Depolymerization to cyclic oligomers

Allcock studied the thermal degradation of poly [bis (trifluoroethoxy )-phosphazene] via gel permeation chromatography and solution viscosity measurements on samples aged at 150°-300°C (II) in sealed, evacuated tubes. The mechanism of degradation was presented as a two-step process (1) chain scission at weak links and (2) depolymerization to cyclic oligomers. Allcock also reported similar studies on the thermal breakdown of poly [bis (phenoxy) phosphazene] and arrived at the same degradation mechanism (12). 

Depolymerization to cyclic oligomers could be occurring concurrent with chain scission or may be initiated by the scission process itself at the ends of the lower molecular weight chains. The formation of depolymerization products (PFAP(II) cyclic oligomers) would not be detected as a change in the solution viscosity which is more dependent on high molecular weight PFAP (II). 

The distribution of products depends on the reaction conditions. If equilibration is carried out in a completely sealed system, decreases in pressure favor depolymerization to cyclic oligomers. Under nonequilibrium conditions (continuous removal of oligomers as they are formed), depolymerization takes place at 260-400°C. The oligomer formed in the largest amounts under these conditions is the cyclic hexamer , (NPCl2)6-... 

Depolymerization to cyclics, followed (57) by chain scission at weak points (i.e., defects in the backbone) followed by rapid depolymerization to cyclic oligomers... 

It was later noticed that the hexachlorocyclotriphosphazene can be polymerized by heating in the melt to yield an uncrosslinked linear high polymer, poly[(dichloro)-phosphazene] (Allcock et al, 1965-1966 Rose, 1968). Further heating results in the formation of an insoluble crosslinked material. This leads in both cases to a transparent, rubbery elastomer which hydrolyzes slowly, when exposed to moisture, forming phosphate, ammonia and hydrochloric acid. At temperatures above 350°C, the polymer depolymerizes to cyclic oligomers. The uncrosslinked species serve as highly reactive polymeric precursor. 

Alkoxy- or aryloxyphosphazene high polymers (where OR = OCH2CF3 or OC H ) depolymerize to cyclic oligomers at temperatures above 200-250 C. This behavior is reminiscent of the reversion reactions that are characteristic of poly(organosiloxanes). 

Depolymerization of the high polymer to cyclic oligomers can occur by back-biting reactions in which active cationic end units attack middle units of the same chain. 

Depolymerization of a high molar mass pol5aner to cyclic oligomers may also be induced by certain catalysts, or it may occur during thermal degradation. ... 

Melchiors, M., Keul, H., and Mocker, H. (1996]. Depolymerization of poly[(R]-3-hydroxybutyrate] to cyclic oligomers and polymerization of the q clic trimer An example of thermodynamic reqrcling. Macromolecules, 29,6442-6451. 

Kawakami, Suzuki and Yamashita showed that compound 7, among many others, could be polymerized to derivatives of the corresponding open-chained species by treatment with boron trifluoride ether complex. Yamashita and Kawakami formed these same sorts of materials by heating the glycols and paraformaldehyde in the presence of toluenesulfonic acid. This led to prepolymers which were then thermally depolymerized to afford the cyclic oligomers which were separated by fractional distillation. 

Cyclic oligomers of condensation polymers such as polycarbonates and polyesters have been known for quite some time. Early work by Carothers in the 1930s showed that preparation of aliphatic cyclic oligomers was possible via distillative depolymerization [1, 2], However, little interest in the all-aliphatics was generated, due to the low glass transition temperatures of these materials. Other small-ring, all-aliphatic cyclic ester systems, such as caprolactone, lactide... 

Reports of depolymerization studies carried out by Semiyen and co-workers at the University of York, UK, typically used much longer reaction times (24-72 h) for the depolymerization of PET [27], This group also reported that the titanates were ineffective catalysts for depolymerization, but it is not known whether measures to prevent titanate hydrolysis were taken. The York group also reported that the relative proportions of cyclic oligomers (trimer, tetramer, etc.) altered as the dilution ratio was changed. Similar results were seen with PBT. They have also reported fast-atom bombardment mass spectrometry (FAB-MS), LC-MS, GPC and X-ray crystallography data. The most recent work reports characterization of the cyclic oligomers from six ester and ether ester systems [29],... 

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