Monomer thermal polymerization using cure

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

Characterization. Infrared spectra of bisdichloromaleimide monomers and polymers in KBr pellets were recorded, using a Perkin-Elmer 180 spectrophotometer. Elemental analyses were provided by Huffman Laboratories. Mass spectra were recorded at 70 eV on a Hewlett-Packard MS 5980 instrument by the direct inlet procedure. A DuPont 990 thermal analyzer was used to evaluate thermal behavior of bisdichloromaleimide monomers and polymers. Reduced viscosity of the polymers was determined in DMF at 30°C with a Cannon viscometer. Thermal polymerization was studied by heating a known weight of the material from room temperature to the desired temperature in a glass tube. The extent of curing was evaluated by extraction with DMF at room temperature. 

One way in which vinyl ether monomers may be used is as reactive diluents in epoxy systems. The addition of a vinyl ether greatly increases the cure speed of the system. The rapid change to a tack-free state appears to be the result of the polymerization of the vinyl ether component of the mixture. A "thermal bump" Is still necessary to cure the epoxy and bring the coating to a fully cured state(16). There is little evidence for any copolymerization of the vinyl ether and the epoxy groups. 

Thermal or photo-polymerization of multi-functional low molar mass monomer results in a high, densely crosslinked network. Liquid crystallinity of monomers or precursors can be frozen in after in situ polymerization. Interestingly, some monomers are not liquid crystalline themselves however, as the curing reaction proceeds, a nematic phase is developed and finally remained. Photopolymerization is divorced from the thermal properties of the material, while thermal polymerization is limited by the finite temperature range over which the liquid crystal phase can exist. But photopolymerization can be only suitable for thin film sample due to the limitation of the penetrating ability of the light used. 

Bisimide polymers were prepared by the thermal polymerization of the olefinic double bonds. A known weight of the monomer was heated in a glass tube (which had a stopper and a side arm) from room temperature to the desired temperature and maintained at that temperature for a known period of time. The cured resin was then taken out and cooled to room temperature. Solubility in DMF was used as a criterion of cross-linking reaction. 

PEKs with pendant thermolabile substituents allowing for thermal cure were studied by two research groups. Polycondensations involving nucleophilic substitution steps were used in all cases. However, in the first case a thermolabile electrophilic monomer (144a) was used [230], whereas alkine substituted diphenols (144b) served as thermolabile monomers in the second study [231]. Finally, a paper dealing with the grafting of anionically polymerized styrene (145) on a bisphenol-A PEK (146) should be mentioned [232]. 

The exact mechanism of acetylene group thermal polymerization is still not clear. It is evident that the adamantane ring stays intact upon polymerization and contributes to the high thermal stability of the cured hydrocarbon networks. We hope to use C labeled monomers to further investigate cure mechanism details. 

The present study reports the synthesis, characterization and thermal reactions of phenyl and carbomethoxy substituted norbornenyl imides. These substrates were designed to model the reactive end-caps of the PMR-15 resin and allow an assessment of the effect that conjugating substituents would have on the high temperature cure of such systems. The effect of these substituents on both monomer isomerization and polymerization is reported and a possible use of the phenyl substituent as a probe of polymer structure is suggested. 

All of the monomers in Fig.46 were polymerized into neat resin castings using a thermal cure cycle with a maximum temperature of 250 °C. The polymers were then subjected to preliminary thermal and mechanical property screening and a summary of their physical properties is shown in Table 21. All of the polymers had Tgs above 200 °C (DSC) and 5% weight losses (TGA) in the range between 430 and 470 °C (air and nitrogen). Room temperature flexural moduli and strengths were 3.10-3.52 GPa and 152-207 MPa respectively. 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

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