Polysiloxanes cyclic oligomers

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

The presence of even small amounts of cyclic oligomers can be detrimental for the utilization of a polymer if the cyclics migrate out of the product during its use. Many commercial processes remove cyclic by extraction (e.g., with steam in polyamide production) or thermal devolatilization (polysiloxane). 

Siloxane polymerization differs mechanistically from the formation of hydrocarbon polymers in that it is essentially an acid-base process, as might be expected from the strong alternation of electronegativites along the het-eroatomic chain, and the radical initiators that catalyze the homocatenation of alkenes do not work for siloxanes. Long, unbranched polysiloxane chains are favored by higher condensation reaction temperatures and basic catalysts such as alkali metal hydroxides. Acidic condensation catalysts tend to produce polymers of lower molar mass, or cyclic oligomers. 

Polysiloxanes are another class of linear or cyclic oligomers that are prepared by a nucleophilic substitution. The first reaction takes place between an organohalosilane (typically dichlorosilane) and water (Eq. (2.52)). To increase the molar mass it is necessary to introduce a catalyst (Eq. (2.53)) ... 

Hall, Patel [30] Polysiloxane rubber thermal degradation Cyclic oligomers PDMS... 

Chlorosilanes are the most common precursor to siloxanes, cyclic or otherwise, because of their ready availability from the direct processing of alkyl chlorides with Si. Upon hydrolysis, chlorosilanes are converted to polysiloxanes and their cyclic oligomers. 

Cyclic oligomers also occur along with polysiloxanes with Cl end groups in polymerizations catalyzed, for example, with HCI/H2O. In the hydrolysis of these end groups, the chain length is doubled on the formation of a new Si—O—Si bond. 

While siloxanes are also amenable to ADMET polymerization, they display trends similar to silanes, in that the steric bulk near the reacting olefin prevents productive metathesis [107]. Specifically, l,l,3,3-tetramethyl-l,3-divinyl-disiloxane proved unreactive, while l,3-diallyl-l,l,3,3-tetramethyldisiloxane almost exclusively cyclized. Still, longer di- and tri-siloxane monomers have been shown to polymerize via ADMET. Ring-chain equilibria are well known for polysiloxanes, and some of these polymers degrade over time via intrachain siloxane exchange reactions to form cyclic oligomers [108]. 

The corresponding cyclic oligomers and polymers from other polysiloxane systems were also prepared and characterised by our group at York including [R(CH3)SiO] where R = H [8], CH2=CH [9] and [10]>[1 ] We then went on to prepare the first cyclic polymer liquid crystals starting with the cyclic [H(CH3)SiO] materials. For all these studies the corresponding linear PDMS materials or fractions were prepared for purposes of comparison. 

Detailed information on the copolymerization of cyclic trifluoropropylmethyl-siloxane trimer and octamethylcyclotetrasiloxane is also very limited in the open literature26 27 . Recently, preparation of various amine terminated (dimethyl-tri-fluoropropyl,methyl)siloxane oligomers with varying molecular weights and backbone compositions has been reported 69115 ll7). Table 11 shows various properties of the oligomers produced as a function of composition. These types of modification play very important roles in determining the solubility characteristics and hence the compatibility of resultant polysiloxanes with other conventional organic monomers... 

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