(239a) Cyclic Polyethylene Furanoate As a Monomer from Renewable Resources for Ring Opening Polymerization
The replacement of oil based
chemicals by renewable resource based chemicals is one of the major issues
chemical industry is facing right now. The alternative polymer poly (ethylene furanoate), PEF, is a possible substitute for poly
(ethylene terephthalate), PET. Among different process alternatives, PEF can be
produced at high polymerization rates and molecular weights via ring opening
polymerization of cyclic oligo (ethylene furanoate), cyOEF. On the other hand, the effective production of these
cyclic oligomers at high purity is still the major obstacle towards the process
industrialization. In this work, different strategies aimed to produce these
cyclic molecules from the two monomers Furan dicarboxylic acid (FDCA) and
ethylene glycol (EG) are explored. And a model is presented to describe the
equilibrium behavior of said cyclic oligomers.
Three pathways towards cyOEF
The three synthetic strategies shown in figure
1 have been investigated. In all cases, high dilutions in organic solvents are
required to achieve high yields; the final product is a distribution of cyclic
oligomers with different number of repeating units.
1 Reaction pathways for the
production of cyOEF.
The first (scheme 1-2-6) is a fast cycle
formation using the rapid esterification of acid chlorides with alcohols.
Moderate selectivity towards cycles of around 50% are in contrast to a complex
reaction setup and toxic chemicals involved for the chlorination. The product
size distribution is kinetically controlled and consists mainly of C3, with C2
and C4 being the second most abundant species.
The depolymerization (scheme 1-3-4-6) and
reactive distillation (scheme 1-3-5-6) are the two more promising routes towards
cycles. For depolymerization FDCA and EG are converted to oligomeric chains of
around 12 repeating units. This prepolymer is subsequently diluted in a high
boiling organic solvent and converted to a mixture of cyclics and linears at
180-200 °C. In case of reactive distillation the FDCA and EG (in excess) are
converted to very short chains (1-2 repeating units). The chains are then
diluted in a high boiling organic solvent and finally converted to cyOEF by
distilling a portion of the solvent together with the excess EG.
Both routes yield a similar distribution of
cyclic species. Possible purification routes include precipitation and
adsorption processes, which have both been applied to yield cycles with high
purity starting from linear species (>99% by HPLC area). The yield of both
reactions lies in the range of 60 to 80 % depending on the process conditions.
One striking feature of this type of reaction is that it is possible to recycle
the solvent and the linear side products in order to increase the yield of the
process and furthermore make it more sustainable.
Modeling of the ring chain equilibrium
The equilibrium composition
of cyclics and linears in 2-Methylnaphthalene and Dichlorobenzene (DCB) could
be modeled using the Jacobson-Stockmayer model (Figure 2). Under high dilutions
this equilibrium shifts towards the formation of cyclic oligomers. It is based
on computing the probability of ring closure using the assumption of randomly
coiled hydrocarbon chains for linear polymers. Two fitting parameters are
involved: one is the equilibrium constant for the step growth polymerization,
which is in equilibrium with the cyclisation reactions, and the second one is
the effective length of the bonds in the polymer chains.
As shown in the figure
below, our own results from depolymerization and reactive distillation
experiments were fitted using the model. These results suggest that it is
indeed possible to use this approach to describe the ring chain equilibrium.
The model is currently under refinement in order to describe system-specific
effects such as the complete absence of cycles with six repeating units.
2 Equilibrium distribution of
cyclic oligomers in DCB.
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