Thermal polymerization inhibitors

Small amounts of additional components may be used in the photopolymer-izable compositions. Dyes, thermal polymerization inhibitors, pigments, adhesion promoters, such as oiganosilane coupling agents, and plasticizers are often employed. Coating aids, such as polyethylene oxides may also be... 

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

The dehydrogenation of the mixture of m- and -ethyltoluenes is similar to that of ethylbenzene, but more dilution steam is required to prevent rapid coking on the catalyst. The recovery and purification of vinyltoluene monomer is considerably more difficult than for styrene owing to the high boiling point and high rate of thermal polymerization of the former and the complexity of the reactor effluent, which contains a large number of by-products. Pressures as low as 2.7 kPa (20 mm Hg) are used to keep distillation temperatures low even in the presence of polymerization inhibitor. The finished vinyltoluene monomer typically has an assay of 99.6%. 

Myrcene Manufacture. An important commercial source for mycene is its manufacture by pyrolysis of p-piaene at 550—600°C (87). The thermal isomerization produces a mixture of about 75—77 wt % myrcene, 9% limonene, a small amount of T -limonene [499-97-8] and some decomposition products and dimers. The cmde mixture is usually used without purification for the production of the important alcohols nerol and geraniol. Myrcene may be purified by distillation but every precaution must be taken to prevent polymerization. The use of inhibitors and distillation at reduced pressures and moderate temperatures is recommended. Storage or shipment of myrcene in any purity should also include the addition of a polymerization inhibitor. 

The inhibitors more commonly used are molecules which in one way or another react with active chain radicals to yield product radicals of low reactivity. The classic example is benzoquinone. As little as 0.01 percent causes virtual total suppression of polymerization of styrene or other monomers. This is true of both thermal and initiated polymerizations. Results of Foord for the inhibition of thermal polymerization of styrene by benzoquinone are shown in Fig. 22. The... 

Fig. 22.—Inhibition of the thermal polymerization of styrene at 90°C by benzoquinone. The log of the viscosity relative to that of pure monomer is here used as a measure of polymerization. The small induction period in the absence of quinone presumably was caused by spurious inhibitors present in the monomer. (Results of Foord. )...

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 . ... 

Among the emerging pollutants of industrial origin, Bisphenol A [2,2 bis(4-hydroxydiphenyl)pro-pane] (BPA) has special relevance since it was one of the first chemicals discovered to mimic estrogens as endocrine disrupters.147 This compound was first reported by Dianin in 1891.1411 BPA is produced in large quantities worldwide, mainly for the preparation of polycarbonates, epoxy resins, and unsaturated polyester-styrene resins.149 The final products are used in many ways, such as coatings on cans, powder paints, additives in thermal paper, in dental composite fillings, and even as antioxidants in plasticizers or polymerization inhibitors in polyvinyl chloride (PVC). To a minor extent, BPA is also used as precursor for flame retardants such as tetrabromobisphenol A or tetrabromobisphenol-S-bis(2,3-dibromopropyl) ether.150 This substance can enter the environment... 

MU2 Muller, C. and Oellrich, L.R., The influence of different inhibitors and ethylenes of differerrt origin on the location of cloud points due to thermal polymerization of ethylene, Acta Polym., 47, 404, 1996. 

Adur A.M., Williams F., Effects of inhibitors on the thermal polymerization of beta-pinene, J. Polym. Sci. C Polym. Lett., 19(2), 1981,53-57. 

The use of phenolic and amino-based antioxidants (ie, thermal stabilizers) by this approach has been limited because they inhibit the free-radical polymerization process (polymerization inhibitor) leading to lower efficiency. One of the few commercial products produced is based on the polymerizable chain breaking antioxidant (AO 12b, Table 3), designed for NBR rubbers (Chemigum HR 665) that has been shown to offer superior antioxidant performance, especially imder aggressive (hot oil/high temperature) conditions, compared to low molecular mass conventional aromatic amine antioxidants (165). In spite of the successful synthesis and copoljmierization of a large number of reactive antioxidants, there is a lack of major commercial development and production of antioxidant systems... 

The kinetic behavior of PMS in thermal and peroxide-initiated polymerizations was compared to that of styrene. The isothermal polymerization rates for PMS and styrene were measured over the conventional thermal polymerization temperature range of 110 C to 150 C. The conversions for inhibitor-free monomers were monitored as a function of time and were very similar (Figure 3). The effects of solvent on polymerization rates were also measured and found to be similar. In 10% solvent, either ethylbenzene or para-ethyltoluene, the conversion rates are about 12% lower than without solvent for both styrene and PMS. The molecular weights and... 

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. 

Once unsaturated polyesters are synthesized, they are diluted in reactive diluents that are capable of polymerizing thermally. Therefore, care needs to be taken at this stage to keep the temperature as low as possible. As an additional precaution during thinning, as well as to have better stability on storage, a polymerization inhibitor such as p-tert-butylcatechol or hydroquinone are necessarily added to the monomers in the thinning tank before resin dissolution. 

More recently, the polymerization of several cyclic disulfides has been investigated by Endo et d. The thermal polymerization of 1,2-dithiane (DT) did not proceed at monomer concentrations below 4.0 mol 1" The polymerization was inhibited by addition of radical inhibitors, indicating that the propagation proceeds by a radical intermediate. The molecular weight of the polymers increased with reaction time. It was proposed that the cyclic polymer is formed mainly by back biting reaction mechanism during the polymerization (Scheme 36). ... 

For example, in the case of thermal polymerization of styrene [1], benzoquinone acts as an inhibitor. When the inhibitor has been consumed, polymerization regains its momentum and proceeds at the same rate as in the absence of the inhibitor. Nitrobenzene [1] acts as a retarder and lowers the polymerization rate, whereas nitrosobenzene [1] behaves differently. Initially, nitrosobenzene acts as an inhibitor but is apparently converted to a product which acts as a retarder after the inhibition period. Impurities present in the monomer may act as inhibitors or retarders. The inhibitors in the commercial monomers (to prevent premature thermal polymerization during storage and shipment) are usually removed prior to polymerization or, alternatively, an appropriate excess of initiator may be used to compensate for their presence. 

U.S. 4,168,982. Photopolymerizable Compositions Containing Nitroso Dimers to Selectively Inhibit Thermal Polymerization. Pazos, Jose F. (E. I. Du Pont de Nemours and Company). September 25, 1979. Cl. 430/281.1 430/917 522/16 522/18 522/28 522/63 522/65 522/76 522/121 522/167 Appl. December 7, 1977. Thermally stable photo-polymerizable compositions comprise (i) at least one nongaseous ethylenicaUy unsaturated compound, (ii) a nitroso dimer which is a noninhibitor of free-radical polymerization but thermally dissociates to nitroso monomer which is an inhibitor of free-radical polymerization, and (iii) an organic, radiation-sensitive free-radical generating system. 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

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