Condensation polymers cyclization

Condensation polymers such as polyesters or polyamides undergo more complex thermal degradation processes where the resulting pathway is a combination of different reactions including scission, elimination and cyclization [75]. 

Note The souree-based name identifies the starting monomer of this ladder polymer, the synthesis of which comprises a multistep reaction involving condensation and cyclization. 

Poly(imide)s first became commercially important with the development of the condensation poly(imide) Kapton [4, 5] in 1965. The two-step reaction of a dianhydride (pyromellitic dianhydride) with a diamine (p-phenylene diamine) to initially form a poly(amic acid), and subsequent thermal cycliz-ation to form the poly(imide), is a common route to the formation of poly-(imide)s, as well as being exploited for the synthesis of oligomeric precursors for addition poly(imide)s. Usually, such condensation polymers are insoluble... 

Synthesis of the hyperbranched poiyethoxysiloxane. Viewed from a standpoint of the chemistry of dendrimers and hyperbranched polymers, triethoxysilanol (regarded in (20) as a primary product of hydrolysis) is no more than a reactant AB3 according to the Flory condition. This signifies that, by generating this product under the conditions of a heterofunctional condensation, one can direct the reaction such that a hyperbranched poiyethoxysiloxane is formed, that is, to make the process structurally selective. It is known that, in hyperbranched polymers, cyclization is a minor contributor to the molecular structuring because of the paucity of A-type functionalities. In other words, with allowance made for structural imperfection of the hyperbranched polymer and for the fact that proportions of the dentritic, linear and end chains depend on a number of factors, it is possible in principle to obtain an end product with desired properties by monitoring structure, rather than process parameters, of the polymer formed. 

At times, it is also useful to identify the starting monomers especially for such complex polymers as double-strand polymers, the synthesis of which is often a multi-step reaction involving condensation, cyclization and crosslinking. 

Other polymers undergo cyclization, but there are no commercial applications. Poly (methacrylic acid) cyclizes by anhydride formation and poly(methyl vinyl ketone) by condensation (with dehydration) between methyl and carbonyl groups. 

Schreiber and co-workers (436) prepared a library calculated to contain 2.18 million polycyclic compounds through the 1,3-dipolar cycloaddition of a number of nitrones with alkenes supported on TentaGel S NH2 resin (Scheme 1.83). (—)-Shikimic acid was converted into the polymer bound epoxycyclohexenol carboxylic acid 376 (or its enantiomer), coupled to the resin via a photolabile linker developed by Geysen and co-workers (437) to allow release of the products from the resin in the presence of live cells by ultraviolet (UV)-irradiation. A range of iodoaromatic nitrones (377) was then reacted with the ot,p-unsaturation of the polymer-bound amide in the presence of an organotin catalyst, using the tandem esterification/ dipolar cycloaddition methodology developed by Tamura et al. (84,85) Simultaneous cyclization by PyBrop-mediated condensation of the acid with the alcohol... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenolic structure renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxylic acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubility is controlled by chemical and polarity differences rather than molecular size. 

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