Polysiloxane copolymers/anionic polymerization
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Alternation of siloxane blocks of different functionality depends on the structure of the starting modified cyclosiloxane Figure 4. Bifunctional cyclosiloxanes with identically substituted silicon atoms are readily prepared from the corresponding dichlorosilanes acting on them with zinc oxide Figure 5. Asymmetrical bifunctional cyclosiloxanes are prepared by co-condensation of the corresponding bifunctional or monofunctional dichlorosilanes with diols or disilanols Figure 6.
Double ring-opening copolymerization is also used in addition to anionic single cyclosiloxane opening polymerization. This results in block copolymers with units from different cycles Figure 7. Polymerization methods considered are effective and well-developed methods for preparation functional POS. With their help it is possible to synthesize directly modified POS, differing by substituents, molecular mass and alternation of units.
However, it is often necessary to encounter difficulties in preparing suitable monomers. Low stability, the complexity of purification and isolation in an individual state of starting compounds for polymerization are the factors that greatly complicate the POS synthesis. Another characteristic example is chlorosilanes, usually prepared from tetrachlorosilane by substituting a certain number of chlorine atoms for functional groups.
As a result of such reactions, usually a mixture of products of different degree of substitution is prepared, and frequent proximity of their boiling points leads to the need for a long process of purification by rectification, which greatly complicates the process of preparing POS based on them and makes it more expensive. Therefore, polymer functionalization approaches are used in cases where it is necessary to prepare functional POS that can not be easily prepared directly by polymerization methods. In more detail, synthetic approaches for the introduction of functional groups into the chain of available POS are exemplified by the introduction of an amino group.
Since POS is of interest with functional amino groups capable of further stepwise polymerization reactions to prepare a polyimide structure, we formulate the requirements for such compounds:. Chemical stability of functionalized POS.
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Amino groups that are part of the POS structure shall, for an unlimited period, be in the starting state under appropriate inert conditions, without reacting with solvents and the POS structure. On the other hand, such amino groups shall have sufficient reactivity for further use. Considered POS with functional amino groups are necessary for the further preparation of polyimides, therefore the range of modifiers under consideration is narrowed to alkylamines, arylamines and modified cyclic imides, which can be used directly in the polyimide synthesis.
Chemical reactions necessary for the POS modification shall not affect the POS chain and other functional groups included in its composition. Hydrosilylation allows the hydride-containing organosilicon compound to be grafted with a wide variety of functional groups linked by a double or triple bond.
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With the help of hydrosilylation, it is possible to first functionalize cyclosiloxane and then open it with one of the polymerization types Figure 9. Directly for the synthesis of polyimides, hydrosilylation can be used in the preparation of a cyclic imide fragment Figure In addition, hydrosilylation can be used directly for the attachment of siloxane chains to the imide cycle Figure This simple but very effective method for preparing low molecular weight bifunctional siloxanes is based on the condensation reaction of dichlorosilanes with derivatives of alcohols or phenols.
Usually this method is applicable for arylsiloxanes in view of greater stability with respect to alkylsiloxanes Figure Various diamine derivatives prepared by this method were used to prepare polyimides of various structures. Condensation method is also applicable for the modification of the side substituents by amino groups. POSs modified with chloromethyl groups in the side chain are usually used as starting compounds Figure There is an alternative method for the preparation of functionalized POS by ring-opening polymerization, originally functionalized by a chloromethyl group.
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The subsequent stages are similar to the previous approach Figure The main methods for the preparation of oligomeric polysiloxanes containing amino groups are considered. Modified polysiloxanes are the starting compounds for the synthesis of polyimide and polysiloxane copolymers obtained by polycondensation of diamines with aromatic dianhydrides of tetracarboxylic acids.
The copolymers are used as a material for proton-conducting membranes of fuel cells.
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Applied researches are carried out with state financial support represented by the Ministry of Education of Russia under the Agreement on granting subsidies No. This work is licensed under a Creative Commons Attribution 4. A New Edition of Web of Science. Table of Content. Journal Archived in:.
Your Name required. Your Email required. Phone Number. Which is bigger, 5 or 8? Type the above text in box below. Article Metrics. Contact Us. Google Page Rank. License This work is licensed under a Creative Commons Attribution 4. Figure 2: Methods for the modification of oligomers prepared by the acidolysis condensation of cyclosiloxane Click here to View figure.
Figure 3: Preparation of POS of a various structures by an anionic ring-opening polymerization Click here to View figure. Figure 4: Various functionalized cyclosiloxanes and structures of block copolymer chain Click here to View figure. Figure 5: Preparation of bifunctional cyclosiloxane with identically substituted silicon atoms Click here to View figure. Figure 6: Preparation of asymmetrical bifunctional cyclosiloxanes Click here to View figure. Figure 7: Anionic copolymerization of two cyclosiloxanes Click here to View figure.
Figure 9: Functionalization of cyclosiloxane by hydrosilylation and ring-opening polymerization Click here to View figure.