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Chemistry Letters Vol.37, No.2 (2008)
A Novel Biomass-based Polymer Prepared from Lignin-derived Stable Metabolic Intermediate
by Copper(I)-catalyzed Azide–Alkyne Click Reaction
Tsuyoshi Michinobu,Ã1;2 Yasunori Inazawa,1 Kenta Hiraki,1 Yoshihiro Katayama,3 Eiji Masai,4
Masaya Nakamura,5 Seiji Ohara,5 and Kiyotaka ShigeharaÃ1;2
1Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
2Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
3Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588
4Department of Bioengineering, Nagaoka University of Technology, Nagaoka 940-2188
5Forestry and Forest Products Research Institute, Tsukuba 305-8687
(Received November 8, 2007; CL-071241; E-mail: tmichi@cc.tuat.ac.jp, jun@cc.tuat.ac.jp)
The CuI-catalyzed, but ligand-free azide–alkyne cycloaddi-
tion reaction was applied to the polymerization of the newly syn-
thesized diethynyl-functionalized 2-pyrone monomer originally
isolated from lignin via metabolic pathway. The polymerization
was monitored by GPC and IR spectra and the resulting high
molecular weight polymer was unambiguously characterized
by 1H NMR, thermal analysis, and optical absorption and emis-
sion spectra.
tive of ‘‘click chemistry’’5 and that there are some reports of
the ligand (base)-free click reactions.6 In this paper, we for the
first time report the successful synthesis and characterization
of the soluble, but high molecular weight polymer of the lig-
nin-derived metabolic substance by the ligand-free click reaction
of the diethynyl-functionalized 2-pyrone monomer and diazide
comonomer.
To transform the carboxylate functions of PDC into the
ethynyl terminals by esterification, the common condensing
agents such as dicyclohexylcarbodiimide (DCC) and 4-dimeth-
ylaminopyridine (DMAP), instead of the direct condensation
upon heating, were employed to give 1 in a moderate yield of
72% (Scheme 1). The di-2-propynyl PDC 1 is stable in the neu-
tral pH, but the ethynyl moieties and the pyrone ring are unstable
in the acidic and basic pH region, respectively. We also tried to
prepare diazide-functionalized PDC monomer for the subse-
quent click polymerization with various diethynyl molecules,
but the esterification of PDC carboxylates with a large excess of
ethylene glycol followed by reaction with sodium azide gave an
unidentified product probably caused by the intramolecular side
reaction between the pyrone ring and azide moieties.
Utilization of biomass in industrial polymer products has at-
tracted considerable attention because the society requires the
change from nonrenewable carbon resources to renewable biore-
sources.1 Lignin is one of the most abundant natural carbon re-
sources, which exists in trees at 15–36% by weight. However,
an advanced system of utilizing it as biomass has not been estab-
lished because of the difficulties in transforming the highly
networked structure of lignin into well-defined versatile small
molecular substances by conventional synthetic methods. There-
fore, we and others have investigated the metabolic conversion
of lignin into stable and functional intermediates, and we recent-
ly succeeded in producing pure 2-pyrone-4,6-dicarboxylic acid
(PDC) on a large scale from lignin via protocatechuate by trans-
formed bacterium.2 PDC was found to possess several attractive
properties such as unique solvatochromic emission, facile subli-
mation under laboratory conditions, and strong affinity to metal
ions.3 Furthermore, PDC consists of the polar pseudo-aromatic
ring system and two carboxylic acids, which can serve as a
bifunctional monomer for polycondensation and polyaddition.
Therefore, we decided to introduce the pseudo-aromatic ring in-
to polymer main-chains for the improvement of mechanical
strength and heat-resistance properties as well as the optical
properties of the bio-based polymers. The serious synthetic prob-
lem was that PDC is unstable under basic conditions, as expected
from the chemical structure, and accordingly base-free reactions
must be employed.
The click polymerization of 1 was carried out in deoxygen-
ated DMF in the presence of CuBr at 20 ꢀC with an equimolar
amount of the diazide comonomer, 1,2-bis(2-azidoethoxy)-
ethane, prepared by the reaction of triethylene glycol with so-
dium azide. Because of the absence of ligand, the polymerization
rate was so slow that the small molecular weight fractions were
observed in the GPC profile even after 15 h (Figure 1a, left).
In the IR spectrum, the azide peak at 2109 cmÀ1 still remained,
O
O
COOH
OH
DCC, DMAP
O
O
O
O
O
COOH
O
(1), 72%
PDC
N
Among many base-free reactions, we recently reported the
direct dehydrated polycondensation of various diol monomers
and PDC.4 The straightforward reaction and the resulting ester
linkage in the main chain are advantageous especially for appli-
cations to biodegradable materials, but the molecular weights of
the resulting polyesters were unfortunately low in this condensa-
tion reaction. We therefore pursued another base-free reaction
and noted that the CuI-catalyzed azide–alkyne cycloaddition re-
action is a remarkably efficient process known as a representa-
N
O
N3
O
O
N3
O
N
O
n
N
N
CuBr, DMF
N
O
O
O
O
O
(P1)
Scheme 1. Synthesis of the linear polymer of PDC by CuI-cat-
alyzed azide–alkyne click reaction. DCC, dicyclohexylcarbodii-
mide; DMAP, 4-dimethylaminopyridine.
Copyright Ó 2008 The Chemical Society of Japan
Michinobu, Tsuyoshi
