Acetate resistance

Alcohols protected as esters, and diols protected as cyclic acetals resist Jones oxidation. 

Protecting groups that are cleaved by an oxidant, like p-methoxyben-zyl201 and dimethoxybenzyl202 ethers or p-methoxybenzylidene203 and dimethoxybenzylidene204 acetals, resist the action of oxalyl chloride-activated DMSO. 

In cases where an isopropytidene acetal resists hydrolysis, the more labile p-methoxybenzylidene acetal can be used. The final steps of Mukaiyama s synthesis of Taxo 346 were impeded by difficulties associated with the esterification reaction by which the N-benzoylphenylisoserine side chain was attached to the Baccatin III core [Scheme 8.159]. Side chains protected as cyclic N 0-acetals were more reactive because of lower steric hindrance even so, the isopropyli-dene derivative of N-benzoylphenylisoserine (159.1) gave incomplete conversion (64%). Moreover, the isopropylidene group was resistant to hydrolysis. However, the corresponding p-methoxybenzylidene derivative (159.2) esterified the Baccatin III core (1593) in 95% yield and 93% conversion and it hydrolysed with aqueous trifluoroacetic acid at 0 cC to give Taxol in 93% yield. 

Cyclic acetals. Derivauz leads to 2-benzenesulfonylnietl cyclic acetals resist acid hydrol... 

Cyclic acetals. Derivatization of diols with this disulfone under basic conditions leads to 2-benzenesulfonylmethyl-l,3-dioxa cycles. In protection of carbohydrates, these cyclic acetals resist acid hydrolysis but can be cleaved by reducing agents. 

Figure 8. Comparison of HFIP/acetal resists showing 1 3 lines at a 100 nm.

Vinyl Vinyl chloride vinyl acetate resistance good chemical resistance Resistance to acids. Poor resistance to... 

By a genetic approach using an acetic acid-sensitive mutant of A. aceti A. pasteurianus), aarA and aarC were identified as an acetate-resistant gene cluster, and they were confirmed to be members of enzymes involved in the... 

The Curtms rearrangement has been used to prepare 5-aminothiazole (11) (60.61), 4-methyl-5-aminothiazole. 2-chloro-5-aminothiazole (58), and 2.4-dimethyl-5-aminothiazole (62) (Scheme 11). Heating the corresponding azides yield carbamates that resist hydrolysis but react with acetic anhydride to give the 5-acetylaminothiazoles. 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

The high crystallinity of acetal resins contributes significantly to their excellent resistance to most chemicals, including many organic solvents. Acetal resins retain their properties after exposure to a wide range of chemicals and environments. More detailed data are available (14). 

Grades of acetal resins specifically designed to enhance the excellent lubricity of the material, without sacrifice in other properties, are known. These include Delrin AF, Ultraform N2311, and Celcon LW materials. Antistatic grades, particularly useful in electronic appHcations, have been developed. At least one recendy introduced new grade, Celcon SR90, claims good scuff resistance. 

Under conditions of extreme acidity or alkalinity, acryhc ester polymers can be made to hydroly2e to poly(acryhc acid) or an acid salt and the corresponding alcohol. However, acryhc polymers and copolymers have a greater resistance to both acidic and alkaline hydrolysis than competitive poly(vinyl acetate) and vinyl acetate copolymers. Even poly(methyl acrylate), the most readily hydroly2ed polymer of the series, is more resistant to alkah than poly(vinyl acetate) (57). Butyl acrylate copolymers are more hydrolytically stable than ethyl acrylate copolymers (58). 

Pentaerythritol is used in self-extinguishing, non dripping, flame-retardant compositions with a variety of polymers, including olefins, vinyl acetate and alcohols, methyl methacrylate, and urethanes. Phosphoms compounds are added to the formulation of these materials. When exposed to fire, a thick foam is produced, forming a fire-resistant barrier (see Elame retardants) (84—86). 

Pentaerythritol in rosin ester form is used in hot-melt adhesive formulations, especially ethylene—vinyl acetate (EVA) copolymers, as a tackifier. Polyethers of pentaerythritol or trim ethyl ol eth an e are also used in EVA and polyurethane adhesives, which exhibit excellent bond strength and water resistance. The adhesives maybe available as EVA melts or dispersions (90,91) or as thixotropic, one-package, curable polyurethanes (92). Pentaerythritol spko ortho esters have been used in epoxy resin adhesives (93). The EVA adhesives are especially suitable for cellulose (paper, etc) bonding. 

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

Electrical Behavior. The resistivity of acetate varies significantly with humidity with typical values ranging from 10 ohm-cm at 45% rh to 10 ohm-cm at 95% rh (16). Because of the high resistivity both acetate and triacetate yams readily develop static charges and an antistatic finish is usually apphed to aid in fiber processing. Both yams have also been used for electrical insulation after lubricants and other finishing agents are removed. 

Chemical Properties. Under slightly acidic or basic conditions at room temperature, acetate and triacetate fibers are resistant to chlorine bleach at the concentrations normally used in laundering. 

Resistance to Microorganisms and Insects. Resistance of triacetate to microorganisms, based on soil-burial tests, is high, approaching that of polyester, acryUc, and nylon fibers. Sod-burial test results on acetate, triacetate, and cotton are shown in Figure 8. Neither acetate nor triacetate fiber is readdy attacked by moths or carpet beedes. 

Since PVA fiber as spun is soluble in water, it is necessary to improve the water resistance of the as-spun fiber (10). Heat treatment followed by acetalization is a classic method to provide high water resistance. 

After the washing, the fiber is dried, and then is heat-drawn in the same manner as in the case of dehydration—coagulation with salt but to a much higher draw ratio. As a result the finished fiber has high strength and modulus and is, without acetalization, sufficientiy resistant to boiling water. Figure 3 shows schematic fiber stmctures (17). 

Dimensional Stability. The wet heat resistance of PVA fiber is indicated by the wet softening temperature (WTS) at which the fiber shrinks to a specified ratio. At one time, the WTS was not more than 95°C for nonacetalized PVA fiber, but improvement of WTS has been achieved by improvement in heat-drawing and -treating techniques other methods proposed include suppression of polymerization temperature of vinyl acetate (36) and employment of alkafi spinning (37). 

---