Hyperbranched macromolecules via olefin metathesis

Article abstract of DOI:10.1021/ja0759040

A facile route to hyperbranched polymers via acyclic diene metathesis is reported. Any molecule functionalized with two or more acrylate groups and one terminal olefin can serve as an AB_n monomer when exposed to animidazolinylidene-based ruthen

Full text of DOI:10.1021/ja0759040

Published on Web 09/29/2007
Hyperbranched Macromolecules via Olefin Metathesis
Irina A. Gorodetskaya, Tae-Lim Choi,§ and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
Received August 6, 2007; E-mail: rhg@caltech.edu
Chart 2. Monomers for Hyperbranched ADMET Polymerization
Hyperbranched polymers are highly branched macromolecules
typically prepared via a one-pot polymerization of ABng2 mono-
mers.1 The A and B functionalities of these monomers readily react
with each other, but neither one reacts with itself. The unique
macromolecular architecture of dendritic polymers gives rise to
attractive features such as multiple end groups, improved solubility,
and lower solution viscosity (compared to those of linear analogues
of the same molecular weight).1 Dendrimerssstructurally perfect
hyperbranched macromolecules2shave already found applications
in medicine,2,3 catalysis,2,4 and nanofabrication.2,5 Unfortunately,
dendrimer synthesis can be rather labor-intensive and, thus,
expensive.2 As such, the preparation and study of hyperbranched
polymers, which typically exhibit properties similar to those of
monodisperse dendrimers,6 have been extensively pursued in recent
years.1 However, many of the current methods for the synthesis of
hyperbranched polymers also have significant drawbacks, including
the need for complex monomers and harsh reaction conditions.
Herein, we report a simple method for the preparation of hyper-
branched polymers via acyclic diene metathesis polymerization
(ADMET)7 of easily synthesized monomers under mild conditions.
Scheme 1. Hyperbranched ADMET Polymerization
Chart 1. Acyclic Diene Metathesis Polymerization Catalyst
The polymerization of each monomer is easily monitored by 1H
NMR spectroscopy.9a,10 For example, Figure 1 shows the 1H NMR
spectra of 4 and the resulting crude polymer 4a. Some peak
broadening due to formation of macromolecules can be observed
in the spectrum of 4a, especially for the backbone proton b. It can
also be seen that the terminal olefins (a) completely disappear during
the polymerization. Moreover, as expected, a new peak (g), a
doublet of triplets, appears at 6.95 ppm due to formation of internal
acrylates (AB olefins). An integration ratio of 1:1 is observed
between g and the remaining unreacted acrylate peak c in the
spectrum of 4a, in accord with the fact that there are twice as many
B groups as A groups in an AB2 monomer.11
Catalyst 1 (Chart 1) was selected for ADMET hyperbranched
polymerization. This imidazolinylidene-based catalyst is tolerant
to many functional groups, stable to air and moisture, and readily
promotes cross metathesis between electron-rich primary olefins.
Furthermore, it can catalyze the cross metathesis involving low
metathesis-reactive olefins, such as electron-deficient alkenes. When
treated with 1, electron-poor olefins do not homodimerize (or do
so very slowly) but do participate in a secondary metathesis reaction
with homodimers of more reactive olefins.8 Therefore, any molecule
functionalized with one electron-rich olefin, such as a terminal
alkene, and two or more electron-poor olefins, such as acrylates,
is an ABn-type monomer (Chart 2) that can be polymerized into a
hyperbranched structure using catalyst 1 (Scheme 1).9
Monomers 2-7 (Chart 2) were utilized for the ADMET
hyperbranched polymerization. They were prepared in one to four
steps from commercially available starting materials such as
glycerol, Pentaerythritol, and 5-hydroxyisophthalic acid. 2 and 3
were prepared as linear analogues to AB2 monomers 4 and 5, as
well as the AB3 monomer 7. To further demonstrate the inherent
flexibility of the presented method, monomer 6 was also synthesized
to make a hyperbranched polymer with a different backbone.
§ Current address: Electronic Chemical Materials R & D Center, Cheil Industries
Inc., Korea.
Figure 1. 1H NMR spectra of monomer 4 and hyperbranched polymer 4a.
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J. AM. CHEM. SOC. 2007, 129, 12672-12673
10.1021/ja0759040 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Results of Polymerizations of 2-7a
of 0.45 (Table 1), closely followed by an R of 0.41 for 3a. This
