In situ hydrogenation of terminal halogen in poly(methyl methacrylate) by ruthenium-catalyzed living radical polymerization: Direct transformation of "polymerization catalyst" into "hydrogenation catalyst"

Article abstract of DOI:10.1021/ja0641507

An in situ, selective, and quantitative hydrogenation of the terminal chlorine (α-haloester) in living PMMA-Cl into PMMA-H was achieved via direct transformation of a "polymerization catalyst" into a "hydrogenation catalyst" in the Ru(II)-catalyzed living radical polymerization, where the polymerization mixture of MMA was directly treated in situ with K₂CO₃ as a base and 2-propanol as a hydrogen donor. The reaction terminated the polymerization and, more importantly, the terminal chlorine was quantitatively hydrogenated, as confirmed by SEC, ¹H NMR, and MALDI-TOF MS. Copyright

Full text of DOI:10.1021/ja0641507

Published on Web 08/09/2006
In Situ Hydrogenation of Terminal Halogen in Poly(methyl methacrylate) by
Ruthenium-Catalyzed Living Radical Polymerization: Direct Transformation of
“Polymerization Catalyst” into “Hydrogenation Catalyst”
Takaya Terashima, Makoto Ouchi, Tsuyoshi Ando, and Mitsuo Sawamoto*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Received June 22, 2006; E-mail: sawamoto@star.polym.kyoto-u.ac.jp.
The so-called “tandem catalysis”1 is an intriguing sequential or
concurrent catalysis where a single catalyst mediates at least two
Scheme 1. In Situ Hydrogenation of Terminal Halogen of Living
PMMA-Cl with Ru Catalysts
reactions in one-pot, requiring neither recovery of an intermediate
of the first step nor an additional use of another catalyst for the
subsequent reaction. Especially, in situ direct transformation of a
catalyst into another by simple ligand modification is attractive,
because sequential multistep reactions are thereby possible with a
single catalyst in a single vessel. Certain ruthenium alkylidene com-
plexes2 can in fact catalyze in tandem such reactions as olefin meta-
thesis and hydrogenation3 or olefin metathesis and isomerization.4
Living radical polymerization5 is another class of metal-catalyzed
reactions of significance, for which a variety of organometallic
catalysts, such as ruthenium, iron, copper, and nickel, have been
evolving (Scheme 1, upper line). Precision control of propagation
has now led to end-functionalized, block, star, and many other
designed polymers and materials.
A remaining subject for refinement thereof is the fact that the
products by definition carry a halogen terminal, because the catalysis
involves a reversible and homolytic cleavage of a carbon-halogen
bond in an initiator or a “dormant” polymer terminal originating
therefrom. While a requisite for fine reaction control, the terminal
halogen is chemically reactive and thermally unstable, possibly
resulting in undesirable reactions upon subsequent use of these poly-
mers, and should thus be removed or transformed.6 Few processes
are by now available, however, such as hydrogenation with possible
hazardous tin reagents and end-capping with expensive and exotic
silyl enolates, none of which seems to be practically versatile.7
Figure 1. In situ hydrogenation of the terminal halogen of PMMA-Cl with
Given these backgrounds, this communication reports a novel
Ru catalysts: polymerization, [MMA]0/[1]0/[2]0/[n-Bu3N]0 ) 2000/20/10/
tandem Ru(II) catalysis for selective hydrogenation of the terminal
halogen in poly(methyl methacrylate) (PMMA) obtained in the
ruthenium-catalyzed living radical polymerization, via in-situ
transformation of a “polymerization catalyst” into a “hydrogenation
catalyst” (Scheme 1). To our knowledge, this is the first example
of halogen-hydrogenation in R-haloesters and their polymer ana-
logues with ruthenium complexes.
In Situ Hydrogenation via Tandem Ru(II) Catalysis. MMA
was polymerized with a ruthenium complex [RuCl2(PPh3)3 (2)]8
as a catalyst in conjunction with a chloride initiator (1: ethyl-2-
chloro-2-phenylacetate)9 and an amine additive (n-Bu3N)10 in
toluene at 80 °C (Figure 1). Monomer conversion reached 49% in
5 h to give PMMA of a controlled molecular weight and a narrow
molecular weight distribution (MWD) [Mn ) 5000; Mw/Mn ) 1.37;
by size-exclusion chromatography (SEC); Figure 1a].
40 mM in toluene at 80 °C; treatment, [3]add ) 125 mM in 4/toluene;
polymer solution/added solution ) 1/4 v/v; final conditions, [MMA]/[1]/
[2]/[n-Bu3N]/[3] ) 400/4/2/8/100 mM in 4/toluene (1/1, v/v).
catalyst (Supporting Information, Figure 1; see the discussion
