10.1002/anie.201908458
Angewandte Chemie International Edition
COMMUNICATION
Cellulose-derived functional polyacetal by cationic ring-opening
polymerization of levoglucosenyl methyl ether
Tapas Debsharma, Yusuf Yagci, Helmut Schlaad*
Abstract: The unsaturated bicyclic acetal levoglucosenyl methyl
ether is readily obtained from sustainable feedstock (cellulose) and
polymerized by cationic ring-opening polymerization to produce a
semi-crystalline thermoplastic, unsaturated polyacetal with relatively
high apparent molar mass (up to ~36 kg mol-1) and decent dispersity
(~1.4). The double bonds along the chain are prone to hydrogenation
and thiol-ene chemistry as well as to crosslinking, making this
polyacetal potentially interesting as a reactive functional material.
polymerize through ring-opening olefin metathesis polymerization
(ROMP) to yield an amorphous thermoplastic polyacetal.[8]
Levoglucosenol should, on a first glance, also polymerize via the
acetal functionality by cationic ring-opening polymerization
(CROP). In fact, molecules with similar or related structures, i.e.,
anhydrosugars[9] and bicyclic ketals[10], have been polymerized
successfully via a Lewis acid-catalyzed CROP. Attempts to
polymerize levoglucosenol 2 via CROP, however, failed. We
therefore reasoned to mask the hydroxyl function of
levoglucosenol by methylation to yield the levoglucosenyl methyl
ether 3 (IUPAC name: 4-methoxy-6,8-dioxabicyclo[3.2.1]oct-2-
ene) (Scheme 1). CROP of 3 would then give the linear
unsaturated polyacetal 4 with the proposed chemical structure
shown in Scheme 1, which is potentially degradable[11] and could
be further modified or crosslinked.[12] It is worth being mentioned
that 3, like its precursor 2,[8] could also be polymerized via ROMP
(preliminary data, not shown).
The exploitation of fossil-based resources gave comfort and
wealth to society and brought the world closer, at the expense of
increasing atmospheric carbon dioxide concentration and other
environmental hazards. The rise in carbon dioxide concentration
increases carbohydrate concentration, therefore, reducing the
overall content of protein in plants.[1] Moreover, the plastic industry
is particularly dependent on fossil-based resources existing in a
limiting amount, and the produced non-degradable plastics create
many environmental problems. It is therefore inevitable to move
towards renewable feedstocks, valorization of biomass, and
environmentally degradable systems.[2] In this respect, bio-
sourced polymers have been of interest among the scientific
community to tackle the above-mentioned problems.[3]
Cellulose, being the most abundant products of biomass on
earth, is an attractive renewable, non-edible resource for the
production of many value-added chemicals such as sugars, lactic
acid, levulinic acid, or furans.[4] Another molecule with relatively
complex bicyclic structure that can be obtained by pyrolysis of
cellulose is levoglucosenone (1, Scheme 1).[5] Nowadays,
levoglucosenone is produced in industrial quantities (50 tons per
year) by the Circa Group Ltd., Australia, and the derivative
dihydrolevoglucosenone (Cyrene™) has been launched as an
environmentally friendly solvent to replace dipolar aprotic solvents
like N-methyl-2-pyrrolidone (NMP).[6] Levoglucosenone is used
for the synthesis of chiral therapeutic agents and molecules with
fixed and known stereochemistry,[7] however it has not yet entered
the field of polymers.
Scheme 1. Synthesis of levoglucosenyl methyl ether 3, starting from
levoglucosenone 1 via levoglucosenol 2, and polymerization via CROP to yield
the polyacetal 4
The overall synthetic procedure for the levoglucosenyl
methyl ether 3 is shown in Scheme 1. Levoglucosenone 1 is
reduced by sodium borohydride in water, and the resulting
levoglucosenol 2 is then deprotonated with sodium hydride and
methylated with methyl iodide to yield 3 (see the experimental
procedures in the Supporting Information). Purification of 3 was
achieved by distillation, and the overall yield was 81%. The
chemical structure of 3 was confirmed by nuclear magnetic
resonance (NMR) spectroscopy and electrospray ionization mass
spectrometry (ESI-MS) (see Supporting Information) to be
(1S,4S,5R)-4-methoxy-6,8-dioxabicyclo[3.2.1]oct-2-ene (major
isomer, 96%). Notably, the synthesis of 3 is far less complicated
and tedious than that of other sugar-based monomers for ROP.[13]
First attempts to polymerize 3 involved the use of triflic acid
(CF3SO3H, TfOH) and boron trifluoride etherate (BF3·OEt2).
Polymerizations were conducted in dichloromethane (DCM)
solution at room temperature or 0 °C for 24 h and were quenched
with triethylamine; results are summarized in Table 1. TfOH
appeared to be a very efficient initiator for the CROP of 3.
Monomer conversion (xp) reached >90% under the chosen
Indeed, the free radical or anionic polymerizations of 1 could
only produce oligomers at best.[8] Its alcohol derivative
levoglucosenol (2, Scheme 1), on the other hand, was found to
*
T. Debsharma, Prof. Dr. H. Schlaad
Institute of Chemistry, University of Potsdam
Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany
E-mail: schlaad@uni-potsdam.de
Prof. Y. Yagci
app
conditions, though a slightly higher molar mass polymer 4 (Mn
Department of Chemistry, Istanbul Technical University,
Maslak, 34469, Istanbul, Turkey
= 18.6 kg mol-1, Ð = 1.4; by size exclusion chromatography (SEC))
was obtained at lower temperature. The attempted
polymerizations of 1 and 2 with TfOH in DCM solution at room
temperature failed; either no reaction occurred or yet unidentified
organic compounds were produced.
Supporting information: All experimental details and procedures. NMR
and ESI-MS data of monomer 3. 1H, 13C, HSQC, COSY NMR spectra,
SEC traces, and TGA/DSC curves of polymer 4 samples. 1H NMR spectra
and SEC traces of polymers 5 and 6.
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