Oligomers, depolymerization cyclic

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... 

POLYESTER CYCLIC OLIGOMERS VIA RING-CHAIN EQUILIBRATION (DEPOLYMERIZATION)... 

Figure 3.4 HPLC trace showing the formation of 5 % PET/PBT cyclic oligomers from the depolymerization of 5% PET/PBT. Cyclic dimers are indicated at 3.0 and 4.0 min, with trimers appearing at 5.0 and 6.0min, etc. the small peaks represent mixed co-cyclics...

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],... 

In many instances the random cleavage of the polymeric chain leads to the formation of oligomers, but this scission is typically followed by secondary reactions. If the oligomers are further decomposed to the monomer level, the overall reaction is still indicated as depolymerization. However, for some polymers such as methylsilicones, the oligomers formed through random chain scission further react, leading to a variety of compounds such as cyclic oligomers. In this case, the pyrolysis reaction is indicated as random scission. 

Little is known about the overall mechanism of cyclic oligomer formation, although the mechanism of the initial stages of the sequence seems fairly clear. The first chemical event is the reaction of formaldehyde (formed in the Petrolite procedures by depolymerization of paraformaldehyde) with phenol to form 2-hydroxy-methyl- and 2,6-6w(hydroxymethyl)phenols in a base-catalyzed process, as shown in Fig. 3. Such compounds were characterized many years ago50), obtained from the action of aqueous formaldehyde on phenol in basic solution at room temperature. Subsequent condensation between the hydroxymethylphenols and the starting phenol occurs to form linear dimers, trimers, tetramers, etc. via a pathway that might involve o-quinonemethide intermediates which react with phenolate ions in a Michael-like reaction, as portrayed in Fig. 4. The condensation of hydroxymethyl-... 

Yamashita has reported the anionic synthesis of a joly-a-methyIstyrene-poly-e-caprolactone (a-MeS-e-C, Mn Ai4 x lo - 5.5 x 10 ), but the second block formation is complicated by a depolymerization equilibrium to give cyclic oligomers (52). 

A silanol terminated PDMS heated under isothermal conditions at different temperatures first shows an increase in molecular weight (MW) as a function of time151 (Figure 9). This process is the condensation of silanol groups on the ends the PDMS chains which leads to an increase of MW. However, the increase in MW is limited probably due to either a lowering in concentration of silanol groups or to the equilibrium nature of the silanol condensation reaction. When the temperature is further increased, a second process is observed which is the depolymerization. A mixture of 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). 

At 200°C the initial volatilization rate does decrease after 50 hr, but not to the extent observed at lower temperatures. The total weight loss after 764 hr is 40%. This weight loss is attributed to the formation of cyclic oligomers produced by depolymerization of PFAP(II). After 50 hr the chain scission degradation process has lowered the molecular weight to ca. 450 repeat units. Thereafter, the depolymerization mechanism is viewed as an unzipping reaction initiated at the ends of the low molecular weight chains. 

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