Thermal stability of copolymers

The chemistry of a wide variety of unsaturateo spiro ortho carbonates and unsaturated spiro ortho esters have been discussed in detail previously (14,19,24,27-X). They not only are useful for monomers that expaiu on polymerization Qa,32-21) but are useful as dental filling materials (38), for the syntHesTs of oligomers capped with hydroxyl groups, pro( ction of biodegradable polymers, and enhancement of thermal stability of copolymers (39-45) ... 

The thermal stability of copolymers of long-chained diol dimethacrylates was investigated [583]. These copolymers included 1,4-butane-, 1,5-pentane-, 1,6-hexane-, 1,8-octane-, 1,10-decane-, or 1,12-dodecanediol dimethacrylates, respectively, as well as 2,2-bis[4-(2-hydroxy-3-methacryloyl-oxypropoxy)-phenyl]propane and triethylene glycol dimethacrylate. The polymers were found to be thermally stable up to 250°C, as shown by the initial decomposition temperature and then-degradation profiles. 

Thermal Stability of Copolymers of Methyl Methacrylate and Dimethylene Spiro Orthocarbonate... 

It may not be appropriate to compare the thermal stability characteristics of VC/VAc copolymer to that of a VC homopolymer (PVC). The copolymerization would involve different kinetics and mechanism as compared to homopolymerization resulting structurally in quite different polymers. Hence, copolymerization of VC with VAc cannot be regarded as a substitution of chlorines in PVC by acetate groups. To eliminate the possibility of these differences Naqvi [45] substituted chlorines in PVC by acetate groups, using crown ethers (18-crown-6) to solubilize potassium acetate in organic solvents, and studied the thermal stability of the modified PVC. Following is the mechanism of the substitution reaction ... 

The effect of substitution of labile chlorines by acetoxy groups on the thermal stability of polymer taken in isolation appears to correspond to the thermal degradation behavior of VCA Ac copolymers over the whole composition range [136,137]. 

This further interesting effect of very low levels of acetoxylation on the thermal stability of PVC may be explained on a basis similar to that applied to the copolymers, with specific reference to labile chlorines taken in isolation. But this can only be done with a certain amount of caution. 

The observed reversal in the thermal stability of the copolymer at a critical composition, which appears to be between 30 and 40 mol% of ethylene, may be explained on the basis of the emergence of phase-separation between the nonpolar ethylene and polar vinyl chloride blocks. Although crystallization of the ethylene blocks in the copolymer is only observed when more than 70 mol% ethylene units are present, the possibility of phase-separation occurring at lower contents of ethylene units cannot be excluded. Also, round about the critical copolymer composition, the Tg of the copolymer may be reduced to a level that would facilitate separation between the unlike phases by increased molecular mobility within the polymer matrix. As has been discussed earlier, occurrence of phase-separation in the copolymer would not only make the mechanism of stabilization due... 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

In most of the studies discussed above, except for the meta-linked diamines, when the aromatic content (dianhydride and diamine chain extender), of the copolymers were increased above a certain level, the materials became insoluble and infusible 153, i79, lsi) solution to this problem with minimum sacrifice in the thermal properties of the products has been the synthesis of siloxane-amide-imides183). In this approach pyromellitic acid chloride has been utilized instead of PMDA or BTDA and the copolymers were synthesized in two steps. The first step, which involved the formation of (siloxane-amide-amic acid) intermediate was conducted at low temperatures (0-25 °C) in THF/DMAC solution. After purification of this intermediate thin films were cast on stainless steel or glass plates and imidization was obtained in high temperature ovens between 100 and 300 °C following a similar procedure that was discussed for siloxane-imide copolymers. Copolymers obtained showed good solubility in various polar solvents. DSC studies indicated the formation of two-phase morphologies. Thermogravimetric analysis showed that the thermal stability of these siloxane-amide-imide systems were comparable to those of siloxane-imide copolymers 183>. 

For example, by using 90 parts of vinyl chloride and 10 parts of vinyl acetate, the random copolymer formed has the toughness of poly (vinyl chloride), thermal stability of poly (vinyl acetate) and solubility akin to poly (vinyl acetate). These combination of properties makes it useful as a paint. 

ISO 305, Plastics - Determination of thermal stability of poly(vinyl chloride), related chlorine-containing homopolymers and copolymers and their compounds - Discoloration method, 1990. 

Table 5. Thermal Stability of Vinylidene Chloride/Methyl Acrylate (Five Mole Percent) Copolymers Generated Using Different Initiators...

In this instance the thermal stability of vinylidene chloride /alkyl acrylate copolymers in which the alkyl groups are isomeric butyl units has been examined by thermogravimetry. The butyl ester comonomers incorporated are shown below (scheme 7). 

JH NMR) studies confirmed the presence of the AB diblocks in the product. This determination was facilitated by the fact that the dendritic nitroxide could be differentiated from the nonnitroxide-bearing dendron by NMR spectroscopy. Careful analytical studies confirmed that the pure ABA copolymers could be separated by column chromatography and that the undesired diblock impurity resulted mainly from the loss of the dendritic nitroxide during the course of the reaction. Obviously, this approach to ABA triblocks has rather limited practical value since the thermal stability of the final product is quite low. 

From the Table IV, it also shows that the low styrene content in the copolymer may relate to the polymerization temperature. As the polymerization temperature was increased from 5° to 70°C, the styrene content of the butadiene-styrene copolymer decreased from 21.7% to 9.1%, respectively. The decreasing in styrene content at higher temperature is consistent with the paper reported by Adams and his associates (16) for thermal stability of "living" polymer-lithium system. In Adams paper, it was concluded that the formation of lithium hydride from polystyryllithium and polybutadienyllithium did occur at high temperature in hydrocarbon solvent. The thermal stability of polystyryllithium in cyclohexane is poorer than polybutadienyllithium. From these results, it appears that the decreasing in styrene content in lithium morpholinide initiated copolymerization at higher temperature is believed to be associated with the formation of lithium hydride. 

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