Siloxanes ring-chain equilibria

As predicted by theory, the position of the ring/chain equilibrium was found to be independent of the nature of the redistribution catalyst employed (acid or base) (4,13,24- ) and of the specific inert solvent used (26). Russian authors (4,27) equilibrated mixtures of eyelosiloxanes comprised of dimethylsiloxane (75 mole %) and either trifluoropropylmethyl, cyanoethylmethyl, or cyanopropylmethyl siloxane (25 mole %) in acetone at a siloxane repeating unit concentration of 0.833 moles/A. They measured the dipole moments of the respective cyclosiloxanes, [(CH3)9SiO]3[Si(CH3)R0j>, to be 2.76 for R = trifluoropropyl, 3.45 for R cyanoethyl, and 3.58 in the case of R cyanopropyl. The equilibrium weight fraction of rings,... 

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 first synthetic route explored to produce cydic polymers made use of ring-chain equilibrium. This approach involves the natural equilibrium that occurs between linear and cydic polymers during condensation polymerizations although, inevitably, this yields linear byproducts and broad polydispersities. As a result, precipitation or preparative gel-permeation chromatography (GPC) was required to obtain cyclic polymers of sufficient purity for further study. This approach is amenable to a broad range of polymerization chemistries, induding the preparation of cyclic polyesters [7,8], polyethers [9], poly(dibutyltin dicarboxylates) [10,11], and poly(siloxanes) [12-15]. 

Both of the above mechanisms are complex processes with pol3rmerisation, depolymerisation, repolymerisation and chain transfer all occurring. Since all the siloxane bonds present in the reaction mixture will stand an equal chance of undergoing reaction whether they are present in D4, oligomers or polymer a ring/chain equilibrium is established. 

Rory (pages 5-9 of ref. 22) reported three types of experiments from which he deduced no evidence for structure (1) stress-temperature coefficients, (2) vapor pressure of a PIB-diluent system, and (3) ring-chain equilibrium constants between cyclic and linear siloxanes. In each case the systems were evaluated far above their respective T/ s. Such results are not pertinent to our present inquiry. We have searched sporadically but without success for physical measurements which span a temperature region across Tu in elastomers. Finally, we note that because elastomers tend to be flexible hydrocarbons, Tu should be weak and may not have a great influence on physical properties. The marked exception to this generalization is PIB with its stiff, stereoregular backbone. Tu in PIB has been discussed recently in great detail, with Tu 250 K, Tip 290 K. 

This illustrative study of ring-chain equilibria in a paraffin-siloxane system demonstrates that the conformational characteristics of chain molecules can be investigated provided suitable labile groups are incorporated into the molecular structures. It should prove within the capabilities of chemists to synthesise a wide variety of molecules suitable for study by the equilibrium cyclic concentration method using the dimethylsiloxane (or other siloxane) linkage as a labile linkage, incorporated into the molecular structure, throu whicJi to effect ring-cJiain equilibration reactions. 

The reaction of chlorosilanes produces hydrolyzates consisting of cyclics and linears with hydroxy and/or chlorine ends, depending upon conditions. Silanol functional linears are easily obtained and can be end-capped by silylation. More pertinent to this discussion, however, is the nature of end blocks that result from siloxane redistribution reactions. Conversion of cyclosiloxanes to equilibrium ring-chain distributions affords chains with ends arising from the catalyst i.e., if KOH is used, the chains have ends bearing silanol and K silanolate functionality. Neutralization with CO2 and HjO then converts the silanolates to silanols. Alternatively, the equilibrates, as well as the above hydrolyzates, can be silylated to convert the silanols and silanolates to other kinds of ends. 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

The molar cyclization equilibrium constants, Kx, of PDMS are measured. Using the Jacobson and Stockmayer equilibrium theory of macrocyclization, the dimensions of PDMS chains with 40-80 chemical bonds in the bulk polymer at 383 K are deduced. Dilution effects in the PDMS systems are contrasted with predictions of the Jacobson-Stockmayer theory, and the experimental molar cyclization equilibrium constants of the smallest siloxane rings are discussed in terms of the statistical properties of the corresponding oligomeric chains using tire RIS model of PDMS of Flory, Crescemi, and Mark [S 116]. 

These reactions indicate that during the early stages of the reaction, tetra-meric siloxane rings enter the linear molecules as a unit. Reorganization, however, occurs rapidly so that, after 0.5 hour, the distribution of the chains from the 11-mer to the 15-mer is approximately random. The distribution of shorter chains becomes increasingly more random as the reaction proceeds until, at equilibrium, the distribution of all linear species is in agreement with the random-reorganization model. 

The system in cationic ring-opening polymerization, which most closely approaches this ideal situation, is the polymerization of cyclic siloxanes [86-88]. The chain, composed of —O—Si— units is highly flexible and provides a reaction sites for cyclization. It is also well suited for experimental studies, because concentrations of cyclic oligomers in equilibrium, due to their volatility, can be determined with accuracy by gas-liquid chromatography. The slope of the experimentally derived plot is indeed very close to the theoretically predicted value of -2.5. 

The random nature of the copolymerization equilibria can be considered a consequence of two concurrent entropically driven equilibria similar to reactions 2 and 3. These copolymerization equilibria, however, would involve the comonomers interacting reversibly with two different chain ends and the reversible transfer of the different comonomer units between chains. Expressed in another way, the equilibria could be written in a manner similar to the Mayo-Lewis model but with rate constants replaced by equilibrium constants, K, K12, K22, and K21, and comonomer concentrations replaced by the total concentrations of the different siloxane units in the system, M and M2, regardless of their locations in the rings or chains. 

Beevers, M. S. Semiyen, J. A., Equilibrium Ring Concentrations and the Statistical Conformations of Polymer Chains Part 10. Cyclics in a Polymeric Par-affin-Siloxane. Polymer 1972,13, 523-526. 

For larger macrocydes, which attain the prediction by the J-S theory proportionality of the equilibrium cydization constants to the ring size in the power - 2.5, the key fartor is the decrease in the flexibilities of siloxane chains in the order... 

Linear polysiloxane can be synthesized by both anionic and cationic polymerizations of cyclic siloxanes such as hexamethylcyclotrisiloxane (n = 3) and octamethyl cyclotetrasiloxane (n = 4). Anionic polymerization is initiated by hydroxide, alkoxides, phenolates, silanolates and siloxoano-lates. The active species in the polymerization is the silanolate anion. Cationic polymerization is initiated by strong protonic acids such as sulfuric acid, trifluoromethane sulfonic acid and trifluoro-acetic acid (equation 53). Both the anionic and the cationic species undergo backbiting reactions during polymerization, such that an equilibrium exists between linear chains and rings. ... 

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