(46b) Ring-Opening Polymerization for 100% Renewables-Based Polyethylene Furanoate (PEF) for the "Green Bottle"

Along the shift of our societies from fossil-fuel based economies towards
more sustainable civilizations, ring-opening polymerization (ROP) can be an
alternative process to synthesize a novel class of renewable resource-based
polymers. Furanic polyesters like polyethylene furanoate (PEF) have been ranked among the top potential
chemicals by the US-DOE and may replace one of the world’s dominant
fossil-based plastics, polyethylene terephthalate (PET). Recent efforts and
first commercial scale production by Dutch company Avantium
and other research groups were invariably based on polycondensation,
which is burdened with the necessary removal of condensation byproducts to
reach high conversions and molecular weights, and thus typically requires
reaction times in the order of days [1,2]. Cyclic monomers do not have endgroups and thus render byproduct removal unnecessary,
which enables ROP as a living chain-growth mechanism to deliver high conversion
to PEF within minutes. We present ROP as an alternative process to reach sufficiently
high molecular weight PEF for commercial applications such as bottles, textiles,
medical grafts, etc. [3,5].

Cyclic PEF monomers (cyOEF) can be derived from
100% renewables-based building blocks furandicarboxylic
acid and ethylene glycol via different synthesis routes that have successfully
been applied to PET, which all exploit the shift of the ring-chain-equilibrium
to cyclics in dilution [4]. Synthesis pathways such
as depolymerizationof short PEF oligomers
in 2-Methylnaphthalene have yielded >80% cyOEF with a ring-size distribution
from dimer to heptamer within 4 hours, as part of
other projects in our group. Maximization of material turnover as well as
minimization of waste is possible by full recycling of unconverted linears and solvents [5]. Purification of cyOEF from
residual linears via silica gel adsorption to yield >99%
cycles is essential for ROP to deliver 1) sufficiently high molecular weights,
2) reproducible reaction control and 3) colorless products. The search for
better and “greener” solvents is ongoing, aided by solubility predictions from
COSMO-RS that show several aromatics and polyethers
to possess promising solvent power and selectivities.

PEF applicable to typical commercial applications of PET can then be
produced from high purity cyOEF. cyOEF were subjected
to catalytic ROP to form PEF chains in yields of >95% and molecular weights
equivalent to a PET bottle (~60’000 g/mol). Since
absolute molecular weight determination is essential for such process
development, all our samples were all analyzed with size exclusion
chromatography (SEC) multi-angle light scattering (MALS) as well as diffusion
(DOSY) NMR. The latter was developed especially for PEF, as it offers advantages
over SEC-MALS such as reduced solvent use, analysis time <2 min and is less
prone to hardware failure, while allowing for simultaneous absolute molecular
weight and conversion analysis due to the different chemical shifts of cyOEF
and PEF [6]. The reaction is usually complete after less than 20 min, which,
even combined with cyOEF synthesis time, outperforms commonly applied
solid-state polycondensation by about a factor of 20.
Since the cyclic raw material exhibits a distribution of ring sizes, the
individual species show melting points ranging from 270^oC to 370^oC
and different reactivity. Successful conversion of each observed cyclic species
from dimer to heptamer was achieved well below their
melting point using 250-280^oC with tin-catalysts such as
FDA-approved tin octoate, the application of which
could facilitate the entry to the FDA-regulated food packaging market. Lower
temperatures are actually beneficial and allow for higher achievable molecular
weights, due to the more limited impact of degrading side-reactions. This
degradation became dominant usually after 20 min after reaching >95%
conversion. Maximum achievable molecular weights also increased with monomer
purity, as linear impurities reduce molecular weight through the introduction
of additional end-groups. After having obtained PEF from ROP with favorable
molecular properties, the material properties essential for bottle manufacture
were analyzed. The higher glass transition temperature (85^oC vs. 73^oC)
and lower melting point compared with PET (215^oC vs. 260^oC)
indicate higher thermal stability and easier processing of the final bottles. An
at  least 
5x  higher  oxygen 
diffusion  barrier
complements  the  advantageous 
properties  of  PEF, 
which  can  be 
explained  with  a higher 
molecular  rotational  energy 
barrier  as  derived 
from  molecular  dynamics simulations that we have recently
performed.

While  the  scale-up 
to  larger  (kilogram) 
volumes  and  processing 
towards  actual bottles  is ongoing, 
the  advantageous  synthesis of 
PEF via  ROP  opens a 
new and promising  pathway  not 
only  towards  the 
highly  anticipated  “green 
bottle”,  but  also enables advanced molecular architecture
control of furan based polyesters through a “living” polymerization, e.g. for
branching and block copolymers, which is infeasible with the current process
based on PC-SSP.

References

[1]
M Gomes, A Gandini, AJD
Silvestre, B Reis, J. Polym.
Sci. A: Polym. Chem. 2011, 49, 3759-3768

[2] L Sipos, E De Jong, MA Dam, JM Gruter, ACS Symposium Series 2012,
1105, P Smith(Ed.), 1-11
[3] D Pfister, G Storti, F
Tancini, LI Costa, M Morbidelli, Macromol Chem Phys 2015, 216, 2141-2146

[4] R R Burch, SR Lustig, M Spinu, Macromolecules 2000, 33, 5053-5064

[5] JG Rosenboom, P Fleckenstein, G Storti, M Morbidelli, 2016, in preparation

[6] W Li, H Chung, C Daeffler, JA Johnson, RH Grubbs, Macromolecules 2012, 9595-9603

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