below). The polymer, recovered after an additional stirring for 25
h, had a molecular weight and an MWD (Mn ) 5400, Mw/Mn )
1.45, Figure 1c) virtually the same as those of a control sample (a)
isolated before the K2CO3 treatment. Without such quenching, the
polymerization proceeded undisturbed to a 85% conversion in 30
h, to give a living PMMA with higher molecular weights (Mn )
8200, Mw/Mn ) 1.29, Figure 1b). In separate runs with addition 3
or 4 alone, the polymerization was not disturbed (Figure 1; see
also Support Information, Figure 2 and Table 1), indicating that
the combination of 3 and 4 is mandatory.
Direct 1H NMR analysis of the polymerization mixture after the
treatment with a 3/4 pair also showed the in-situ generation of
hydrogenated MMA (5) from the remaining MMA (44% in 5 h
and 47% in 25 h, with n-octane as an internal standard). Presumably,
such a fast and concurrent hydrogenation of unreacted MMA is
one of the essential factors for the efficient quenching of the
polymerization.
At this point, the polymerization solution (unquenched) was
added into a K2CO3 (3) solution in a 2-propanol (4)/toluene
mixture11 at 80 °C under an inert gas atmosphere. The originally
red-brown solution immediately turned red-purple and then yellow
upon continuous stirring for ca. 5 min. The color change indicates
the formation of a Ru(II) hydride(s), known as a hydrogenation
9
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C O M M U N I C A T I O N S
2C: the same sample for Figure 2A), consistent with the chlorine-
capped PMMA to be formed in the living polymerization.6
Accordingly, the mass difference between spectra C and D was
34.7, close to the mass difference (34.5) between chlorine and
hydrogen.
Reaction Mechanism. The mechanism of the in situ hydrogena-
1
tion13 was further investigated by H NMR model reactions. For
example, treatment of the Ru dichloride complex (2) with 3 in
4/toluene gave signals at -7 and -10 ppm characteristic of the
hydride RuH2(PPh3)3 and also at -18 ppm of RuHCl(PPh3)3.
Simultaneously, the signal due to acetone via reduction of 4 was
also observed. When deuterated 2-propanol (d8) was used instead
of 4 in the polymer treatment, the signal intensity of the ω-end
methine proton decreased, which suggests 2-propanol is the
hydrogen source (Supporting Information, Figure 4). From these
results, the proposed mechanism is as follows: the Ru hydride
complexes are first generated directly from the polymerization
catalyst 2 by the K2CO3-mediated hydrogen transfer from 2-pro-
panol (solvent), and these hydride complexes hydrogenate the
terminal chlorine of dormant PMMA-Cl and MMA. The details
are now under investigation.
In conclusion, a selective, quantitative, and in situ hydrogenation
of the chlorine terminals in PMMA-Cl was achieved in the Ru-
catalyzed living radical polymerization, via direct transformation
of a polymerization catalyst into a hydrogenation catalyst with
K2CO3 and 2-propanol.
Figure 2. 1H NMR (A and B in CD2Cl2 at 25 °C) and MALDI-TOF-MS
(C and D) spectra of PMMA-Cl and PMMA-H. The asterisks (*) indicate
satellite peaks. The two samples (A/C and B/D) were fractionated to remove
the catalyst residues; see main text and Supporting Information for molecular
weight data.
Acknowledgment. This work was supported by the Japan
Society for the Promotion of Sciences for Young Scientists.
Supporting Information Available: Experimental details for the
polymerization, hydrogenation, the characterization data, and 1H NMR
and spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
Polymer Characterization. The terminal structure of the
polymers was analyzed by 1H NMR (Figure 2A,B) after purification.6a
The control sample (a), without the 3/4 treatment, exhibited the
characteristic signals of a PMMA main chain and the terminal
MMA unit adjacent to the ω-end C-Cl bond, methyl (a′: 1.6 ppm),
methylene (b′: 2.4 ppm), and methoxy (c′: 3.7 ppm), along with
the methylene (d: 4.0 ppm) and methine (e: 3.3 ppm) protons
derived from the initiator, indicating the formation of a chlorine-
capped PMMA-Cl (Figure 2A).
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In contrast, PMMA obtained with the 3/4 mixture was completely
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The in situ, selective, and quantitative hydrogenation was also
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Figure 2B). The end-capped sample gave only a single major series
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