Polysiloxanes

Polysiloxanes are generally prepared, as illustrated in Fig. 10, by reacting equimolar amounts of a mesogenic monomer containing allylic functions with a-dimethylsila-nyl-a-hydrogen-oligodimethylsiloxanes in the presence of a chloroplatinic catalyst or 

These functional polymers were prepared, according to Fig. 11, by reacting a, -di-acrylates containing different mesogenic groups with secondary bisamines, such as piperidine, 2-methylpiperidine, 1,6-dime- 

Polysiloxanes are inorganie polymers that eontain alternate silieon and oxygen atoms in their baekbone [1-6]. While the silicon atom has two side-groups the oxygen atom carries none. In this regard they are similar to polyphosphazenes that we have seen in Chap. 3. The most widely studied polymer of this family is poly(dimethylsiloxane) (PDMS) which contains methyl groups as the substituents on silicon. PDMS is isoelectronic with poly(isobutylene) as well as poly(dimethylphosphazene) although its properties are dramatically different (Fig. 6.1). 

PDMS has several unusual properties [1,5]. These are listed as follows. 

These properties have made PDMS and related pol5uners the most important inorganic polymers from a commercial point of view. In fact, they are the only inorganic polymers that have achieved a remarkably high level of utility and consequently are produced in large quantities throughout the world. Thus, it is estimated that nearly 800,000 metric tons of this family of polymers is manufactured every year with a commercial value of over 4 billion U.S. dollars [7]. The applications of these polymers are in ex- 

Many other members of the polysiloxane family are known. These have not yet achieved the level of commercial success of the original systems. These include polymers with double-strand type structures (Fig. 6.3), hybrid polymers containing alternate siloxane-organic backbones, or examples with siloxane-phosphazene hybrids etc. Some of these polymers will also be described in this chapter. 

The cyclic structural unit [Si309] and the organosiloxane analogue [Me2SiO]6 

In their properties, polysiloxanes are intermediate between purely organic polymers and inorganic silicates the structure may be varied in numerous ways to shift the pattern of properties of silicones either in one direction or the other. Commercial polysiloxanes generally contain methyl substituents. Whereas in the scientific literature the name polysiloxanes is used, the name silicones (silicone oil, silicone grease, silicone rubber) is preferred in the technical literature. 

Various functional silanes (e.g., R2SiCl2 or RSiCl3) can be used as starting materials for the preparation of polysiloxanes. The silanes are first hydrolyzed to the corresponding silanols, which are very unstable and easily undergo polycondensation with the elimination of water and the formation of -Si-O-Si- link- 

Linear polysiloxanes obtained by the hydrolysis of dichlorosilanes are of relatively low molecular weight they can, however, be condensed further through the terminal OH groups by thermal after-treatment. 

The polysiloxanes obtained by anionic initiation have considerably higher molecular weights than those obtained by cationic initiation. 

Ring-opening polymerization of cyclic siloxanes with cationic initiators allows the possibility of introducing stable end groups by the use of suitable chain transfer agents. Thus, polysiloxanes with trimethylsilyl end groups are formed when the cationic polymerization of octamethylcyclotetrasiloxane is carried out in the presence of hexamethyldisiloxane as transfer agent  

The superior thermal stability of polysiloxanes has made them attractive candidates for use at elevated temperatures. The most common member of the family, poly(dimethylsiloxane) 

PDFS blends because hydrogen abstraction is not possible due to the absence of methyl groups in the chains. 

Camino and co-workers [a.309] combined kinetic formal treatments and computer simulations to analyse the thermal degradation of PDMS. It was shown that PDMS 

Reprinted from [a.309] with permission from Elsevier 

UV irradiation causes rapid degradation of polysiloxanes, but its extent depends on the type of side-group substituents [333,569,1485,1987,2267]. 

The main reaction in the case of polydimethylsiloxane (4.101) is photocleavage of side methyl groups and the formation of very reactive silyl radicals 

The silyl (4.103) radical may also react with oxygen, giving a polysilylperoxy radical which abstracts hydrogen from a methyl group, and gives polysilyl hydroperoxides  

In the presence of oxygen, the methylene side radical (—CHj) is also simultaneously oxidized [569]  

Poly(dimethylsiloxane) oils with a high content of silicon hydride, (—Si—H) groups located inside the chain (4.106) and/or at the end of the chain (4.107) are much more unstable towards UV irradiation than the corresponding poly(dimethylsiloxanes) [1053]. 

Polysiloxane fluids and resins are obtained by hydrolysis of chlorosilanes such as dichloro-dimethyl-, dichloromethylphenyl-, and dichlorodiphenylsilanes [Brydson, 1999 Hardman and Torkelson, 1989]. The chlorosilane is hydrolyzed with water to a mixture of chlorohy-droxy and dihydroxysilanes (referred to as silanols), which react with each other by dehydration and dehydrochlorination. The product is an equilibrated mixture of approximately equal 

The advantages of these catalysts are, especially, their high structural stability, 

D3 is the only coplanar molecule and thus the only strained one among cyclic siloxanes. 

Both D3 and D4 can be polymerized either anionically or cationically. The main advantages of the anionic polymerization of D3 are formation of a narrow MWD polymer and formation of linear polymer virtually free of cyclic products (the process can be stopped before the linear-cyclic polymer equilibrium is reached). The cationic polymerization is better suited for the preparation of telechelics, particularly with acetoxy or alkoxy end-groups. Moreover the polymerization of monomers containing functional groups like =Si—H, can also be performed only cationically, on account of the lability of the = Si—H bond toward bases. 