observation confirms that 3a is a linear polymer despite its low
viscosity. It also validates that polymers 4a-7a are not simply
linear, alternating AB comb-shaped polymers. Branched AB2-based
polymers 4a, 5a, and 6a all have R parameters indicative of a
spherical shape in solution. Moreover, polymer 7a yielded the
lowest R value, which is in agreement with the AB3-based polymer
having the lowest intrinsic viscosity and, thus, the most branching.
Overall, the R values found strongly suggest a spherical shape in
solution and, therefore, hyperbranching for polymers 4a-7a.
In summary, we have demonstrated that olefin metathesis can
be used to prepare hyperbranched polymers with a variety of
backbones. The monomer synthesis is simple, and the polymeri-
zation is conducted under very mild conditions. It is easy to envision
that functionalization of these hyperbranched polymers could lead
to a number of applications. Therefore, as an extension of this work,
we are currently investigating functionalization of the peripheral
groups (acrylates) of the hyperbranched polymers presented here.
1
polymer
M
w (kDa)
M
n (kDa)
PDI
R × 10-
2a
3a
4a
5a
6a
7a
4.31
21.43
3.61
14.77
10.24
30.90
2.07
4.44
0.55
3.08
3.17
5.00
2.1
4.8
6.5
4.8
3.2
6.2
4.45 ( 0.01
4.12 ( 0.02
3.82 ( 0.02
3.24 ( 0.02
3.34 ( 0.03
2.69 ( 0.02
a Polymerization conditions: 0.5 mol % of 1 was used and the
polymerizations were conducted in near-refluxing methylene chloride (45
°C) with venting. Mw, Mn, and PDI were calculated from triple-angle laser
light-scattering and refractive index measurements. R was measured with
the help of an on-line differential viscometer.
Acknowledgment. We gratefully acknowledge Materia, Inc. for
the generous gift of catalyst 1. We also thank Wyatt Technology
Corp., especially Dr. Michelle Chen and Dr. Jeffrey Ahlgren for
help with the SEC analysis and instrumentation. This research was
supported by the office of Naval Research (ONR) through the
Multidisciplinary University Research Initiative (MURI) program
and NSF.
Figure 2. Mark-Houwink-Sakurada plots for polymers 2a-7a.
A multiangle light-scattering (MALS) detector combined with
a differential refractometer and an on-line viscometer (all from
Wyatt Technology Corporation) following size exclusion chroma-
tography (SEC) was used to determine the molecular weights and
PDIs of the obtained polymers. Additionally, viscometer data helped
to characterize branching of the macromolecules resulting from
ADMET of 2-7. Table 1 summarizes the typical crude polymer-
ization results. The PDI values observed are quite high, which is
not surprising for a hyperbranched step-type polymerization.12
Figure 2 compares the plots of intrinsic viscosity (IV) vs
molecular weight (Mark-Houwink-Sakurada plots) for polymers
2a-7a. As expected, the IV of branched polymers 4a-7a is much
lower than that of the linear polymer 2a for any given molecular
weight. Interestingly, the supposedly linear polymer 3a has a
drastically reduced intrinsic viscosity compared to that of 2a,
although not quite as low as the viscosities of branched polymers.
We attribute this property of 3a to the presence of a methoxy-
methyl pendant group in each monomer unit. This group is inert
during the polymerization, but its length is comparable to the
monomer’s overall size. Such an architecture results in a “comb”-
type polymer with a lower than expected IV (relative to that of a
linear analogue).13 Across the molecular weight range studied, the
viscosity of polymer 7a, based on an AB3 monomer 7, is even lower
than that of AB2 polymers 4a-6a. This observation indicates even
more branching in the AB3-based polymer. On the other hand, the
intrinsic viscosity does not change dramatically with slight varia-
tions in the backbone; it can be seen from Figure 2 that the Mark-
Houwink plots for 4a-6a completely overlap.
Supporting Information Available: Experimental procedures with
full characterizations for the synthesis of 2-7 and the polymerization
procedure. This material is available free of charge via the Internet at
http://pubs.acs.org.
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To extend our analysis, we compared the Mark-Houwink shape
parameter R ([η] ) KMR) for polymers 2a-7a (Table 1). An R
parameter of 0.5-1.0 is typical for randomly coiled linear
polymers.14 Polymers with a rigid-rod shape have an R of 2.0, and
spherically shaped macromolecules are expected to have an R <
0.5.14 The linear polymer 2a was found to have the highest R value
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