Polysiloxane specific forms

The silicones (mainly polydimethyl siloxanes) are considerably more expensive than the polysulfides therefore, silicone use is restricted to more specialized applications where specific attributes of the materials are required. For example, virtually every U.S. missile or satellite contains some form of silicone sealant because of its long use life, low volatile content, and heat resistance. They are also used around aircraft windows and doors. High-temperature fuel tank sealants based on fluorocarbon-containing polysiloxanes have also been the subject of considerable government-sponsored research (21). 

In some cases, cyclic and linear PDMS have been combined to form conetworks, and unsaturated cyclic side-chain fragments have been placed into polysiloxanes to make them thermoreactive. Interactions between ring polymers have also been analyzed and related to loops in chromatin. Also, some cyclic oligosiloxanes having polar end groups show liquid-crystal-line behavior, specifically smectic A and E phases. Finally, several PDMS cyclics have been exposed to vacuum pyrolysis and the products analyzed by matrix-isolation spectroscopy. The pyrolysis products obtained under a variety of conditions identified the radical reactions that were involved. 

Synthetic polymers are an integral part of everyday life. Synthetic polymers in the form of polyolefins such as polypropylene and polyethylene and other synthetic polymers such as poly(acrylate)s and poly(styrene)s (PSs) are predominantly used in packaging materials. Condensation polymers are produced in smaller amounts than addition polymers. Nevertheless, some condensation polymers such as polyesters with specific reference to poly(ethylene terephthalate) (PET) and aliphatic polyethers, aliphatic polyamides (i.e., nylons), and polysiloxanes are quite important. 

While siloxanes are also amenable to ADMET polymerization, they display trends similar to silanes, in that the steric bulk near the reacting olefin prevents productive metathesis [107]. Specifically, l,l,3,3-tetramethyl-l,3-divinyl-disiloxane proved unreactive, while l,3-diallyl-l,l,3,3-tetramethyldisiloxane almost exclusively cyclized. Still, longer di- and tri-siloxane monomers have been shown to polymerize via ADMET. Ring-chain equilibria are well known for polysiloxanes, and some of these polymers degrade over time via intrachain siloxane exchange reactions to form cyclic oligomers [108]. 

While the basic chemistry and physical behavior of linear polysiloxane polymers is well studied and, generally speaking, well understood, simple unmodified polysiloxanes are rarely encountered in real world applications. The majority of commercial silicone products are complex multicomponent formulations which take the form of elastomeric rubbers, adhesives, sealants, coatings, fluids or gels. Silicone elastomers are chemically crosslinked networks of linear polysiloxanes which exhibit poor mechanical properties in their native, unmodified state [1]. In order to obtain the desired combinations of mechanical, physical and chemical properties for a specific application, commercial high-performance silicone elastomers... 

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