Catalyst Free PET and PEF Polyesters Using a New Traceless Oxalate Chain Extender

Plastic material performance is strongly correlated to the polymer's molecular weight. Obtaining a sufficiently high molecular weight is therefore a key goal of polymerization processes. The most important polyester polyethylene terephthalate (PET) and the new polyethylene furanoate (PEF) require metal catalysts and time-consuming production processes to reach sufficiently high molecular weights. Metal catalysts, which are typically antimony or tin for polyesters, end up in the plastic products which may result in sustainability and ecological challenges. When the less reactive comonomer isosorbide is introduced to produce (partly) biobased materials with enhanced thermal properties, such as polyethylene-co-isosorbide furanoate (PEIF), reaching high enough molecular weight becomes even more challenging. This study presents an easily implementable approach to produce high molecular weight PET and PEF polyesters and their isosorbide copolyesters PEIT and PEIF by coupling lower molecular weight polymer chains by the reactive diguaiacyl oxalate (DGO) chain extender. DGO is so reactive, that the use of metal catalysts can be completely avoided and it helps avoiding an extra solid-state polymerization step. In addition, DGO distinguishes itself from typical chain extenders by its ability to be completely removed from the resulting polymer, thereby avoiding the inherent drawbacks associated with typical chain extenders.


Overview S1 | Reaction mechanisms of DGO with the polymer chain and ethylene oxalate formation.
When DGO is added to PET with ethylene glycol end groups present.First the transesterification reaction between ethylene glycol end group and DGO takes place, releasing one guaiacol: Next, the second transesterification reaction between ethylene glycol end group and DGO end group takes place, releasing another guaiacol: Now, the polymer chains are connected and the chain length is increased: The oxalate can change position within the polymer chain by transesterification, resulting in different repeating structures: TPA-EG-TPA, TPA-EG-OX or OX-EG-OX: These structures can then undergo internal ring formation to form the six membered ring component ethylene oxalate: The BHET used for polymer synthesis contains 1.3 mol% DEG (based on total TPA), this results in 0.7 mol% DEG from BHET in the final polymer.b.

Overview S2 | ICP analysis limit of detection and calibrated wavelength of the measured metals
The commercial Rama PET also contains a portion of isophthalic acid (4%), which should be kept in mind when comparing them to the synthesized polymers.The FDCA decarboxylation is highlighted; 6.54 ppm in TCE-d2 (residual solvent signal is set at 6.00 ppm).The decarboxylation is likely reduced by the absence of a catalyst and mild conditions.According to the NMR integrals, the DEG content of the polymer is 4.1% (based on total FDCA).See experimental section [H] for the synthesis details.According to the NMR integrals, the DEG content of the polymer is 3.6 mol% relative to the total TPA content.Signals 7a and 7b show that the commercial Rama PET also contains a portion of isophthalic acid (~4%), which should be kept in mind when comparing them to the synthesized polymers.As the correction factor for the purity analysis is lower than 5% we can accept the value of 99.9 mol%.However, this value excludes the impurities which are insoluble in the melt.See experimental section [A] for the synthesis details.

Fig.S6 | 1 HFig.S7a | 1 H
Fig.S6 | 1 H-NMR of PEF (produced via FDCA) in TCE-d2 .The corresponding signals of PEF and common side products are assigned.Note that the decarboxylation of FDCA can be observed by the shift at 6.58 ppm when the TCE residual solvent shift is set at 6.04 ppm.According to the NMR integrals, the DEG content of the polymer is 4.1% (based on total FDCA).See experimental section [G] for the synthesis details.

Fig.S7b | 13 C
Fig.S7b | 13 C NMR of PEF after stepwise addition of 3.5 mol% DGO.The expected region where the oxalate would show up highlighted by overlaying the 13 C NMR of PETO (157.0 ppm).No oxalate is observed.See experimental section [H] for the synthesis details.

Fig. S8 | 2 oxalate
Fig.S8 | DSC of PEIF and PEIT produced in the autoclave.The black line represents the PEIT with 16% isosorbide and has a Tg of 93 °C.The blue line represents the PEIF with 26.3% isosorbide and has a Tg of 110 °C.

Fig.S10 | 1 H
Fig.S10 | 1 H-NMR of PET without booster in TCE-d2According to the NMR integrals, the DEG content of the polymer is 4.5 mol% relative to the total TPA content.A part (0.7 mol%) of this DEG content comes from the starting material BHET, which contains 1.3 mol% DEG (see supplementary information).See experimental section [D] and [E] for the synthesis details.

Fig.S11 | 1 H
Fig.S11 | 1 H-NMR of PET with 2.2 mol% booster in TCE-d2.According to the NMR integrals, the DEG content of the polymer is 4.2 mol% relative to the total TPA content.A part (0.7 mol%) of this DEG content comes from the starting material BHET, which contains 1.3 mol% DEG (see supplementary information Overview S4).See experimental section [D] and [E] for the synthesis details.

Fig.S12 | 1 H
Fig.S12 | 1 H-NMR of commercial RAMAPET N180 in TCE-d2 (IV: 0.8 dl/g)According to the NMR integrals, the DEG content of the polymer is 3.6 mol% relative to the total TPA content.Signals 7a and 7b show that the commercial Rama PET also contains a portion of isophthalic acid (~4%), which should be kept in mind when comparing them to the synthesized polymers.

Fig.S13 | 1 H
Fig.S13 | 1 H-NMR of PEIT in TCE-d2 produced by the autoclave experiment.According to the NMR integrals, the DEG content of the polymer is 3.3% and the isosorbide content is 16%, both are percentage of the total TPA content.A part of this DEG content comes from the starting material BHET, which contains 1.3 mol% DEG (see supplementary information Overview S4).See experimental section [I] for the synthesis details.Ratio TPA : diol = .−.

Fig.S14 | 1 H
Fig.S14 | 1 H-NMR of PEIF in TCE-d2 produced by the autoclave experiment.* indicates the decarboxylation of FDCA.According to the NMR integrals, the DEG content of the polymer is 2.6% and the isosorbide content is 26.3%, both are percentage of the total FDCA content.See experimental section [J] for the synthesis details.Ratio FDCA : diol = .−.

Physical properties of the PET (without catalyst) produced by boosting and without boosting. Additionally, the physical properties of a commercial PET with catalyst and SSP (RAMA PET N180) is given. The raw data can be found below. See experimental section [D] and [E] for the synthesis details.
ICP analysis results with an overview of the metals analyzed.(n.d.) = not determined

data tensile testing of commercial RAMA PET N180:
as PETO contains two diacids (TPA and oxalate), the sequence of the polymer backbone can be organized in various ways: at the top you can see the different polymer repeating structures of PETO with the presence of ethylene oxalate repeating unit highlighted.B, The 1 H NMR of PETO with 48% oxalate in TCE-d2; all of the signals could be assigned to the above mentioned PETO polymer structures.Since the diols reacted to create an ester with either terephthalate or oxalate (a1, a2, a3, a4, c1, d1), the amount of oxalate in the polymer can be quantified using the signals of terephthalate and the diols.See experimental section [B] for the synthesis details.A B Fig.

S2 | 13 C-NMR of PETO in TCE-d2 A, the
different polymer repeating structures of PETO with the carbons numbered for assigning the NMR.B,13C NMR of PETO with 19% oxalate.C,13C NMR of PETO with 48% oxalate.As seen with the 1 H NMR of PETO with 48% oxalate, the polymer backbone mainly consist of the alternating sequence OX-EG-TPA.Only small amounts of the other sequences are seen.Note that the oxalate signal shifts at 157.2 ppm.See experimental section [C] for the synthesis details.