Polycondensation of butenediol: Synthesis of telechelic 2-butene-1,4-diol oligomers

Article abstract of DOI:10.1021/ja207465h

The catalytic condensation of cis-2-butene-1,4-diol with CpRu(MQA)(C ₃H₅) (Cp = cyclopentadienyl, MQA = 4-methoxyquinoline-2- carboxylate) generates poly(2-butenediol), an unsaturated telechelic polyether diol with molecular weights between 400 and 4600 g/mol. This Ru(IV) allyl catalyst enchains 2-butene-1,4-diol primarily as the linear trans-2-butenyl ether (92%) along with vinyl branches (8%). These telechelic oligomers are useful chain extenders and macromonomers, as demonstrated by their use in the synthesis of poly(lactide)-b-poly(butenediol)-b-poly(lactide) triblock copolymers. Model studies support a proposed mechanism involving the formation of Ru(IV) allyl intermediates from allylic alcohols and chain growth by selective nucleophilic displacement at the terminus of the Ru(IV) allyl to generate trans-2-butenyl ether linkages.

Full text of DOI:10.1021/ja207465h

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pubs.acs.org/JACS
Polycondensation of Butenediol: Synthesis of Telechelic
2-Butene-1,4-diol Oligomers
Matthew K. Kiesewetter, Justin A. Edward, Hyunuk Kim, and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S Supporting Information
b
ABSTRACT: The catalytic condensation of cis-2-butene-
1,4-diol with CpRu(MQA)(C3H5) (Cp = cyclopentadienyl,
MQA = 4-methoxyquinoline-2-carboxylate) generates poly-
(2-butenediol), an unsaturated telechelic polyether diol
with molecular weights between 400 and 4600 g/mol. This
Ru(IV) allyl catalyst enchains 2-butene-1,4-diol primarily as
the linear trans-2-butenyl ether (92%) along with vinyl
branches (8%). These telechelic oligomers are useful chain
extenders and macromonomers, as demonstrated by their
use in the synthesis of poly(lactide)-b-poly(butenediol)-
b-poly(lactide) triblock copolymers. Model studies support
a proposed mechanism involving the formation of Ru(IV)
Figure 1. Ring-opening polymerization of furans and step-growth
polymerization of butene-1,4-diol.
allyl intermediates from allylic alcohols and chain growth by
selective nucleophilic displacement at the terminus of the
Ru(IV) allyl to generate trans-2-butenyl ether linkages.
suggested a novel strategy for generating polyethers by the
CpRu(IV) allyl-catalyzed condensation of 2-butene-1,4-diol
(Figure 1).
A model study was first carried out by reacting the Ru(IV) allyl
complex 1 with a stoichiometric amount of cis-2-butene-1,4-diol
at room temperature in acetone-d6 (eq 1). After 72 h at room
temperature, 18% of the butenediol was converted to the allylic
ether cis-4-(allyloxy)but-2-en-1-ol, indicating that butenediol is a
competent nucleophile for displacing the allylic fragment from
Ru(IV) complex 1. In the presence of 5 mol % 1, cis-2-pentenol
was converted (53% conversion) in CD3OH after 24 h to an 80:20
mixture of the linear trans-1-methoxy-2-pentene and branched
3-methoxy-1-pentene (7% yield for the branched product and
46% yield for the linear product; eq 2). These results indicate that
allylic alcohols can function both as nucleophiles and electro-
philes in allylic substitution reactions21,22 mediated by Ru(IV)
allyl complex 1.
elechelic polymers are valuable building blocks for the con-
T
struction of an array of polymeric materials that are useful as
adhesives and biomedical and elastomeric materials.1À7 Polytetra-
hydrofuran (PTHF) (Figure 1) is a commodity telechelic polymer
of which ∼200 000 tons are produced annually.8 In contrast, the
corresponding unsaturated analogue poly(1,2-dihydrofuran) is
unknown. This is a potentially attractive material, as unsaturated
telechelic polymers such as hydroxytelechelic butadiene (HTPB)9
are widely used commercially as components for elastomers
and as binders for solid rocket fuels.10À12 However, poly(1,2-
dihydrofuran) cannot be readily produced from 1,2-dihydrofuran
because of side reactions that occur under the conditions typically
used in the Lewis acid-mediated ring-opening polymerization
(ROP) of THF (Figure 1).13,14 Herein we report a strategy for
obtaining unsaturated telechelic oligoethers by a different route
involving the Ru-catalyzed step-growth polymerization of 2-butene-
1,4-diol.
