Which polymerize or decompose

TABLE 11.52 Chemicals Which Polymerize or Decompose on Extended Refrigeration... 

Chemical vapor deposition (CVD) is a chemical process used to produce high-purity and high-performance soUd materials. In a typical CVD process, the wafer (substrate) is exposed to one or more volatile precursors, which react or decompose on the substrate surface to produce the desired deposit. The CVD method is widely used in microfabrication processes to deposit materials in various forms through polymerization of functionalized [2.2] paracyclophanes. The CVD method is also used to make reactive coatings for PDMS. Figure 10.7 shows one of these processes used for PDMS surface modification. 

One of the earliest examples of this methodology involves the reaction of a polymeric anion (formed by living anionic polymerization) with molecular oxygen to form a polymeric hydroperoxide which can be decomposed either thermally or, preferably, in a redox reaction to initiate block polymer formation with a second monomer (Scheme 7.25). However, the usual complications associated with initiation by hydroperoxides apply (Section 3.3.2.5). 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

The reaction takes place in CS2 at —78 °C (yield 35 %). Sjq forms intense yellow crystals which polymerize at about 60 °C If only small amounts of Sj, are needed and Sg and S7 are available, the oxidation of either one with trifluoroperoxy-acetic acid provides Sm in a reaction of unknown mechanism. The homocyclic oxides SgOj and S7O decompose at 5 °C in CSj or CHjClj solution within several days to give SjQ, insoluble sulfur and SO ... 

The free radical polymerization is probably initiated by the reaction of the peroxide with a metal—carbon bond which has been modified through complexation, solvation, or even chemical interaction with a proper monomer. This "site is interacted with the peroxide molecule, which is then decomposed in a metal-catalyzed manner to form a free radical terminus on the polymer chain along with an inert metal—peroxide interaction product. Whether the metal in question is aluminum, titanium, or a complex of the two is uncertain since the mechanism of Ziegler type reactions is still uncertain and since all three have been found in separate studies to promote the polymerization of methyl methacrylate in the presence of peroxides. However, the complex between AlEt>Cl and TiCl3 has been observed to have a much greater effect in accelerating the polymerization of methyl methacrylate than either component by itself hence, the complex appears to be the most likely species. 

Catalysts comprising a metal-carbon double bond (metallocarbenes, or metallocenes) are efficient. With these initiators, the polymerization mechanism appears to involve coordination of the C=C double bond in the cycio- or dicycloalkene at a vacant d orbital on the metal. The metallocyclobutane intermediate which is formed decomposes to produce a new metal carbene and a new C=C bond. Propagation consists of repealed insertions of cycloalkenes at the metal carbene. 

Viscose Silk.—III. From Cellulose Xanthate. We have referred to the solvent action of xanthic acid, which is the ethyl ether of di-thio-carbonic acid, viz., HS-CS-OC2H5. When sodium cellulose is dissolved in xanthic acid the cellulose is in the form of sodium cellulose xanthate. A solution properly prepared by treating cellulose with sodimn hydroxide and carbon di-sulphide in the presence of benzene or carbon tetra-chloride, in which polymerization of the cellulose compound is effected, is decomposed by forcing capillary streams of the solution into a solution of ammonium sulphate. The cellulose is thus obtained as in the other processes in the form of fine filaments of a hydrated cellulose possessing silk-like properties. Artificial silk of this type is known as viscose silk and is made in large quantities. In 1914 about 20,000,000 pounds of artificial silk were made, of which about 3,000,000 pounds were made in the United States. Most of this product was viscose silk. 

A sequence of H-abstractions due to the very reactive chlorine radical and / -decomposition gives rise to the formation of polyene molecules and HC1 which is released from the melting phase. Cross-linking reactions between polyenes lead to alkyl aromatic intermediates which can further decompose, releasing tar compounds and/or can polymerize with char formation. These transformations are experimentally observed as a second weight loss in PYC thermo gravimetric analysis. 

The exchange reaction between ethylzinc methoxide and enol acetates affords ethylzinc enolates, which decompose by polymerization or by reaction with the methyl acetate produced in the exchange reaction (equation 40). ... 

Classification by Decomposition Behavior. The decomposition mechanism is a reasonable way to classify polymers. They can either depolymerize upon irradiation, for example, poly(methylmethacrylate), or decompose into fragments such as poly-imides or polycarbonates. This method of classification is closely related to the synthesis of polymers. Polymers that are produced by radical polymerization from monomers, which contain double bonds, are likely to depolymerize into monomers, while polymers that have been formed by reactions like polycondensation will not depolymerize into monomers upon irradiation, but will be decomposed into different fragments. The second group cannot be used to produce films with the same structure or molecular weight as the original material with methods such as PLD. 

Some substances which polymerize readily or decompose spontaneously are generally encountered in stabilized form, the form under which they are listed in Annex I. Such substances are occasionally marketed in non-stabilized form however, In which case the manufacturer, or other person marketing them, must state the name of the substance on the label, followed by the designation (not stabilized) . 

Many solvents and their solutes can decompose, polymerize or react very rapidly with oxygen or water, thereby creating a cloud of gas or vapour. If confined, this vapour will cause a high pressure, which may lead to the confining vessel bursting. 

Usually the monomer (as the starting compound is called) is liquid and can be polymerized in bulk or in solution. The initiator of polymerization is not oxygen, but some compound which, on heating, decomposes into free radicals (e.g. organic peroxides. Chapter 16). The growth of the polymer chain is terminated by one of the following occurrences ... 

Organic peroxides n. Peroxides used in the plastics industry are thermally decomposable compounds analogous to hydrogen peroxide in which one or both of the hydrogen atoms are replaced by an organic radical. As they decompose, they form free radicals, which can initiate polymerization reactions and affect cross-linking. 

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