Monomer stabilization thermal polymerization

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

The heat of reaction for vinyl polymers affects the thermal stability of the polymer during extrusion, and the thermal stability is related to the ceiling temperature. The ceiling temperature is the temperature where the polymerization reaction equilibrium is shifted so that the monomer will not polymerize, or if kept at this temperature all the polymer will be converted back to monomer. From thermodynamics the equilibrium constant for any reaction is a function of the heat of reaction and the entropy of the reaction. For PS resin, the exothermic heat of reaction for polymerization is 70 kj/gmol, and the ceiling temperature is 310 °C. Ceiling temperatures for select polymers are shown in Table 2.5. 

Materials. Styrene (BDH Chemicals Ltd.) was stabilized with 0.002 % t-butyl catechol. The stabilizer was removed by washing successively with 10 % potassium hydroxide solution and water, drying over calcium chloride for 24 hr, and vacuum distilling. The monomer was kept in a refrigerator until required. Effective removal of inhibitor was checked by gas chromatography and by dilatometric measurement of the rate of bulk thermal polymerization at 60 °C this was 0.080 % hr which compares with a literature value (17) of 0.089 % hr . ... 

Azabicyclo[2.2.1]heptan-3-one (75) is more strained and therefore should be more reactive than the bicyclic lactams described in the foregoing sections. The monomer 75 was polymerized in bulk or in solutions in dimethyl sulfoxide and tetrahydrofuran at temperatures between 40 and 120 °C. [75] The resulting polyamide (76) having cyclopentane rings in the main chain has good thermal stability at temperatures up to 300 °C. Its melting point and decomposition temperature are about 307 and 335 °C, respectively. 

Most Grignard reagents are inert toward styrene (up to the temperature of spontaneous thermal polymerization). This is a significant difference from lithium alkyls, which are readily able to initiate styrenic monomers [123]. The only reported exception is p-vinylbenzyl magnesium chloride, which polymerized styrene in THF at O C, but not at — 78X [50,51]. Substitution at the puru-position of a phenyl ring may stabilize the benzyl anion, owing to the delocatlization of electrons, and favor ionic dissociation of... 

Phenol, the simplest and most important phenolic compound in industrial fields, is a multifunctional monomer for oxidative polymerization, and hence, conventional polymerization catalysts afford an insoluble product with uncontrolled structure. On the other hand, the peroxidase catalysis induced the polymerization in aqueous organic solvent to give a powdery polymer consisting of phenylene and ox-yphenylene units showing relatively high thermal stability (Scheme 2).5,6 In the HRP and soybean peroxidase (SBP)-catalyzed polymerization in the aqueous 1,4-dioxane, the resulting polymer showed low solubility the polymer was partly soluble in N,N-dimethylformamide (DMF) and dimethyl sulfoxide and insoluble in other common organic solvents.5 On the other hand, the aqueous methanol solvent af-... 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

Chloral, CCI3CHO, can be polymerized cationically or anionically. Phosphine and lithium t-butoxide are especially suitable initiators for anionic polymerization, whereas tertiary amines produce poly(chlorals) of lower thermal stability. The polymerization is first initiated above the ceiling temperature (58°C) and is subsequently allowed to proceed well below the ceiling temperature however, yields of only 75-80% are obtained because of the unfavorable polymerization equilibria. Since the unconverted monomer cannot be removed completely by heating, heating must be supplemented by extraction to remove the residual monomer. 

Thermal polymerization kinetics that obey the Diels-Alder mechanism were studied. Expressions for rate of propagation and molecular weight were derived. The rate of propagation varies as the cube of monomer concentration. Subcritical damped oscillations can be expected under certain conditions. Reaction in a circle representation of free radical reactions was used to analyze the stability of reactions. Oscillations in concentration were discussed for systems of three and four reactions in a circle and the results generalized to n reactions in a circle. 

Butadiyne, H-CSC-C C-H, as a polymerizable monomer, has received very little attention from polymer chemists although its discovery dates back to Bayer in 1885. This structurally simple, highly reactive bifunctional molecule would be expected to have been a monomer of considerable interest in the field of polymer chemistry. Possibly, limited butadiyne stability may account for the small amount of polymerization research. The The compound is a liquified gas at room temperature (BP = 10 C), discolors slowly in sealed vessels at 20 C and may explode if heated. Storage and instability problems may be circumvented. Prevention of explosion may e accomplished by addition of an inert diluent such as butane. The monomer may also be stored in t e form of a labile complex with N-methyl-pyrrolidone. Its thermal condensation or polymerization was briefly recorded as an observation by Bayer and described in a little more detail by Miiller in 1925. Prevention of this thermal polymerization has been the subject of several patents with methylene blue, pyridine and vinylpyridine claimed as inhibitors. 

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