Our approach was inspired by Kitamura’s pioneering studies15À17
on the formation of allyl ethers and our own investigations of the
catalytic behavior of Ru allyl complex 1,18 a modified version of a
Ru(IV) (de)allylation catalyst first reported by Kitamura.15À17
Complex 1 catalyzes the hydrolysis or alcoholysis of allyl
carbonates in either aqueous or alcohol solution.19 Kinetic and
mechanistic studies indicated that the Ru(IV) allyl complex can
be generated from allyl alcohols and that the corresponding
Ru(IV) allyl species are readily attacked by alcohols to generate
allyl ethers. While these catalysts are known to catalyze the
hydrolysis of allyl ethers,20 our kinetic studies implied that allyl
ether hydrolysis is slower than allyl ether formation. This reactivity
The catalytic oligomerization of cis-2-butene-1,4-diol (BD)
was carried out by treating a 50/50 acetone/cis-2-butene-1,4-diol
solution with Ru complex 1 (0.10 mol % relative to diol) at
room temperature (Table 1, entry 1). Analysis of aliquots by
Received: August 8, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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Table 1. Catalytic Oligomerization of cis-2-Butene-1,4-diol
entry
[BD]0 (M)
[Ru] (mM)
solvent
time (h)
temp
yield
Mn (Mw/Mn)a
1
6.1
7.2
12
5
acetone
72
22
48
72
25 °C
55 °C
reflux
reflux
1.71 g, 80%
0.183 g, 65%
2.99 g, 70%
1.90 g, 57%
415 (2.2)
499 (1.4)
2300 (2.6)
5970 (2.2)
2
1.24
8.1
acetone
3
4b
CH2Cl2/Et2O
CH2Cl2/Et2O
8.1
5
a Determined by GPC vs polystyrene standards. The units of Mn are g/mol. b Reaction performed as in entry 3 followed by 24 h under high vacuum.
Figure 2. 1H NMR spectra of (top) cis-2-butene-1,4-diol and (bottom)
Figure 3. Chain extension of 2 with L-lactide to yield 3. Asterisks
indicate PLA end-group resonances.
poly(2-butene-1,4-diol). Each inset above the bottom spectrum is a 10Â
enlargement of the section of the spectrum below that inset.
sieves,26 the molecular weight of the polymer was 2300 g/mol as
gel-permeation chromatography (GPC) revealed an increase in
molecular weight with time. Under these conditions, a molecular
weight plateau (Mn = 700 g/mol) was reached at 48 h (as deter-
mined by GPC), which corresponds to 80% conversion from
determined by GPC. Continuing the reaction for an additional
24 h under high vacuum further increased the molecular weight
to 5970 g/mol (Table 1, entry 4), and linear enchainment
of butene-diol predominated (linear/branched = 93/7). These
results suggest that higher-molecular-weight telechelics can be
generated by removing the water byproduct of the condensation
reaction.
1
monomer (as determined by H NMR). Evaporation of the
reaction solvent yielded a mixture of products that was shown
by electrospray ionization mass spectrometry (ESI-MS) to be
composed of a mixture of cyclic [ESI-MS: (C4H6O)n, n = 3À10;
for (C4H6O)3 + H+, m/z 210.82 vs 211.12 g/mol) and linear
polymer chains spaced by the mass of a butene ether repeat unit
(Figure S2 in the Supporting Information). Dissolution of the
crude material in CH2Cl2 followed by treatment with hexanes
and drying yielded poly(2-butene-1,4-diol) [poly(BD), 2] [GPC:
Mn = 770 g/mol, Mw/Mn = 2.97. ESI: HO(C4H6O)nH, n = 5À27;
for HO(C4H6O)16H + Na+, m/z 1161.59 vs 1161.64 g/mol]. The
concentrated supernatant contained primarily cyclic oligomers, as
determined by ESI-MS (Figure S4). These cycles are the unsatu-
rated versions of the 5n-crown-n series of crown ethers.23À25
To characterize the structure of poly(BD) 2, the crude sample
isolated by precipitation (Mn = 415 g/mol, Mw/Mn = 2.2)
was purified and fractionated by dialysis in CH2Cl2/MeOH to
afford a higher-molecular-weight fraction (Mn = 5000 g/mol,
Mw/Mn = 1.3) that could be analyzed by NMR spectroscopy
(Figure 2 and Figure S3). Analysis of the 1H NMR (Figure 2) and
13C NMR (Figures S5 and S6) spectra of 2 revealed that the
butenediol is enchained predominantly as linear trans-2-butenyl
ether (94%) and 2-vinyl ethyl ether (6%) repeating units (linear/
branched ratio = 94/6).
