Siloxanes cyclic oligomer formation

The propagation reaction in the acid-initiated polymerization of hexamethyl-cyclotrisiloxane was described" as concurrent monomer addition, coupling of linear chain fragments, and cyclic oligomer formation by cleavage of siloxane bonds. The cationic polymerization of cyclic siloxanes was reviewed. ... 

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. 

In the polymerization of the cyclic siloxanes, D3 and D4, the formation of linear polymer is accompanied by cyclic oligomers. Thus, polydimethylsiloxane prepared at temperatures not exceeding 200 °C contains up to 15-18 vol% cyclic compounds. This gives [D] 2.2 mol l-1 [where D = -(-Si(CH3)20—] and therefore this is the lowest... 

The ring-opening polymerization of D4 is controlled by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cyclization equilibrium constants of dimethylsiloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equilibrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equilibrium constant of the formation of each cyclic is approximatdy equal to the equilibrium concentration of this cyclic, Kn [(SiRjO) J. Thus the total concentration of cyclic oligomers in the equihbrium is independent of the initial monomer concentration. As a consequence, the amount of linear polymer decreases until the critical dilution point is reached, at which point only cyclic products are formed. 

The suggested mechanisms to explain the formation of the same products (cyclic trimer and higher cyclic oligomers) from hydroxyl-ended, end-capped and KOH-containing polydimethyl-siloxane are shown in Scheme 40. 

In 1976, Hory et al. [7-9] returned to the problem of macrocyclization equilibria, presenting a new mathematical approach called Direction Correlation Factor theory, to improve the agreement between calculated and experimental data for rings of DP < 15. Cyclization equilibrium constants ( ) for the formation of cyclic poly(dimethyl siloxane)s, PDMS, were calculated for 6 < c < 31 and compared with experimental data. Similar calculations were performed for cyclic oligomers of Nylon-6, but the agreement with the experimental data was not satisfactory for the smaller rings. Remarkably, Hory presented a positive citation of two papers of Andrews and Semiyen (see below) who reported that equilibrated Nylon-6 contains around 12 % of cyclic species in the melt at 500-560 K. Unfortunately, he did not comment, how this relatively large value fits in with the much lower values he had previously calculated from the J - - S theory (see text above and p. 328 in Ref. [5]). 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

Non-polymerizable 1,3-dioxane non-polymerizable Non-polymerizable Non-polymerizable Formation of cyclic dimer and Siloxanes 5) tetramer was reported 31 . Hexamethyltrisiloxane 1,3,5-tnoxane formation 32 331 of Considerable cyclization. Distribu- cyclic oligo- and polymers report- tion Gf cycjjc oligomers as predicted ed no quantitative data. by the J-S theory... 

Figure 13.3 Base catalyzed ring opening of organo-siloxane oligomers to form high molar mass linear polysiloxanes. The rearrangement of cyclic monomers (usually mixtures of D3 and D4 cyclics) in the absence of chain terminate rs or crosslinking agents favors the formation of high molecular weight linear silicones at temperatures < 110°C.

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