Linear/cyclic equilibrated polymer

Cyclic or ring molecules occur in a wide range of polymers, including linear and nonlinear polymers prepared by condensation or by ring-to-chain equilibration reactions. Cyclics also occur in many biological macromolecules, sometimes in the form of loops as in certain proteins and nucleic adds. 

JS presented a general theory of ring-chain equilibria as early as 1950. Their theory has been extensively used by polymer chemists to describe the cyclic populations of linear polymeric equilibrates in concentrated solutions... 

Other novel initiators include the use of an aluminium-tetraphenylporphyrin complex Et—Al(TPP)12. Only D3 is polymerized at 25 °C over 6 days to give a monodispersed (.Mn/Mw = 1.12, Mn = 1550) polymer. D4 and D5 do not react under similar conditions and linear equilibration is suppressed. In addition, they report that this initiator will oligomerize functional cyclics such as (SiMeHO)4 and (SiPh20)3. 

The ratio of linear to cyclic molecules is shifted heavily in favor of cyclic compounds in equilibrated 3,3,3-trifluoropropylmethylsiloxane polymers (31) which consist of 86 wt % of rings and 14% of chains. 

Fig. 1. Gel permeation chromatogram (g-p.c.) of an undiluted equilibrate of polyCphenybnethyl-riloxane) showing r onsoorreqmndii to small cyclics, macrocyclics and linear polymer...

The kinetics and thermodynamics of this process show close similarities to the polymerization of D4 initiated by CF3SO3H [3,4]. Both processes involve simultaneous formation of cyclic oligomers and polymer and lead to equilibrium, with similar proportions of cyclic-to-linear fractions [3-5], They also show similar thermodynamic parameters and a similar effect of water addition on the initial rate of polymerization. The specific feature of the polymerization of >2 is that cyclic oligomers 03 and D4 are formed simultaneously with the polymer fraction, but they equilibrate with monomer much faster than the polymer fraction. This behavior is best understood assuming formation of the tertiary oxonium ion intermediate, which isomerizes by ring expansion-ring contraction [3]. These kinetic features of the polymerization of make this monomer an interesting model for deeper studies on the cyclic trisilyloxonium ion question. 

These ring-opening polymerizations are referred to as equilibration reactions. Since a variety of Interchange reactions can take place, a quantitative conversion of the tetramer to high polymer Is not achieved and there Is, at thermodynamic equilibrium, a mixture of linear and cyclic species present. Scheme III shows examples of the types of redistribution reactions thought to be occurring In these systems. It Is generally convenient to use D" to refer to a dlfunctlonal slloxane unit and M to refer to a monofunctional slloxane unit. Thus, D4 represents the cyclic slloxane tetramer and MM represents the linear hexamethyldlslloxane. 

The complete conversion of the dimethyldichlorosilane to only linear oligomers is also possible in the continuous hydrolysis operation. In this operation, the cyclics are separated from linear oligomers by a stripping process and are mixed again with dimethyldichlorosilane. This mixture undeigoes equilibration to chloro-terminated oligomers and is subsequendy hydrolyzed. The silanol-stopped linear oligomers are direcdy used in the manufacture of silicone polymers. 

Experimental molar cyclization equilibrium constants for cyclics in the undiluted equilibrate (containing 81% linear polymer) and the solution equilibrate (containing 14% linear polymer) are plotted as tog Kx versus tog x in Fig. 11. The Kx values are believed to be correct to within 10% for cyclics with x = 2—5, and it is concluded... 

The product after the first stage consists of an approximately equal mixture of cyclic compounds, mainly the tetramer, and linear polymer. To achieve a more linear polymer and also to stabilize the viscosity, it is common practice to equilibrate the products of hydrolysis by heating with a catalyst such as dilute sulfuric acid. For fluids of viscosities below 1000 cs, this equilibrium reaction is carried out for hours at 100—150°C. 

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