f
2
+
e
[Fe(bpyI )
2 3
]
f
macroligands
Ru(bpy)
bpyBr
22
22
[
2
(bpyI
2
)]2
+
g
2
46
41
bpyI
2
In a typical polymerization, the ligands (1) or the metalloini-
a
GPC MW characterization in CHCl
3
using RI and MALLS
tiators (2 and 3) were combined with 2-ethyloxazoline in CH3-
CN at 80 °C for 1 day (eq 1). Analysis of the resulting polymers
detectors. Values reported for M
w
and PDIs represent averages of at
b
least two runs. Calculated molecular weight based on monomer and
1
by H NMR indicated the disappearance of the halide initiator
initiator loadings. c [Monomer]/[halide initiator sites]. d PDI )polydis-
e
functionalities. Glassy polymer products were obtained which
retain the color of the respective initiators (UV-vis): 1, pale
yellow; 2, violet; 3, orange. MW data for the polymers were
determined by gel permeation chromatography (GPC) (Table
persity index ) M /M . Error: (0.02 to (0.06. Sample fragments
w
n
f
after injection. Macroligand arms are detected. Liberated from Fe star
polymer with K
quenched prior to completion.
2
CO
3
, in CH
3
CN/H
2
O, 80 °C, 14 h. g Reaction was
1
). The labile Fe star polymers fragment during GPC which
allows for independent characterization of the liberated mac-
14
roligand arms. (Polymer arms of Fe star: [monomer]/[initiator
site] ) 125; calcd MW ) 25k; found Mw ) 28k.) MW data
for the Fe polymer were also obtained using multiangle laser
light scattering (MALLS). (Fe star from 2c: [monomer]/
(2)
1
5
[initiator site] ) 125; calcd MW ) 76k; found Mw ) 77k.)
The kinetically inert Ru-centered polymer remains intact during
GPC analysis, and the chromophores are detected in the eluted
polymer. Low polydispersities (PDIs) were observed for all of
the polymers. However, since the MWs for metal-free polymers
prepared from 1 are approximately two times the anticipated
values, a polymer coupling reaction may be occurring after the
reaction is complete.16
to previously liberated bpy-centered polyoxazolines regenerates
2
+
the distinctive violet color of [Fe(bpy)3] (UV-vis). It was
also observed that films of the Fe polymer undergo thermal
bleaching when heated to ∼210 °C. This process is at least
partially reversible since the violet color returns upon cooling.
2+
Blends of [Fe(bpy)3] and linear polyoxazolines exhibit a
In addition to developing methodologies for growing poly-
mers from metalloinitiators, a second important goal is to
identify ways in which MCSPs might be disassembled. Such
similar, even sharper violet to clear transition which is also
reversible. This behavior is not observed for the [Fe(bpy)3]2
complex alone under the same conditions.
+
“
star burst” reactions which separate the metal from its
These results demonstrate the viability of the multifunctional
initiator strategy as a way into a variety of metal-centered
macromolecular structures. Extension of this concept to other
metal complexes and reactions promises to broaden the scope
of living polymerization methodologies and to yield new
materials with intriguing and useful properties.
macroligands are not only important for the characterization of
MCSPs but also allow for their chelation to different metal
centers. Metal complexation can protect sensitive functionality
during polymerization. And since these reactions may be
accompanied by dramatic changes in color or other physical
properties, they could be important for the application of MCSPs
as environmentally responsive materials. For the Fe star
polymer, disassembly was achieved under a variety of different
conditions (eq 2). Since the Fe(II) core polymers fragment
during GPC, these materials may be sensitive to shear. In
addition, aqueous basic and acidic reagents can also effect “star
burst” reactions. While the Fe(II) cores remain intact upon
Acknowledgment. We acknowledge the University of Virginia and
the National Science Foundation for support for this research. Special
thanks is extended to the Finn, Marshall, and Harman groups for helpful
discussions and for use of equipment and chemicals and to Dr. R. E.
Burnett who has facilitated this work in innumerable ways.
Supporting Information Available: Additional experimental de-
tails, UV-vis spectra of 2 and Fe polymer, GPC data vs PS standards,
GPC chromatogram, and Zimm plot of Fe polymer with reports (10
pages). See any current masthead page for ordering and Internet access
instructions.
10b,17
termination of the living chains with amines,
aqueous K2CO3 yields a colorless solution with concomitant
deposition of a rust-colored solid. Addition of (NH4)2Fe(SO4)2
reaction with
(
14) In some cases, a high MW shoulder is observed with a UV-vis trace
and a MW corresponding to the intact Fe star.
15) Microbatch MALLS MWs represent an average for the entire sample
and vary with both experimental technique and sample handling.
JA962834G
(
(17) (a) Kobayashi, S.; Masuda, E.; Shoda, S.-I.; Shimano, Y. Macro-
molecules 1989, 22, 2878-84. (b) Chujo, Y.; Ihara, E.; Ihara, H.; Saegusa,
T. Macromolecules 1989, 22, 2040-4.
+
(
16) Similar coupled products were obtained using [Zn(bpyX2)3]2 which
may fragment during polymerization.