Ceiling temperature poly

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymeri2ation (see Table 1) (24). When corrected for the heat of fusion, it is significantly less than the theoretical value of —83.7 kJ/mol (—20 kcal/mol) for the process of converting a double bond to two single bonds. The steric strain apparentiy is not important in the addition step, because VDC polymeri2es easily. Nor is it sufficient to favor depolymeri2ation the estimated ceiling temperature for poly (vinyhdene chloride) (PVDC) is about 400°C. 

This method was first applied by McCormick27 and by Bywater and Worsfold11 to the system a-methylstyrene/poly-a-methyl-styrene, and the free energy, entropy and heat of polymerization as well as the ceiling temperature were determined. Similar studies concerned with the system styrene/polystyrene are being carried out in our laboratories. 

Fluorothioketones are more difficult to polymerize. There are two reasons. First, agents that promote polymerization also catalyze dimerization to dithi-etanes, which is a very fast reaction. Second, ceiling temperature of polymerization is low with the result that polymer decomposes back to monomer as it is being isolated. However, poly(hexafluorothioacetone) can be formed at very low temperatures by initiation with dimethylformamide or BF3 etherate, even though at — 78° C the only product isolated is 2,2,4,4-tetrakis(trifluoromethyl)-l,3-di-thietane. 

Ito has also extended this type of photochemistry to the electron-beam-induced catalytic acidolysis of acid-labile main chain acetal linkages in polyphthaldehyde. These polymers, like the poly(2-methylpentene-l-suIfone) (PMPS) sensitizer in NPR resist described earlier have ceiling temperatures on the order of -40 °C. As normally used, the polyaldehydes are end-capped by acylation or alkylation and are thus quite stable. The main chain bonds are very sensitive to acid-catalyzed cleavage which in turn allows the whole chain to revert to monomer in an unzipping sequence similar to that occuring in irradiated PMPS. Irradiation of polyphthaldehyde containing 10% of a suitable sensitizer such as triphenylsulfonium hexafluoroarsenate with either deep UV... 

Cyclopolymerization of dialdehydes was extensively studied by Aso and his coworkers (50). It was remarkable that o-phthalaldehyde could be polymerized readily (5Z-53), because aromatic aldehydes such as benzaldehyde, isophthalaldehyde and terephthalaldehyde did not polymerize with common ionic catalysts. In addition, the poly[o-phthal-aldehyde] obtained was composed of only cyclic structural units. These results suggested that the driving force for the polymerization of o-phthalaldehyde was apparently attributable to the formation of the five-membered ring in the course of cyclopolymerization. The ceiling temperature of the polymerization of o-phthalaldehyde was calculated to be — 43° C from the relationship between the equilibrium concentration of the monomer and the polymerization temperature (51,52). 

Synthesis of monomers for the preparation of the methyl ester polyisocyanides begin with a-amino acid esters. These polyisocyanides were developed to yield optically active polymers which could be characterized first as non-electrolytes, and, after hydrolysis, as polyelectrolytes in aqueous media. As predicted, poly[a(carboxymethyl) alkyl isocyanides] are soluble in various solvents. Unfortunately, a low ceiling temperature, some instability to alkali and especially to... 

Under most conditions, only the simple polypropylene ketone) is formed in propylene/carbon monoxide alternating copolymerisation. Isomerisation of poly(ketone) to poly(spiroketal) can occur, and it may be assisted by cationic palladium species and protonic acids. It must be emphasised that a low reaction temperature favours the formation of a spiroketal structure [107]. At a temperature above the ceiling temperature, the poly(spiroketal) depolymerises to the more flexible and entropically favoured poly (ketone) [481]. 

The most relevant early work in the context of this study is the radiation induced depolymerization of poly(phtalaldehyde) [9]. In this case, depolymerization is due to a ceiling temperature phenomenon whereby radiation induced cleavage of the polymer causes it to revert fully to monomer. Poly(phtalaldehyde) is a material with a very low ceiling temperature which is only rendered stable at room temperature through the device of capping its chain-ends after low temperature polymerization, thereby preventing its spontaneous degradation when heated. 

Anionic polymerization of EtG leads to a new synthetic biodegradable polymer, namely poly(ethyl glyoxylate). Because of a low ceiling temperature and transfer reactions, PEtG has hydroxyl ends that can be end-capped with phenyl isocyanate. 

Poly(2-methyl-1-pentene sulfone) (PMPS) is an alternating copolymer of 2-methyl-l-pentene (2MP) and sulfur dioxide. The formation of PMPS occurs only by a free radical polymerization mechanism and is complicated to a degree by ceiling temperature considerations. For all exothermic addition polymerization reactions there is a critical temperature called the ceiling temperature (Tc) above which no reaction occurs. The precise Tc depends upon the monomer concentration according to the expression (i)... 

For polymerization of tetrafluoroethylene, and A5 values at 25°C are given as —37 kcal/mol and —26.8 cal/°K-mol. Calculate the ceiling temperature (Tc) from these two values. Account for the fact that in practice poly(tetrafluoroethylene) is found to undergo fragmentation well below the calculated Tc-... 

Poly( y-mercapto acids). y-Thiobutyrolactone did not polymerize in bulk at 155°C with potassium ferf-butoxide as initiator (17). This is probably because of an unfavorable monomer-polymer equilibrium, resulting in a low ceiling temperature. It might be possible to overcome these difficulties by using lower polymerization temperatures or high pressure techniques, or both. A similar behavior has been reported for y-butyrolactone (18), which could be polymerized at elevated temperatures at 20,000 atm. It is likely that substituted y-thiolactones are even more difficult to polymerize. 

In a previous study, Bowmer and O Donnell (3) had found that radiation product yields were correlated to the ceiling temperature for a number of poly(olefin sulfone)s. Their results are summarized in Figure 5 which shows that the lower the Tc for a particular poly (olefin sulfone), the higher is the gas yield from irradiation, i.e., the lower the Tc the more strongly the right-hand side of the equilibrium ... 

G(S02) versus ceiling temperature (Tc) for a variety of poly (olefin sulfone)s. Figure 3 in reference 3. 

A widely studied commercially available polymer is poly(methylmethacrylate) (Fig. 14.2). It is a polymer that can be completely depolymerized by heating above the ceiling temperature (7c). It is possible only to achieve 100% monomer as product by laser irradiation with a C02 laser (2 9.6 or 10.6 pm) [71]. About 1% monomer can be detected in the ablation products after irradiation with 248 nm laser light, and about 18% monomer can be produced with 193 nm [71,72]. 

Though useful polymers can be made by these reactions, their low ceiling temperatures (see p. 599) and consequent tendency to undergo facile depolymerization by an unzipping mechanism pose serious limitations. To overcome this problem the technique of end-capping or end-blocking may be used. Thus poly-oxymethylene (polyacetal), an engineering plastic, prepared from the cyclic acetal... 

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