Polymerization with acid catalysis

Vinyl-type addition polymerization can also be carried out with acidic catalysis such as boron tnfluoride or tin tetrachloride and with basic catalysis such as alkali melals or alkali alkyls. An example of the first ease is the low-temperature polymerization of isobutene, which gives Vistanex" and butyl rubber an example of the second type is the polymerization of butadiene with sodium, which leads to buna rubber. 

An important question to be explained by the chemical mechanism is why base catalysis often leads to condensed structures. In 1950, Aelion et al. (13) pointed out that the condensation reaction in base-catalyzed systems was faster than with acid catalysis and that the microstructure of the final product was different. Many of these facts are due to the opposite effects of, for example, substituents, on silicon on the stabilization of the transition state in base- and acid-catalyzed reactions (15). For the base-catalyzed condensation reaction to take place, a silicon atom has to be attacked by a deprotonated silanol oxygen the nucleophile. The acidity of the silanol proton increases as the basicity of the other groups bonded to the silicon decreases. This feature implies that poly silicic acid is a stronger acid than Si(OH)4 (20). Therefore, monomers react preferentially with higher polymerized species. 

Polymerization Chemistry. The amine groups of urea react very readily with formaldehyde, forming methylol nreas (Fig. 3.9). The A-stage reaction is controlled by the urea/formaldehyde ratio (1/1.3 to 1/2.2), an alkalme buffer at pH 7.5-8.0, and refluxing up to 8 hr, to produce a mixture of mono-, di-, and trimethylol ureas. These condense to form oligomers and Anally, with acid catalysis and heat, highly cross-linked thermoset polymers. 

The flow sheet of the technical process is shown in Figure 13.4. For a comprehensive description of critical steps of this technology, see Refs. 14 and 15 the main points are control of the solid-liquid reaction between the glucose and the fatty alcohol with acidic catalysis avoiding a fast polymerization of glucose, the good water removal, the economic separation and recycle of the excess fatty alcohol, and the treatment of the highly viscous APG melt to get the desired product quality with low polydextrose content. 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

The other class of acrylic compatible tackifiers includes those based on ter-penes. Terpenes are monomers obtained by wood extraction or directly from pine tree sap. To make the polyterpene tackifiers, the monomers have to be polymerized under cationic conditions, typically with Lewis acid catalysis. To adjust properties such as solubility parameter and softening point, other materials such as styrene, phenol, limonene (derived from citrus peels), and others may be copolymerized with the terpenes. 

Indole itself forms a dimer or a trimer, depending on experimental conditions the dimer hydrochloride is formed in aprotic solvents with dry HCl, whereas aqueous media lead to dimer or trimer, or both. It was Schmitz-DuMont and his collaborators who beautifully cleared up the experimental confusion and discovered the simple fact that in aqueous acid the composition of the product is dictated by the relative solubilities of the dimer and trimer hydrochlorides/ -This, of course, established the very important point that there is an equilibrium in solution among indole, the dimer, the trimer, and their salts. It was furthermore demonstrated that the polymerization mechanism involves acid catalysis and that in dilute solution the rate of reaction is dependent on the concentration of acid. 

The mechanism of chemical modification reactions of PS were determined using toluene as a model compound with EC in the presence of BF3-0(C2H5)2 catalyst and the kinetics and mechanism of the alkylation reaction were also determined under similar conditions [53-55]. The alkylation reaction of toluene, with epichlorohydrin, underwent polymerization of EC in the presence of Lewis acid catalysis at a low temperature (273 K) as depicted in Scheme (9). 

Acid anhydride-diol reaction, 65 Acid anhydride-epoxy reaction, 85 Acid binders, 155, 157 Acid catalysis, of PET, 548-549 Acid-catalyzed hydrolysis of nylon-6, 567-568 of nylon-6,6, 568 Acid chloride, poly(p-benzamide) synthesis from, 188-189 Acid chloride-alcohol reaction, 75-77 Acid chloride-alkali metal diphenol salt interfacial reactions, 77 Acid chloride polymerization, of polyamides, 155-157 Acid chloride-terminated polyesters, reaction with hydroxy-terminated polyethers, 89 Acid-etch tests, 245 Acid number, 94 Acidolysis, 74 of nylon-6,6, 568... 

So far, various dicarboxylic acid derivatives, dicarboxylic acids, their activated and non-activated esters, cyclic acid anhydrides, and polyanhydrides have been polymerized with glycols through lipase catalysis to give polyesters. 

The gel point is defined as the point at which the entire solid mass becomes interconnected. The physical characteristics of the gel network depends upon the size of particles and extent of cross-linking prior to gelation. Acid-catalysis leads to a more polymeric form of gel with linear chains as intermediates. Base-catalysis yields colloidal gels where gelation occurs by cross-linking of the colloidal particles. 

Z- Configuration is typical of the majority of a-aryl(hetaryl)-/V-alkylaldo-nitrones. The isolation of -isomers in the condensation of aromatic aldehydes with iV-j3-]ihenyletli Tliydroxylamine has been described (155). The synthesis of a, N -diary lnitrones gives best results if acidic catalysis is employed (156), or when clay is used as a catalyst (157). Significant reduction of reaction time and increase in the yields of nitrones can be achieved if microwave irradiation is used (158, 159). On the basis of polymeric arylaldehydes, the synthesis of polymeric a,-diarylnitrones has been described (160). 

One process that may offer benefits of acid catalysis without the drawbacks of H2S04 is the use of carbonic acid. The pH of carbonic acid is determined by the partial pressure of C02 in contact with water, and thus it can be neutralized by releasing the reactor pressure. Carbonic acid is relatively mild and hence does not offer the same hydrolytic capability of H2S04. However, van Walsum (25) has demonstrated that at temperatures on the order of 200°C, carbonic acid does exhibit a catalytic effect on the hydrolysis of xylan. Van Walsum (25) observed an enhanced release of xylose and low-degree-of-polymerization xylan oligomers compared to pretreatment... 

The labeling of furan (Table XII) is similar to thiophene, since the most active of the borohydride-reduced catalysts is iridium. However, self-activated iridium oxide is the most active of all for general deuteration, whereas nickel chloride is the most efficient and selective for the a. position, only slight polymerization being observed. Acid catalysis should be used with caution for exchange labeling, since furan readily polymerizes above 30° in the presence of hydrochloric acid and chlorides. 

MF5 and MCI5 are strongly electrophilic see Electrophile and Electrophilic Reaction) and catalyze Friedel-Crafts reactions. The HF/TaFs system is a superacid catalyst and has been used in the selective acid-catalyzed isomerization and hydrogenolysis of cycloalkanes. Oligomerization and polymerization of alkynes with Nb and Ta halides as catalysts have been reported see Oligomerization Polymerization by Homogeneous Catalysis) ... 

The rate equation with strongly acidic catalysts is also second order in silanol and first order in catalyst (75). The mechanism is proposed to proceed via protonation of silanol, followed by an electrophilic attack of the conjugate acid on nonprotonated silanol. The condensation processes outlined in reactions 16a and 16b for sulfonic acids is also an applicable mechanism for the acid catalysis. The condensation polymerization in emulsion catalyzed by dodecylbenzenesulfonic acid is second order in silanol, but the rate has a complex dependence on sulfonic acid concentration (69). This process was most likely a surface catalysis of the oil-water interface and was complicated by self-associations of the catalyst-surfactant. 

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