Silicones condensation polymerization

Condensation polymers commercial, 20 392-393t phenol-formaldehyde, 70 409 Condensation polymerization, of silicone fluids, 22 573... 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45],... 

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 ADMET step condensation polymerization of carbosila- and carbosiloxa-dienes has generated a new class of unsaturated silicon-containing polymers when monomer structure/reactivity relationships, as described earher for the polar functionalities, are obeyed (equation 22). 

The synthesis of oligomeric peralkylated polyaminophosphazenes based upon the P3 and P4 templates is an elegant (but complicated), hazardous, and expensive multi-step process. To exploit catalysts of this type in silicone synthesis on an industrial scale would require a much simplified and lower-cost synthetic protocol. One of the simplest and lowest-cost routes to a conjugated -P=N- template that forms the framework for the synthesis of phosphazene base materials is via phosphonitrilic chloride oligomers (Fig. 3). These are well-known acidic catalysts used in the silicone industry for the condensation polymerization of silanol-terminated polydimethylsiloxanes. Catalysts of this type are most commonly prepared by the reaction of PCI5 with NH4CI or HMDZ,... 

The chemical reactions that occur when water and TEOS are dissolved in ethanol are hydrolyzation and condensation polymerization. The solution is activated once the water reacts with alkoxy groups on the silicon to form hydroxyl groups and alcohol. This hydrolyzation produces complex silanols and ethanol with TEOS, but this never goes to completion. That is the water is not used up to form silicic acid. Instead condensation polymerization takes partially hydrolyzed units and makes larger units with bridging oxygens. This condensation polymerization regenerates water. 

For a nonmolecular sihcon source, silica is obtained as a gel formed from a non-homogeneous solution and subsequently treated hydrothermally. Silica can be prepared, for instance, by acidification of a basic aqueous solution, and when the reaction conditions are properly adjusted, a porous silica gel is obtained. Most frequently, two types of chemical reactions are involved silicate neutralization producing silicic acids, followed by condensation polymerization of the silicic acid species. In the case of molecular silicon sources, solvent and catalyst are usually first combined to form a homogeneous solution to which a silicon alkoxide is then added. In both... 

Both the heat-cured and RTV silicones are formed by condensation polymerization and are sometimes referred to as alkoxy- or acetoxy-cured systems. The basic hydroxysilane starting materials are produced by the hydrolysis of mixtures of mono-, di-, and trichlorosilanes as depicted in Figure 3.7. " Condensation with elimination of water produces hydroxyl-terminated alkylpolysiloxanes. These resins can be further polymerized in the presence of alkoxysilanes such as propylorthosilicates to yield the RTV silicones (Figure 3.8). Catalysts such as organotitanates or dibutyltindilaurate may accelerate the curing process. 

Silicone rubber, developed by Dow Corning company, is one of the few polymers developed for medical use. The repeating unit is dimethyl sUoxane which is polymerized by a condensation polymerization. Low molecular weight polymers have low viscosity and can be cross-linked to make a higher molecular weight, rubber-like material. Medical grade silicone rubbers contain stannous octate as a catalyst and can be mixed with a base polymer at the time of implant fabrication. 

Rapid, spontaneous condensation polymerization then yields a silicone polymer. 

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. 

Condensation-cured silicones. Both the heat-cured and RTV silicones are formed by condensation polymerization and are sometimes referred to as alkoxy- or acetoxy-cured systems. The basic hydroxysilane... 

Supersaturated solutions of silicic acid in pure water are thermodynamically unsta ble because condensation polymerization through dehydration takes place. All higher polymers of whatever size, molecular weight, or state of hydration can be represented by a general formula containing n silicon atoms. The polymerization of additional monomer molecules or the deposition of silica can be represented as follows ... 

The simple organosilicon derivative (CH3)2SiCl2, which can be prepared by several methods, serves as a starting point synthesis of an important class of polymers called silicones. Hydrolysis of (CH3)2SiCl2 followed by condensation-polymerization results in long-chain molecules with a host of valuable and interesting properties ... 

TABLE II Condensation Polymerization of Alkylhalosilanes and Related Silanes to Alkylpolysiloxanes (Silicones)... 

Some topics involving polymerizations and related chemical reactions which were covered at the "Silicones and Silicone Modified Materials" symposium are conversions from silicon itself to semi-inorganics (contribution by Lewis), ring-opening polymerizations (contributions by Chojnowski, Soum, Kress, Jallouli, and Komuro), atom-transfer radical polymerizations (Matyjaszewski), hydrosilation polymerizations (Kaganove, Tronc, Narayan-Sarathy), polymerizations with controlled stereochemistry (Kawakami), condensation polymerizations (Fu), and polysilane syntheses (Newton) (25). 

Silanols are utilised for silicon-based polymeric materials and also find use as nucleophilic coupling partners in organic synthesis. Traditional synthetic methods utilise toxic reagents and are non-environmentally friendly, and other recently reported synthetic methods, in the absence of organic solvents, suffer the main drawback of the production of disiloxanes. Recent results by Kaneda et al. overcome this by using water as the solvent, with silver supported in hydroxyapatite with Mtde condensation to the disiloxanes. They show that the reaction can also be catalysed by homogeneous silver, although the supported nanoparticles were superior and reusable without any loss of activity or selectivity. 

In the condensation polymerization of two-component silicones, byproducts are released. These systems are less likely to be inhibited and can be used on a greater variety of substrates. However, reversion of polymerization is a potential problem. 

Synthesis of linear silicone aliphatic polyesters by the condensation polymerization of l,3-bis(3-carboxypropyl)tetra-methyldisiloxane with alkanediols (1,4-butanediol, 1,6-hex-anediol and 1,8-octanediol) using an immobilized lipase B from Candida antarctica (Novozym-435) as a catalyst, as shown in Scheme 2.2, has been reported [21]. These reactions were performed in the bulk (without the use of solvent) in the temperature range 50-90°C under reduced pressure (50-300 mmHg, vacuum gauge). 

Silcones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones, ranging from liquids to hard, glasslike solids with many useful properties. 

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