Provision with additional aromatic and aliphatic lactones

Cyclic 1,4-butylene terephthalate (cBT) is the only aromatic cyclic monomer commercially available. Two aliphatic lactones and three other aromatic MCOs with a structurally similar repetition unit compared to cBT (Scheme 5) were additionally chosen. The similarity was supposed to facilitate mixing and to gain insight into the effect of structure on the thermal properties of the mixtures at the same time (chapter 5.2.2). A short linear diol with three or four carbon atoms was therefore combined with a linear aliphatic diacid with four or six carbon atoms or with a linear or an angled aromatic diacid.

5.2.1.1 Preparing of the aromatic and aliphatic lactones

Pseudo-high dilution synthesis and depolymerization in bulk as well as in dilution were evaluated as routes to partly aromatic MCOs (Scheme 6). A successful preparation was achieved by pseudo-high dilution synthesis. The depolymerization in dilution was

successfully carried out, as well, but dismissed because of a lengthy reaction and complicated purification.

The aliphatic monomers cyclic butylene 1,4-adipate (cBA) and cyclic 1,4-butylene succinate (cBS) (Scheme 5a and b) were readily prepared by cyclo-depolymerization in bulk of the corresponding polymers in a 5 L steel reactor. The polymers had been prepared by polycondensation from the respective diol and diacid with utilization of Ti(OnBu)₄ as transesterification catalyst directly before their cyclo-depolymerization. The method is basically the same as published by CAROTHERS et al. in the 1930s for various monomers^35,130–133. It has recently been described by BEREZINA et al. for cBS⁶² and extensively for cBA²³¹.

al. in the 1990s. High yields (70 – 87 %) and pure products (according to ¹H NMR and MALDI spectra) were obtained when solutions of the reactants, diol and dicarboxylic acid chloride in case of oligoesters, are dosed to a solution of a base like triethylamine or pyridine and a catalytic amount of a sterically less hindered amine as DABCO or DMAP, following the literature.^19,154–157 Providing dicarboxylic acid chloride in the reaction flask led to a high yield (94 %) with a good purity, as well. Cyclic compounds were mainly formed in these cases because of the low concentration of reaction partners during the whole reaction time. In contrast to reports in literature¹⁹, DMAP did not show a significantly reduced catalytic activity relative to DABCO. Similar yields with rather different degrees of purity were obtained with both catalysts on the different preparation routes explored here for the different aromatic MCOs (e.g. dosing of just the diol or of both reactants simultaneously).

Further routes for the synthesis in pseudo-high dilution were evaluated but dismissed for reasons of yield and product purity. These comprise reactions with heating to accelerate the reaction and/or to remove side products. The transesterification of dimethyl isophthalate with BDO in presence of Ti(OnBu)₄ in oDCB gave the desired cycles only in almost negligible yield with mainly opened cycles present. The reaction of terephthaloyl dichloride with BDO under removal of HCl gave a high amount of linear esters (according to the amount of BDO end groups (above 20 %)) and almost exclusively polymers (compared to CBT100 in chapter 5.1.1.2 ¹H NMR spectroscopy, p. 37 ff.), questioning the formation of cyclic structures. The same is true for the utilization of sodium hydroxide instead of amines at an elevated temperature.

Depolymerization in dilution was screened in preparing cBI in oDCB. The low yield of the batchwise preparation combined with a slow transformation of the previously prepared PBI to cBI (>72 h at 10 wt-% of cBI in oDCB) and the cumbersome drying because of the high-boiling, hazardous solvent made this method less favorable. Hence, it was not further pursued. CBI was chosen as model compound instead of cBT in this case because of the easier handling of the corresponding polymer regarding its lower melting temperature and better solubility in organic solvents.

No successful preparation of cyclic aromatic oligomers was established by depolymerization in bulk. The resulting partly aromatic MCOs of this study are not volatile enough to be distilled off. Depolymerization of aromatic polyesters in bulk was explored for PBT and PBI. The first was chosen because of its central meaning for this project, the latter because of its close relation to PBT combined with the promising yields in comparable experiments with poly(1,2-ethylene isophthalate) in literature.¹³⁴ Additionally, the much lower melting temperature of PBI polymer and corresponding MCO promised a less demanding

evaporation of the monomer. Neither the utilization of a temperature above 300 °C nor of a diffusion pump nor of the stripping gas nitrogen (in case of cBI) led to an isolation of mentionable amounts of MCOs according to GC-MS. The thermal degradation was usually high after the extensive heating necessary according to appearance, odor and ¹H NMR spectra.

5.2.1.2 Melting and crystallization behavior of the monomers

The thermal behavior of the monomers relevant for this work has been determined by DSC measurements. Iterative measurements have been carried out to evaluate the changes of the observed behavior over time. The observed key figures for melting, crystallization and glass transition (Table 4) were analyzed regarding the influence of chemical structure of the monomers and compared to literature.

