Stepwise Addition Polymerization Polyaddition

The mixture is then heated for 10 min in the oven at 110 °C to bring about crosslinking. The solubilities of the mixture and the resulting silicone rubber are tested in toluene. 

Equilibration of a Silicone Elastomer to a Silicone Oil with TrimethylsiiyI End Groups 

Safety precautions Before this experiment is carried out, Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. 

60 g of distilled octamethylcyclotetrasiloxane are mixed with 0.1 g of very finely powdered potassium hydroxide and 0.5 ml DMSO as solubilizer in a 250 ml conical flask, which is then placed in an oil bath at 140°C. The increase in viscosity of the mixture can easily be observed by occasional swirling. After 20-30 min, the liquid has reached the consistency of thin honey. Half the product is taken out and cooled [further work-up under (c) and (d)]. The residue is heated again until, after 2-3 h, a plastic, putty-like mass is produced. It is allowed to cool, whereby a rubbery polymer is obtained. Yield 90-95%. 

The polysiloxane from experiment (b) is soluble in toluene. It can be converted by hot vulcanization into an insoluble silicone rubber. Using a small blender, 10 g of the polymer are kneaded with 10 g of quartz powder or 7.5 g of ground kieselguhr, and 0.6 g of dibenzoyl peroxide paste (50% in silicone oil). To work the additives into the silicone rubber without a mechanical blender is very tedious and difficult to achieve completely. 

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If the equilibrium constant K has a value between 1 and 10, less than a thousandth of the total amount of water formed in the reaction mixture is sufficient to prevent the formation of really high-molecular-weight condensation polymers. Hence it follows that it is extremely important to remove as completely as possible the low-molecular-weight reaction products, for example, water, eliminated during a polycondensation. In principle, these equilibriums are also known in stepwise addition polymerizations (polyaddition) like the back-reactions of urethane groups. Since they mostly occur at higher temperatures only, they can be neglected. 

Stepwise addition polymerizations (polyaddition) or condensation polymerizations (polycondensation) are possible polyreacticais for the first step. The two latter combinations attained interest in the technical synthesis of polyimides and polybenzimidazoles. 

Polymerization is a chemical reaction in which the molecules of monomers are linked together to form polymers. The two principle kinds of polymerization are addition and condensation polymerization. Addition polymerization has two subgroups chain propagation polymerization and stepwise chain growth. (It should be noted that much of the literature, especially in Emope, uses polymerization only for the chain propagation process. Stepwise growing is referred to as polyaddition, condensation polymerization is named polycondensation.)... 

Quantitative conversion is one of the essential preconditions to achieve a significant molecular weight in stepwise polymerization process. Consequently, an iron-catalyzed Michael reaction would be a suitable elementary step for a polyaddition. Bis-donor 24c and bis-acceptor 41b, readily accessible from common starting materials [69], were converted with FeCl3-6H20 to yield a poly-addition product... 

In addition to the repeat unit sequence, another area of current interest in polymer structural control (Fig. 1) may be the spatial or three-dimensional shapes of macromolecules. In fact, the recent development of star [181-184] and graft [185] polymers, as well as starburst dendrimers [126], arborols [186,187], and related multibranched or multiarmed polymers of unique and controlled topology, has been eliciting active interest among polymer scientists. In this section, let us consider the following macromolecules of unique topology for which living cationic polymerizations offers convenient synthetic methods that differ from the stepwise syntheses (polycondensation and polyaddition) [126,186,187]. 

The hrst section covers the basic principles and characteristics necessary for polymer preparation by polymerization, being either (a) stepwise polymerization of bifunctional monomers by polycondensation, stepwise polyaddition and ringopening processes, or (b) chain polymerization of vinyl monomers by free radical, cationic, anionic, and coordination addition processes. Both of these polymerization techniques are used for polymer preparation from monomer. The goal of the polymerization technique is to obtain polymers with specific structures and properties -this generally requires specialized polymerization conditions. Also described are the factors affecting the rates of homo- and copolymerizations and the reactivity ratios of different comonomers. 

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