Silicones addition polymerization

Webster OW, Hertler WR, Sogah DY, Eamham WB, Rajanbabu TV (1983) Group-transfer polymerization. 1. A New concept for addition polymerization with organo-silicon initiators. J Am Chem Soc 105 5706-5708... 

Triethylsilyl acrylate can be induced lo undergo hydrolysis of the ethoxyl radicals to a desired extent forming linear or cross linked polymers. Addition polymerization will also lake place on the double bond of the acrylate radical. More stable monomers result from the use of allyl or vinyl groups instead of acrylates. The latter contain a silicon-oxygen-carhon linkage which is always more or less susceptible to hydrolysis. 

Borane polymers were prepared by the Rohm and Haas Corp. by using the Friedel-Crafts addition to an olefinic linkage followed by a typical silicone hydrolytic polymerization. 

Materials Synthesis and Characterization. In addition to the requirements of etching resistance, sensitivity, solubility and high glass transition temperature (Tg), one of the criteria used in designing both a negative and positive electron-beam resist system was synthetic simplicity. The trimethylsilylmethyl appendage allows the incorporation of silicon into polymeric resists without adverse synthetic complications. Standard free radical or condensation polymerization techniques can be employed with appropriately substituted monomers that are readily available. 

The addition polymerization of silicone-thermoplastic semi-IPNs is ideal... 

Use Stabilizing and rust-inhibiting agent for silicones, lubricant and fuel additive, polymerization catalyst, hydrogen chloride scavenger. 

FIGURE 8.4 Silicone rubber from addition polymerization. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Silicone elastomers are either room-temperature vulcanization (RTV) or heat-cured silicone rubbers, depending on whether cross-linking is accomplished at ambient or elevated temperature. [The term vulcanization (see Chapter 1 and Chapter 2) is a synonym for cross-linking. While curing is also a synonym for cross-linking, it often refers to a combination of additional polymerization plus cross-linking.] RTV and heat-cured silicone rubbers typically involve polysiloxanes with degrees of polymerizations of 200-1500 and 2500-11,000, respectively. 

A second class of silicones cures by addition polymerization initiated by a catalyst Polymerization occurs by a free-radical mechanism involving a vinyl, allyl, or other unsaturated group of a silicone monomer (Figure 3.9). Homopolymers can be formed... 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

The root canal sealers based on silicone are a variation on the addition-polymerized silicone used clinically as an impression material [62]. They have been modified to improve their flow, but their essential chemistry is the same. They consist of two-paste systems comprising a base and a catalyst paste. The silicone components are a polydimethylsiloxane polymer blended with paraffin oil and filled with finely divided zirconium dioxide. This latter substance both reinforces the set silicone material and confers radiopacity. 

Storage Store in cool, dry place keep tightly closed Uses Base polymer for addition cure silicone elastomers polymerized prods, provide noncorrosive shield against moisture, dirt, and contaminants for elec, components mold-making elastomers, encapsu-lants, and sealants for elec, components ManufJftistrib. Gelest... 

The -Si-O-Si-O- backbone of silicones is referred to as siloxane. The silicon atoms may be linked to a wide variety of aliphatic or aromatic groups, as shown in Fig. 3.6 where the R groups are commonly methyl (-CH3), phenyl (C6H3-), allyl (-CH2-CH=CH2), or vinyl (-CH=CH2). Silicones used in electronic assembly and packaging may be either room-temperature vulcanizing (RTV) that cure by condensation polymerization or vinyl types that cure by addition polymerization initiated by a catalyst. Table 3.4 lists some formulations for each type. 

Commercially, silicones are thus not produced by polymer analog reaction, but are exclusively manufactured by the polycondensation or addition polymerization of low-molar-mass compounds. In addition to the poly(dialkyl siloxanes), the polycondensation of dialkyl silane dichloride with... 

Liquid injection molding (LIM) A process of injection-molding thermosetting resins in which the uncured resin components are metered, mixed, and injected at relatively low pressures through nozzles into mold cavities, the curing or polymerization taking place in the mold cavities. The process is most widely used with resins that cure by addition polymerization such as polyesters, epoxies, silicones, alkyds, diallyl phthalate, and (occasionally) urethanes. 

ROMP, ROP, AND ADDITION POLYMERIZATION OF SILICON-CONTAINING CYCLIC MONOMERS AS A WAY TO NEW MEMBRANE MATERIALS... 

Keywords ROMP, addition polymerization, silicon containing norbomenes, silacyclobutanes, disilacyclobutanes. 

Depending on the substituents on the silicon, the polymerization can achieve conversions up to 90% [18, 19]. Additionally, there is no control over the product molecular weight, which varies from below 1,000 to several million grams per mole, having polydispersities from 1.5 up to 10 for the separated fractions [19]. 

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