Cyclic PDMS oligomers

Linear polymers are the most commonly found, and consist of chains of D units endblocked by a variety of functionalized M units. Branched-chain silicones consist mainly of D units, with a D unit being replaced by a T or a Q unit at each point of branching. Cyclic PDMS oligomers are also common and can play a role in adhesion. They are usually found as mixtures of structures going from three siloxy units, to four, five, and higher siloxy units. A whole range of analytical techniques can determine the detailed molecular structures of these materials [20,21],... 

FIGURE 19. The siloxane bond interchange reaction for cyclic PDMS. The intermolecular process (a) results in a much higher molecular weight cyclic polymer than that of the starting material. The intramolecular process (b) gives a mixture of cyclic oligomers. Reproduced from Bannister, D. J. and Semiyen, J. A., Polymer 1981, 22, 377, by permission of the publishers, Butterworth and Co. (Publishers) Ltd. 

For the most common siloxanes, polydimethylsiloxanes (PDMS), R=Me. Thus, most common cyclic siloxanes would be referred to as Dn, for example, D3, D4, D5,. .. linear oligomers are often called MD M, for example, MM, MDM, MD2M,. .. and, T and Q units constitute parts of many highly branched oligomers and polymers, or cross-linked networks. 

Silanols are stronger acids than their hydrocarbon homologues. Condensation reactions are governed by the polarity of the Si O and -OH bonds. The most widely used product is polydimethylsiloxane, PDMS, R1 = R2 = -CH3. The formation of cyclic oligomers always competes with that of linear chains. 

Another method of preparing PDMS is the ring-opening polymerization of cyclic oligomers (Sec. 2.3). 

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

A silanol terminated PDMS heated under isothermal conditions at different temperatures first shows an increase in molecular weight (MW) as a function of time151 (Figure 9). This process is the condensation of silanol groups on the ends the PDMS chains which leads to an increase of MW. However, the increase in MW is limited probably due to either a lowering in concentration of silanol groups or to the equilibrium nature of the silanol condensation reaction. When the temperature is further increased, a second process is observed which is the depolymerization. A mixture of cyclic oligomers... 

TABLE 8. Yields of cyclic oligomers from a5ro-dihydroxy-PDMS. M = 111500. GLC quantitative analysis (T = 275-405 °C, 0-80% degradation) (data from Reference 151 reproduced by permission of Pergamon Press)... 

Liquid polydimethylsiloxanes (PDMS) are stable up to 180 °C while liquid polyphenylmethylsiloxanes are stable up to 250 °C. PDMS decomposes under vacuum at 100 °C to form the cyclic oligomers hexamethylcyclotrisiloxane and oxamethylcyclotetrasiloxane. This process occurs at lower temperatures in the presence of terminal Si-OH groups or additives of electrophilic or nucleophilic character. The terminal OH groups interact with the Si-O bond by the reaction shown in Equation 7.1 ... 

Poly(dimethyl/methylphenyl siloxane)s and poly(dimethyl/diphenyl siloxane)s have been prepared with a range of phenyl contents and their thermal degradations studied. The products of degradation are shown to be benzene and a complex mixture of cyclic oligomers, the latter being analysed by GLC, mass spectrometry, and n.m.r. spectrometry. The mechanism of degradation is discussed in relation to the degradation reactions previously described for (PDMS) and poly(methylphenyl siloxane)s. A study of soluble and insoluble heat stability additives for (PDMS) has been reported and a mechanism of stabilization proposed. ... 

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. 

In contrast to the cyclic alkanes, comparison of the solid state and SiNMR spectra of cyclooctamethyltetrasiloxane (n = 4) with its solution CNMR spectra demonstrates that the conformational mobility of such polar oligomers is significantly determined by the molecular packing in the crystal, and that the s-s transition (265 K) is only observed in solid state experiments and not in solution. The DSC behavior of both cyclic (M = 3.7-24.4 x 10 ) and linear polydimethylsiloxanes (PDMS) (M = 0.16-25.5 x 10 ) has been studied. " In addition to melting and the s-s transition, a and a cold crystallization exotherm were observed in cyclic and linear oligomers with Mn>3130 and Mn>2460, respectively. Furthermore, the temperatures of these transitions were much low er than those of octamethyltetrasilqxane. 

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]). 

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