Cyclo-depolymerization (CDP)

CDP uses the thermodynamically controlled equilibrium between polymer chain and MCOs to depolymerize an existing polymer to MCOs. Its principles have already been mentioned above (introduction of chapter 3.2). Strategies for CDP differ depending on whether it is carried out in bulk or in dilution. CDP is however usually catalyzed in both cases, mostly by alkoxide metal complexes as named above for catalysis of ROP or transesterification.

Especially titanium(IV) alkoxides or dibutyl tin alkoxides have found to be very efficient catalysts, so far.²⁴

The fraction of MCOs in dilution is significantly higher than in bulk. Actually, at concentrations below 3 % (w/v) the equilibrium is clearly on the side of MCOs while in bulk usually less than 2 wt-% are MCOs.¹² In dilution, the intramolecular CDP leading to cycles is firstly statistically much more favored than the intermolecular transesterification and is secondly driven by the gain in entropy. This is in agreement with both the RUGGLI-ZIEGLER

dilution principle^93–95 and the findings of STEPTO et al.^96,97 and of GORDON and TEMPLE^98,99

cited above.

3.2.1.2.1 CDP in bulk with isolation of MCOs by distillation

The preparation of MCOs in bulk is only effectively possible if they are distilled off during the process because the equilibrium lies heavily on the side of the polymer chain (usually 95-99 wt-% are linear).¹² Therefore, only volatile MCOs can be prepared by this route. Hence, mostly aliphatic MCOs are accessible via CDP in bulk. Aromatic or partly aromatic MCOs are usually not distillable at a temperature below the degradation temperature even at ultra-high vacuum (judging from the respective melting temperatures).36,69,123–129

CAROTHERS, mostly accompanied by SPANAGEL, prepared over 40 aliphatic MCOs in the 1930s with yields of raw distillate between 30 and 85 %.^35,130–133 In contrast, there are few examples of aromatic MCOs isolated by depolymerization in bulk reactions. The first

CBT oligomers, mainly the cyclic dimer, were identified for the first time at a source for mass spectrometry (MS) with electron impact ionisation.^88,136 At 200 °C, first oligomers were observed, at 330 °C, a sudden thermal decomposition took place freeing mainly cyclic dimer and at 380 °C, mainly larger cyclic oligomers were present. Similarly, the pure cET dimer was collected at the surroundings of processing equipment for PET by NAGAHATA in 2000.¹³⁷

This route has mostly been disregarded because of the generally limited numbers and quantities of aromatic MCOs accessible combined with the high demands on the equipment, despite the high product purity obtainable by CDP in bulk with distillation.

3.2.1.2.2 CDP in dilution

CDP in dilution is usually carried out by refluxing or heating the respective polymer in a suitable, high-boiling solvent in a concentration of 2 % (w/v) or less. As catalyst, tin complexes are common as they convert the polymer fast and are not too water sensitive.

However, they are not readily quenchable and have to be removed after CDP for avoiding unintentional ROP. The other commonly used catalysts are based on titanium and are quenchable by hydrolysis which, on the other hand, makes strictly anhydrous conditions during CDP necessary. Other metal complexes, on basis of e.g. antimony have been explored as well.⁷⁰

The conversion of polymer to MCOs is generally substantially higher than that in CDP in bulk with a yield of up to 85 %. On the other hand, CDP in dilution is paired with high amounts of solvent, with the necessity of separating the MCOs from the usually high-boiling solvent as well as from polymeric and oligomeric linear species.¹² Additionally, a relatively long reaction time has been reported with values between 30 min (for a few reactions utilizing tin or titanium catalyst) to 8 d, most being between 1 and 4 d.^24,138–141

It has been shown that the solubility of the polymer in the solvent used for CDP is crucial as the rate of depolymerization is depressed in case of not complete solubility.^24,70 The dilution ratio of the solvent itself alters the ratio of the different oligomers being formed during CDP, which has been reported for PET.¹¹⁶ The temperature during CDP has no influence on the depolymerization rate or the ratio of the cyclic oligomers formed above a threshold, as investigated by BRUNELLE et al. for cBT.²⁴

Different solvents have been successfully applied for CDP of different partly aromatic polyesters. Mostly aromatic and halogenated solvents are used, particularly dichlorobenzene (oDCB) has been employed widely. Further solvents in publications are 1,2-xylol or 1-methylnaphthalene, e.g.

BRUNELLE et al. from Cyclics Corporation patented the CDP 1995 and 1997.20,138,139,142 The depolymerization of PBT, PET and poly(1,2-ethylene 2,6-naphthalene dicarboxylate) was herein emphasized in diluted solutions of less than 0.3 M (in respect to the structural units) with yields of 33 – 90 %. They were carried out in the aromatic solvents 1,2-xylene, chlorobenzene, naphthalene, toluene, tetramethylbenzene, methylnaphthalene, oDCB and their mixtures, with 1 – 5 mol-% tin or titanium complexes as catalysts (Bu₂SnO or (Ti(OiPr)₄, e.g.) and with heating to a temperature of 140 to 280 °C. Preferentially, the depolymerization was carried out in a plug-flow tube reactor.¹³⁹ Interestingly, they found the molar concentration of cycles in solution to be the same (about 0.05 mol∙L^-1) for every experiment, except for very low concentration, where the presence of end-groups limited the number of cycles formed.²⁰

In 1997 SEMLYEN et al. prepared cBT by CDP in oDCB in yields of up to 70 %. They admittedly diluted the PBT by a relatively high ratio of 1.4 % (w/v) and used relatively long reaction times of 72 h.¹¹⁶

Furthermore SEMLYEN et al. published CDPs of PET in 1-methylnaphthalene in different dilution ratios of 3 or 10 wt-% with different catalysts in 24 h reaction time.¹⁴³ The highest yield of 30 wt-% cET was obtained in higher dilution – in accordance with the theories cited before89,90,93–99 – and with zinc(II) acetate as catalyst. The presence of a second cyclic oligomer species with ether defects was noticed in these experiments.

HODGE et al. published the preparation of various cyclic alkylidene isophthalates via CDP in polymer solutions of 1 – 2 % (w/v) in oDCB or chlorobenzene with Bu₂SnO as catalyst in 2000.¹⁴⁴ After 12 h, an almost maximal yield was observed, but not a thermodynamically equilibrated system, which was only seen after up to 10 d of reflux (monitored by gel permeation chromatography (GPC) in chloroform). The formation of cyclic diol/diacid adducts occurred for cycles smaller than 16 ring atoms with a yield close to 90 % and for larger cycles with a yield of about 72 %. The working group around HODGE prepared cyclic

1,2-suspension with saturated hydrocarbons as hexadecane. CDP in solution led to a kinetic distribution of ring sizes because of the high dilution of the cycles and hence to broadly distributed ring sizes (in accordance with the theory of JACOBSON and STOCKMAYER^89,90).

They showed a lower melting temperature of the MCOs. CDP in suspension was thermodynamical controlled and resulted in a narrow distribution with a higher melting temperature.

After findings of enrichment of cET oligomers in supercritical fluids, especially in supercritical carbon dioxide (scCO2),^146,147 the CDP of PBT and extraction of the resulting cBT by supercritical fluids was established by WEIJERS et al. in 2006.¹⁴⁸ Fast cycle formation and solvation was observed at comparatively high temperatures and pressures of 230 °C and 250 bar, while the mass transfer of the loaded supercritical phase was rate determining.

Supercritical pentane showed an even higher solubility power for cBT in that study.