Oligomer siloxane

The position of the equiUbrium depends on a number of factors, such as concentration of siloxane units and the nature of substituents on the sihcon, but is independent of the starting siloxane composition and the polymerization conditions (81,82). For a hulk polymerization of dimethyl siloxane, the equihbrium concentration of cycHc oligomers is approximately 18 wt % (83). The equiHbrium mixture of cyclosiloxanes is composed of a continuous population to at least but D, D, and make over 95 wt % of the total cycHc fraction (84). 

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

The competition at 200°C between an aliphatic and an aromatic amine toward the formation of an imide is a very selective reaction (Fig. 5.1) for the formation of an aliphatic imide.141 This reaction suggests that the reactive processing in extruder, for example, could be used to transform a melt-processable polyimide with an oligomer end capped with an aliphatic amine. In order to get a perfecdy alternate block polyimide-block siloxane, Rogers et al. used low-temperature transimidization.142 An oligomeric aromatic imide was end capped... 

The main factors determining the reactivity of these siloxane oligomers towards other reactants are the type and nature of the terminal functional groups. Due to the fundamental differences in their structures, chemical reactivities and overall properties,... 

Siloxane Oligomers with Functional Groups Directly Bonded to the Terminal Silicon Atoms (Si—X)... 

General structure of the (Si—X) terminated siloxane oligomers and a list of important reactive functional end groups (X) are given in Table 2. 

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. 

Preparation of Functionally Terminated Siloxane Oligomers via Controlled Hydrolysis of Organo-silanes... 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The Major Route to the Functionally Terminated Siloxane Oligomers... 

An important advantage in the preparation of a,eo-functionally terminated siloxane oligomers, over the other telechelic systems, is the flexible polymerization chemistry of cyclic organosiloxane monomers and intermediates. This is mainly due to the partial... 

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