A number of cationic polymerizations of D3 and D4 have been described in the literature. More recently CF3S03H (TfOH) has been used in CH2C12 solvent for both D3 and D4 providing satisfactory conditions of the cationic synthesis of polydimethylsiloxanes. It seems, on the basis of analysis of literature data, that TfOH is the most suitable initiator, at least for small-scale laboratory synthesis. In this system the formation of linear polymer is accompanied by cyclic products and their proportion, as it will be discussed later in this section, depends on the initial monomer concentration ([DJ0). At higher [DJ0, the proportion of cyclic products is lower. Thus, it is desirable to conduct polymerizations at highest possible [D4]0. 

The proportion of cyclic products is unaffected by other variables (i.e., [TfOH, temperature). It may depend on the nature of the solvent used, due to the differences in the interactions between solvents and D4 or PDMS. However, as mentioned above, for high yields of linear polymer as little solvent as possible should be used. 

practically, a concentrated solution of purified D4 in CH2C12 solvent is polymerized by CF3S03H. Traces of water should carefully be removed. The ratio [D4.]o/[CF3S03H]o yields the required polymerization degree. The polymer is precipitated in CH3OH and the solvent is removed from the oily or solid polymer under vacuum. Monofunctional chain-stoppers (like hexamethyldisoloxane) can also be used to control the polymer Mn. 

The starting materials can be prepared through hydrolyses of alkyl or arylsilicone halides [156-160]. Qrganosilicone halides, in turn, are made commercially by heating alkyl or aryl halides with silicon at 250-289°C. Copper catalyzes this reaction  

The same materials can also be formed by the Grignard reaction  

Alkyl silanes can also be prepared by additions of trichlorosilanes to ethylene or acetylene HSiClj + 

Trichlorosilane reacts with aromatic compounds in the presence of boron trichloride  

The siloxane linkages can result from hydrolysis of the halides. The products of hydrolyses, silanols, are unstable and condense  

---

Cased W, Sauer T and Wegner G 1988 Soluble phthalocyaninato-polysiloxanes—rigid rod polymers of high molecular-weight/Macromo/. Chem. Rapid Commun. 9 651-7... 

Sauer T, Arndt T, Batchelder D, Kalachev A A and Wegner G 1990 The structure of Langmuir-Blodgett-films from substituted phthalocyaninato-polysiloxanes Thin Solid Films 187 357-74... 

Crockett R G M, Campbell A J and Ahmed F R 1990 Structure and molecular-orientation of tetramethoxy-tetraoctoxy phthalocyaninato-polysiloxane Langmuir-Blodgett-films Po/yme/ 31 602-8... 

The silanols formed above are unstable and under dehydration. On polycondensation, they give polysiloxanes (or silicones) which are characterized by their three-dimensional branched-chain structure. Various organic groups introduced within the polysiloxane chain impart certain characteristics and properties to these resins. 

Gyclodextrins. As indicated previously, the native cyclodextrins, which are thermally stable, have been used extensively in Hquid chromatographic chiral separations, but their utihty in gc appHcations was hampered because their highly crystallinity and insolubiUty in most organic solvents made them difficult to formulate into a gc stationary phase. However, some functionali2ed cyclodextrins form viscous oils suitable for gc stationary-phase coatings and have been used either neat or diluted in a polysiloxane polymer as chiral stationary phases for gc (119). Some of the derivati2ed cyclodextrins which have been adapted to gc phases are 3-0-acetyl-2,6-di-0-pentyl, 3-0-butyryl-2,6-di-0-pentyl,... 

Fig. 2. Molecular structures of selected photoconductive polymers with pendent groups (1) poly(A/-vinylcarba2ole) [25067-59-8] (PVK), (2) A/-polysiloxane carbazole, (3) bisphenol A polycarbonate [24936-68-3] (4) polystyrene [9003-53-6] (5) polyvin5i(l,2-/n7 j -bis(9H-carba2ol-9-yl)cyclobutane) [80218-52-6]...

The reaction is of practical importance in the vulcanization of siUcone mbbers (see Rubber compounding). Linear hydroxy-terrninated polydimethyl siloxanes are conveniently cross-linked by reaction with methyldiethoxysilane or triethoxysilane [998-30-1]. Catalysts are amines, carboxyflc acid salts of divalent metals such as Zn, Sn, Pb, Fe, Ba, and Ca, and organotin compounds. Hydroxy-terrninated polysiloxanes react with Si—H-containing polysiloxanes to... 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Fig. 7. Schematic description of a polysiloxane at the monolayer—substrate surface (4). The arrow points to an equatorial Si—O bond that can be connected either to another polysiloxane chain or to the surface. The dashed line on the left is a bond in a possible precursor trimer where the alkyl chains can occupy...

Maxillofacial polymers include the chlorinated polyethylenes, polyethemrethanes, polysiloxanes (see Elastomers), and conventional acrylic polymers. These are all deficient in a number of critical performance and processing characteristics. It is generally agreed that there is a need for improved maxillofacial polymers that can be conveniently fabricated into a variety of prostheses (218,227,228). 

Information on the synthesis of the polyetherimide—polysiloxane block copolymers has not been disclosed. Many other synthetic methods for preparing block copolymers have been described (19,20,25) but are currendy not beheved to be commercially important. 

---