Depolymerization to cyclic

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). 

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. 

Diol-functionalized telechelic polymers have been desired for the synthesis of polyurethanes however, utilizing alcohol-functionalized a-olefins degrades both 14 and 23. Consequently, in order for alcohols to be useful in metathesis depolymerization, the functionality must be protected and the oxygen atom must not be /3 to the olefin or only cyclic species will be formed. Protection is accomplished using a/-butyldimcthylsiloxy group, and once protected, successful depolymerization to telechelics occurs readily. 

Conversely, if the polymer could be made by some other route (for example, by macromolecular substitution), it might be stable at moderate temperatures where the rate of depolymerization is very slow, but would depolymerize to the cyclic trimer or tetramer when heated to higher temperatures. In fact, this behavior is found for uncross-linked polymers such as [NP(OPh)2] , that appear to be kinetically stabilized at moderate temperatures, but are sufficiently destabilized thermodynamically by the bulky aryloxy side groups that they depolymerize when heated above 150-200 °C. 

Spiro orthoesters (92, R = Me, Ph, and H) show typical equilibrium polymerization behavior at or below ambient temperature. [92] The poly(cyclic orthoester)s derived from 92 depolymerize to the monomers, although they have sufficient strains to be able to undergo ring-opening polymerization. The polymerization enthalpies and entropies for these three monomers were evaluated from the temperature dependence of equilibrium monomer concentrations (Table 5). The enthalpy became less negative as the size of the substituent at the 2-position in 92 was increased H < Me < Ph. This behavior can be explained in terms of the polymer state being made less stable by steric repulsion between the bulky substituents and/or between the substituent and the polymer main chain. The entropy also changed in a similar manner with the size of the substituents. 

Depending on the method of preparation (cf. Section 4.33.4.2.1), 1,3,2-dioxathioIane 2-oxide (16) or its derivatives are obtained at first in the form of low molecular weight polymers which at elevated temperatures depolymerize to the corresponding monomers (57USP2798877, 61ZOB1332). The polymerization of such cyclic sulfites is catalyzed by bases (e.g. dialkylanilines or pyridines). The formation of polymers on thermolysis of 1,3,2-dioxathiolan-4-one 2-oxides has been described in Section 4.33.3.1.2. 

In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. 

Cyclic polysilanes are more stable than chain polymers. Attempts to produce high polymers of polysilanes by ring opening of cyclic compounds are unsuccessful, but conversions from linear polymers or chain oligomers to cyclic compounds are feasible. Most of the limited work in this area is done with permethylpolysilanes. As mentioned in 15.2.4.1.2, the preparation of methylcyclosilanes from Me2SiCl2 and alkali metals proceeds through polymer formation followed by depolymerization, at least in some cases . ... 

Polymer yields are optimized by avoiding the use of diluents. If solution polymerization is desired, then in order to avoid the formation of increased amounts of equilibrium cyclics it is necessary to employ more reactive cyclic reactants and selective polymerization conditions, e.g., strained cyclics such as (Me2SiO)j or Me2SiCH2CH2SiMe20 can be selectively transformed to linear polymers using conditions that are so mild that little of the resulting polymers are depolymerized to other cyclics. ... 

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. 

As discussed earlier, polydichlorophosphazene depolymerizes to a spectrum of cyclic oligomers when heated at 250-350°C jn helium flow-tube apparatus at 400°C, species (NPCl2)3 g are identified, with the cyclic hexamer predominating. It should be noted that these results are for nonequilibrium reaction conditions under which all or most of the polymer is converted to oligomers. 

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