Open-chain siloxanes

Open-chain compounds which have the requirements for more than one of the structures already defined are named, if possible, in terms of silane, silazane, silthiane, 01 siloxane containing the hugest number of silicon atoms. Examples ... 

The combination of monofunctional units, M, with difunctional units, D, leads to straight chain polyorgano-siloxanes terminated by M-units. Combination of only difunctional units produces cyclic polyorgano-siloxancs or open-chain polydiorganosiloxanes with, for example, a hydroxy or alkoxy end group. The incorporation of T- and optionally also Q-units leads to branched polyorgano-siloxanes. 

This section is concerned with polymers derived from open-chain phosphazenes, phospha(thia)zenes and related cross-linked materials. Cyclolinear and cyclomatrix materials as well as carbon-chain polymers with cyclophasphazene substituents are covered in Section 3. General and specific reviews have appeared including an overview of recent development in inorganic polymers including poly(phosphazenes), a comprehensive survey of hybrid siloxane-phosphazene systems, water-soluble phosphazenes and related hydrogels, a brief survey of polymerization reactions and mechanisms, and sulfur containing poly(phosphazenes). ... 

Cyclic siloxanes can undergo a ring-opening chain-growth polymerization. Free radicals are not useful as initiator species, because of the nature of the siloxane bond, but cationic initiators are very effective. The reaction is illustrated using the most common cyclic oligomers, the timer hexa-methylcyclotrisiloxane, or the tetramer octamethylcyclotetrasiloxane " ... 

Si(CHs)2—. All the compounds examined by Wright and Hunter [ 1 ] show absorption in the 800 cm" region when —Si(CH3 )2 — groups are present. This is a methyl rocking mode, and a second band is always associated [5] with it near 855 cm". The 800 cm" band appears in the range 814—802 cm" in the cyclic polymers, and is practically invariant at 800 cm" in the open-chain materials and methyl silanes. Richards and Thompson [3] have shown that the band persists at this point also, in the dimethyl siloxanes. The position of the Si—C stretching bands in this series is less well defined but in dimethyl silane [17] these occur at 730 and 650 cm". ... 

In the chemoselective polymerization of monomer 1 sequencing of siloxane units in the polymer chain depends exclusively on the way in the which monomer is opened and added to the end of the growing chain. There are three non-equivalent places of opening of 1 marked by a, b, c in equation 4, which lead to three different arrangements of siloxane groups in the open chain monomer units. 

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

Polymerization. The polymer chain is formed by a ring-opening reaction caused by the action of alkaUes on the monomer, a cycHc siloxane ... 

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

Figs. 5 and 6 demonstrate that, contrasting to some NMR based expectations [5] Raman spectroscopy indicates significant structural differences between the overall structures of dissolved silicate molecules when they contain different alkaline metals. Since there have not been siloxane rings associated with the 460 cm 1 centred large Raman shift in Lithisil-25 it is reasonable to assume that this dilute solution contains open siloxane chains. 

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization (ROP) of cyclic monomers such as cyclic ethers, acetals, amides (lactams), esters (lactones), and siloxanes. Ring-opening polymerization is of commercial interest in a number of systems, including the polymerizations of ethylene oxide... 

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 ... 

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