Table 4: Data from DSC measurement of the studied cyclic monomers (data from 3^rd DSC run of each sample if not stated otherwise).

1^st heating segment 1^st cooling segment 2^nd heating segment Lowest T_m

[°C]

Highest T_m [°C]

Total H_m [J∙g^-1]

High T_c

[°C] T_g [°C] ΔC_p [J∙g^-1∙K^-1]

Lowest T_m [°C]

Highest T_m [°C]

Total H_m [J∙g^-1] cBT 126.7 182.7 39.7 --- 22.3 0.224 129.5 183.7 42.7

cBI 102.3 131.3 2.1 --- 18.5 0.435 --- --- --- cPT 188.7 221.6 87.8 172.4 37.6 0.147 184.1 221.4 76.1

cPI 65.8 ¹⁾ 120.4 ¹⁾ 57.7 ¹⁾ --- 30.0 0.415 --- --- --- cBA --- 95.5 145.5 53.2 -41.8 ²⁾ 0.055 ²⁾ --- 95.2 137.9 cBS --- 69.4 5.7 --- -71.9 ²⁾ 0.480 ²⁾ --- 45.7 78.0

1) Data from 1^st DSC run, not observed in further runs.

2) Data from DSC run for T_g determination with decreasing of the temperature to -90 °C.

effects of different oligomers in the MCOs. The presence of isophthalic acid in the monomer results in a smaller T_m (i.e. the highest observed melting endotherm) and ∆H_m than of the respective MCO with terephthalic acid instead. The reason is probably a distortion of the supposed zigzag structure caused by the “kink” of isophthalate and leading to a suppression of crystallization reported before for other aromatic polyesters.232,234,235 The effect was enhanced by presence of PDO, the diol with the odd number of methylene groups. A melting endotherm was observed for cBI during the first heating cycle of all DSC measurements, for cPI only during the very first heating, but not for any reheating of the same sample even after crystallization at room temperature for more than 21 d. Crystallization could not be observed for neither of the MCOs during cooling in DSC.

An odd number of carbon atoms in the repeating unit of the monomers does not seem to disturb the chain alignment during crystallization per se significantly. This is in contrast to the observations for polyesters, where a general lowering of the melting temperature by odd-numbered diols was recognized.^232,233 The expected lower melting temperature of MCOs with PDO instead of BDO was only observed in combination with the obstructive isophthalate.

The higher T_m and ∆H_m of cPT compared to cBT already known from literature²³⁵ can be explained by the presence of the rigid terephthalate, which “forces” the chains in an oriented structure. The length of the diols and hence the distance between the polar groups and of the π-electron system of the aromatic determines the strength and hence the T_m of the crystals.

The order of T_m and ∆H_m for the aliphatic monomers cBA and cBS follows the rules previously set up:^233,236 A longer chain makes the monomer more like polyethylene and causes a comparatively higher T_m. The shorter diacid chain prevents an easy alignment and hence crystallization during cooling in the DSC measurements made. ∆H_m during the subsequent heating phase has hence decreased compared to the initial melting (<80 J∙g^-1 instead of 104 J∙g^-1 initially). A higher crystallinity was interestingly observed in general for cBS during second heating of the DSC measurements than during the first heating of subsequent measurements days later.

5.2.1.3 Glass transition

The T_g is determined by the chain flexibility. Consistently, it was found to be higher for shorter diols and for the more rigid, aromatic MCOs than for the more flexible, aliphatic monomers. This is in good agreement with previous observations regarding polyesters mentioned before.^232,233 A difference between terephthalate- and isophthalate-containing oligomers was observed. The increase of free volume granted by the bulky isophthalate

possibly gains importance in monomers with only short diols. The density of polar groups in these and hence the attractive forces between the molecules highly limit the free volume.

Surprisingly, a longer diacid in the two studied aliphatic monomers has the opposite effect on the glass transition compared to the diols and heightens the T_g. This is probably caused by the longer distance between the ester groups that facilitates a short-range orientation and possibly reduces the free volume.

The amount of amorphous phase was higher for monomers with an isophthaloyl than a terephthaloyl moiety, judged from the ΔC_p. It is caused by the alignment of the linear terephthalate-containing monomers.232,237,238 The higher amorphous fraction in MCOs with BDO compared to those with PDO can be explained similarly. This dependency of the vitreous amount on the diol length is again not transferable on the length of an aliphatic diacid, where a significantly smaller ΔC_p was found for the longer adipate than for succinate.

Obviously, the longer chain remarkably enhances the alignment of the monomers in crystals.

5.2.2 Effect of chemical structure on thermal behavior in mixtures of