Analysis of the 13C NMR spectra of low-molecular-weight
samples provided clear evidence for allylic alcohol end groups
(Figure S6). The presence of alcohol end groups on both ends of
oligomers 2 was confirmed by a chain-extension experiment in
which oligomer 2 was used as a macromolecular initiator for ROP
of L-lactide (L-LA). ROP of L-LA (0.417 g; 2.89 mmol) initiated
with a sample of 2 (202 mg, 0.035 mmol; Mn = 6000 g/mol, Mw/
Mn = 2.2) in the presence of 1,5,7-triazabicyclo[4.4.0]dodec-1-
ene (TBD) (0.004 mmol, 2.9 mM)27,28 in CH2Cl2 afforded the
poly(LA)-b-poly(butenediol)-b-poly(LA) triblock copolymer 3
(Mn = 12 700 g/mol, Mw/Mn = 1.97).4 The 1H NMR spectrum
of 3 exhibited a quartet at 4.4 ppm consistent with the assignment
1
1
(assisted by HÀ H COSY) as poly(LA) (PLA) methine end
groups (* in Figure 3). Integration of this end-group signal versus
the sum of the signals for the internal olefinic Hs of the enchained
butenediol yielded 1.99 end groups/chain and Mn = 6000 g/mol
for the butenediol block, in close agreement with that measured
for 2. Each of the PLA blocks had Mn = 5500 g/mol, giving an
overall Mn of 17 000 g/mol, in good agreement with that
measured by GPC for 3 (Mn = 12 700 g/mol). Integrations of
the resonances of the PLA versus the resonances due to the butenediol
gave a poly(BD):PLA ratio of 1.13:1.00, consistent with a poly(LA)-
b-poly(butenediol)-b-poly(LA) triblock copolymer.4
Small-scale screening experiments revealed that for oligomer-
izations carried out in acetone solvent at 25 or 55 °C, the mole-
cular weights (300À500 g/mol) and yields (60À80%) were
modest. However, when a solution of cis-2-butene-1,4-diol
(3.9 mL, 0.047 mol) and 1 (14.8 mg, 0.025 mmol) in a mixture
of ethyl ether (1.9 mL) and CH2Cl2 (3.8 mL) was refluxed for
48 h beneath a condenser column packed with 4 Å molecular
The internal butenediol block can be readily degraded by
catalytic olefin metathesis.29,30 Treatment of 3 (102 mg,
0.0080 mmol) with 1-hexene and the Grubbs-II catalyst
(1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro-
(phenylmethylene)(tricyclohexylphosphine)ruthenium (2 mg,
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used as macromonomers for the synthesis of triblock copolymers
and should also prove useful as cross-linkable chain extenders in
polyurethanes and polyetheresters.
’ ASSOCIATED CONTENT
Supporting Information. Experimental details; 1H, 13C,
S
b
1
1
and HÀ H COSY NMR spectra; and ESI-MS and differntial
scanning calorimetry traces. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Figure 4. Polymerization mechanism (PF6 anions have been omitted for
clarity).
Corresponding Author
waymouth@stanford.edu
0.0024 mmol) in anhydrous 1:1 hexene/benzene (4 mL) for 12 h
afforded PLA with Mn = 6700 g/mol, and H NMR analysis
1
’ ACKNOWLEDGMENT
We acknowledge support from the NSF (GOALI CHE-0645891
and NSF-CHE-0910729). We thank Purac for a generous donation
of lactide. This work was also supported by the NRF of the Korean
Ministry of Education, Science and Technology (NRF-2010-357-
C00059 to H.K.).
indicated the absence of poly(butenediol). Taken together, these
results suggest that the poly(butenediol)s generated from con-
densation of cis-2-butene-1,4-diol are oligomeric telechelic diols
that can function as macroinitiators for the generation of triblock
copolymers.4
Shown in Figure 4 is one proposed mechanism for the
oligomerization. On the basis of previous studies15,18,31 and
model reactions (eqs 1 and 2), we propose that the Ru(IV) allyl
precursor 1 reacts with cis-2-butene-1,4-diol with the elimination
of cis-4-(allyloxy)but-2-en-1-ol to generate a solvated Ru(II)
species. This Ru(II) complex can react with either the monomer
or an allylic alcohol chain end to generate Ru(IV) allyl 4 with
concomitant elimination of water. Nucleophilic displacement of
the allyl ligand of 4 by either monomer or ROH chain ends
results in chain extension.18,31À33 Attempts to isolate or char-
acterize intermediate 4 have proven unsuccessful, but we propose
that the endo-syn Ru allyl species 4s is formed from Ru(II) and
cis-2-butene-1,4-diol. As we observed only trans-butenyl ethers
in the polymer chain, we propose that syn-anti isomerization of
the Ru(IV) allyl21,22 is competitive with chain growth. This was
corroborated by model studies in which an aqueous solution
of cis-2-butene-1,4-diol was observed to isomerize to trans-
2-butene-1,4-diol in the presence of 1.5 mol % 1.
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