Small-scale screening of novel biobased monomers: the curious case of 1,3-cyclopentanediol

In this work, we report on the small scale polycondensation and consecutive analysis of novel polyesters based on the potentially renewable 1,3-cyclopentanediol (CPdiol). To avoid evaporation of monomers during thin-film polymerization reactions, trimer pre-polyesters have been synthesized from the corresponding acid-chlorides with diol monomers. Polymerization of these trimers was explored by thermogravimetric analysis to identify potential side reactions, and to assess the ideal polymerization temperature. In general we observe that trans-1,3-cyclopentanediol exhibits good thermal stability up to 200 °C, whereas thermal dehydration of the alcohol end-groups occurs upon further heating. In contrast, for cis-1,3-cyclopentanediol, the ester bonds of the cyclopentane end-groups become labile, thereby generating carboxylic acid end-groups, and 3-cyclopentenol already at 180 °C. The thermal dehydration reactions yield double bond end-groups, which in turn facilitate cross-linking through cross-coupling and Diels–Alder reactions, leading to an increase in molecular weight. Despite the limited thermal stability of CPdiol, here we demonstrate that polymerization of CPdiol can successfully be achieved in thin-film polycondensation conditions at 180 °C, yielding molecular weights well above 10 kg mol−1.


Reactor block design
The temperature consistency in the reactor was tested by measuring the temperature on 3 different points in the reactor: temperature sensor in the bottom part, in the side part, and inside in oil in a HPLC vial.
There was consistently a small deviation in temperature inside the reactor as compared to the set & bottom temperature, which is caused by heat loss of the reactor itself to the surrounding environment. This temperatureloss was compensated for in the settings: i.e. for a desired polycondensation temperature of 220 ºC, the temperature was set to 228 ºC. The reactor was validated for polycondensation experiments by the polymerization of a known polymer: poly(ethylene terephthalate). First a consistency check was performed by polymerizing batches of 10 mg over 3 runs (Table 3), and secondly the effect of loading on the polycondensation was checked (Table 4). Overall, the polycondensation is considered to perform consistently well in order to perform polycondensation reactions on the novel polyesters described in this work. SI-4

GC data
Chiral GC data for cis/trans ratio determination of 1,3-cyclopentanediol.

Detailed synthesis of trimers
A generally applicable synthesis method for the preparation of trimer pre-polyesters has been developed based on previously reported synthesis method. The syntheses were performed in anhydrous conditions. A molar ratio of 3:1 diol:di-acid was used. A typical reaction is described: A solution of 9 mmol diol, 7 mmol pyridine, and catalytic amount of DMAP in 5 mL anhydrous THF was stirred in an ice bath in a 25 mL 2-neck round-bottom flask equipped with a condenser. At 0 °C, under N2-flow, a solution of 3 mmol di-acid chloride in 3 mL anhydrous THF was added dropwise. After stirring the reaction mixture overnight, the THF was removed in vacuo. The product was extracted with CHCl3 (3x 50 mL), and washed with 10w% aq. Cu2SO3 (3x 50 mL), and with 0.01 M aq. HCl (3x 50 mL). The CHCl3 extract was dried with MgSO4, filtered, and reduced in vacuo. Typically the extracts could be used as is, however some could be further purified by precipitation from CHCl3 in ether.
Typically the trimers from FDCA and terephthalic acid were obtained as white solids. Purity was confirmed by NMR, and/or LC-MS. The pre-polyesters were mostly isolated as trimers, however also some pentamers and heptamers are present.

Trimer 1,3-cyclopentanediol-furanoate, -20% cis
The product was obtained as off-white solids (1.34 gr, 80% yield). After drying at 110 ºC discoloration occurred, the product was isolated by dissolving the mix in CHCl3, filter out the brown solids, and reduce the CHCl3 in vacuo, to re-obtain the product as off-white solids (0.88 gr, 52% yield).

Trimer 1,3-cyclopentanediol-furanoate, -30% cis
The product was obtained as light brown solids (1.37 gr, 82% yield). After drying at 110 ºC discoloration occurred, the product was isolated by dissolving the mix in CHCl3, filter out the brown solids, and reduce the CHCl3 in vacuo, to re-obtain the product as light brown solids (0.91 gr, 54% yield

LC-MS data
The trimers of 1,3-CP-F with various cis content were dried at 110 ºC, after which significant discoloration was visible ( Figure 3). The obtained trimers where analyzed via LC-MS ( Figure 4) to assess the degradation products (Table 2) prior to a second purification step.    SI-11   SI-14

SI-25
Detailed polymerization of small-scale screening

MALDI-ToF-MS -side reactions with cyclopentene group
One expected side reaction is the reaction between two cyclopentene groups, which are generated by the thermal dehydration of pendant cyclopentanol groups. These double bonds can undergo (thermal) radical addition reactions, forming a Cp-Cp bond ( Figure 38). The expected structures are diol terminated (SI_07), and dehydrated (SI_08, SI_09). Further degradation can lead to loss of CPol group (SI_10), combined with dehydration (SI_11), eventually leading to di-acid terminated chains (SI_12). Table 9. Calculated m/z of linear chains after Cp-Cp addition reaction depicted in Figure 38, with the weight of a potassium ion.

MALDI-ToF-MS -other possible side reactions
Two possible Diels-Alders 2+4 cycloaddition reactions are possible between the furan-group and the generated cyclopentene group. The first one (Figure 39, top) is the reaction of cyclopentene with a normal furan group of FDCA. The second one (Figure 39, bottom) is the reaction of cyclopentene with a decarboxylated furan group. The decarboxylation of FDCA is a well-known side reaction, and free acid groups on the furan ring are generated by the degradation mechanism described in this work. Furthermore. Diels-Alder reactions are also known to occur with these furan groups. A trace of SI_15 has been found in the samples with 30% cis 1,3-CPdiol in the polymer.

Obtained MALDI-ToF-MS spectra 8% cis
In the following MALDI-ToF-MS spectra, and the text, the distributions are numbered in accordance to the structures in Figure 37 and Figure 38, e.g. linear diol-terminated chains SI_01, is denoted as 1. The major ticks are aligned to the M0 of the repeat unit CP-F (m/z 222.1), ranging from n=2 to n=6 of the linear diolterminated chains 1.