Multifunctional condensation

Polycondensation Methods of Preparing Ladder Polymers (Multifunctional Condensation)... 

In order to ensue a clear presentation of the results the authors decided to segregate both synthetic principles All synthetic strategies developed from the multifunctional condensations of Stille and Marvel were assigned to this general type of reaction. At the same time the first multistep sequences (polymer-analogous cyclization of poly(methyl vinyl ketone) and polyacrylonitrile) are used as point of reference for the classification of the other type of synthesis (stepwise procedures). 

Ladder polymers have their backbone made from an interrupted series of condensed rings. In such polymers, very frequently the rings contain heteroatoms, and the polymer can be considered as part of the class of polymers with heterocycles in the main chain. One of the first synthesized polymers from this class was obtained from the oxidation and heating of polyacrylonitrile [1, 2]. Multifunctional condensations also can lead to ladder polymers. For example, a poly(phenoxazine) is formed from the reaction of a substituted quinone and a diaminodihydroxybenzene as shown below ... 

Expressions for molecular size distributions in multifunctional condensation leading to three-dimensional polymers are derived (Flory, 1941, 1946, 1953 Stock-mayer, 1943, 1952, 1953) by following much the same approach as for linear polymers, though with much more difficulty. Only the results of these derivations will, however, be considered here. The derivations are based on three simplifying assumptions of ideal network formation (1) all functional groups of the same type are equally reactive and independent of size of molecules to which they are attached (2) all groups react independently of one another and (3) no intramolecular reactions (cyclization) occur. 

Certain polymeric stmctures can also be blended with other coreactive polymers or multifunctional reactive oligomers that affect curing reactions when exposed to ir radiation. These coreactive polymers and cross-linking oligomers undergo condensation or addition reactions, which cause the formation of network stmctures (Table 9) (4,5,47). 

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

Most network polymers (the epoxies and the polyesters, for instance) are made by condensation reactions. The only difference is that one of the two reacting molecules is multifunctional (polyester is three-functional) so the reaction gives a three-dimensional lacework, not linear threads, and the resulting polymer is a thermoset. 

If the molar ratio of the multifunctional unit to the bifunctional unit is Q/f, then at completion of the condensation Q moles of end... 

For this purpose, perfluorooctanesulfonyl-tagged benzaldehydes were reacted with 1.1 equivalents of a 2-aminopyridine (or 2-aminopyrazine), 1.2 equivalents of an isonitrile, and a catalytic amount of scandium(III) triflate [Sc(OTf)3] under micro-wave irradiation in a mixture of dichloromethane and methanol (Scheme 7.85). A ramp time of 2 min was employed to achieve the pre-set temperature, and then the reaction mixture was maintained at the final temperature for a further 10 min. The fluorous tag constitutes a multifunctional tool in this reaction, protecting the phenol in the condensation step, being the phase tag for purification, and serving as an acti-... 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

By 1960 it was clear that acetyl CoA provided its two carbon atoms to the to and co—1 positions of palmitate. All the other carbon atoms entered via malonyl CoA (Wakil and Ganguly, 1959 Brady et al. 1960). It was also known that 3H-NADPH donated tritium to palmitate. It had been shown too that fatty acid synthesis was very susceptible to inhibition by p-hydroxy mercuribenzoate, TV-ethyl maleimide, and other thiol reagents. If the system was pre-incubated with acetyl CoA, considerable protection was afforded against the mercuribenzoate. In 1961 Lynen and Tada suggested tightly bound acyl-S-enzyme complexes were intermediates in fatty acid synthesis in the yeast system. The malonyl-S-enzyme complex condensed with acyl CoA and the B-keto-product reduced by NADPH, dehydrated, and reduced again to yield the (acyl+2C)-S-enzyme complex. Lynen and Tada thought the reactions were catalyzed by a multifunctional enzyme system. 

Atkinson KR, Hawkins SC, Huynh C, Skourtis C, Dai J, Zhang M, et al. Multifunctional carbon nanotube yarns and transparent sheets Fabrication, properties, and applications. PhysicaB Condensed Matter. 2007 May 15 394(2) 339-43. 

When Paul Flory wrote his famous book Principles of Polymer Chemistry in 1952, he indicated an alternative scheme for polymer synthesis [1]. He theorized about synthesizing condensation polymers from multifunctional monomers. These polymers were predicted to have a broad molecular weight distribution and to be non-entangled and non-crystalline due to their highly branched structure. However, they were considered to be less interesting since they would provide materials with poor mechanical strength, and at that time Flory did not feel it was worthwhile pursuing this line of research. 

It is apparent that the mode of reaction of the hyperbranched polyesteramides must be distinctively different from those of the known commercial crosslinkers. In order to explain these results, the hyperbranched polyesteramides should in our view not be regarded as simply multifunctional polymeric crosslinkers but rather as precondensed forms of two-functional crosslinkers (the addition product of diisopropanolamine and the cyclic anhydride), as depicted in Fig. 22, left. Bearing in mind the chemical fate of benzoic acid (2.2.1, Fig. 11) which was condensed with a polyesteramide resin and which appeared to transesterify at least as fast as it esterified, the mode of reaction of polyesters comprising aromatic acid end groups must be in accordance and comprised of both transesterification and esterification. 

In order to keep polyamides soluble in relatively apolar solvents, the use of flexible (macro)monomers such as a, co-(diaminopropyl)polydimethylsiloxane [52] or oligoethyleneglycol-based diamines [53, 54] has been proven to be a successful approach (Fig. 10). Poly condensations of dimethyl adipate with a variety of diamines were successful in bulk and at moderate temperatures between 60 and 100 °C (reaction A in Fig. 10). The low temperatures (60-100 °C) that suffice in these polymerizations also allow the use of monomers that are thermally instable, such as diethyl fumarate [53]. Moreover, multifunctional amines could be regioselectively polymerized up to molecular weights of 9 kDa, making lipase catalysts a valuable tool for the preparation of well-defined polyamides that can be further functionalized with active groups. 

Fig. 4 Schematic illustration of synthesis of multifunctional nanoparticles starting from a w/o microemulsion, b solubilization of fluorescent dye in the microemulsion core, c formation of silica nanoparticle and encapsulation of fluorescent dye, d condensation of silane ligand and chelation of Gd(lll), e post coating with silica, and f extraction of nanoparticles...

A variety of laboratory studies of the organic condensed-phase products of model hydrocarbon-NO -air systems have established that multifunctional compounds such as those observed in ambient air are... 

Multifunctional organics are also possible contributors. Nielsen et al. (1998) have examined the possible contribution of multifunctional compounds to missing NOy in both the gas and particle phases. As discussed in Chapter 9, compounds with sufficiently high vapor pressures (> 2x 10 5 Torr) exist essentially completely in the gas phase, those with low vapor pressures (< 2 X 10 9 Torr) in the condensed phase (i.e., on or in particles), and those in between the two extremes as both gases and particles. Nielsen and co-workers have developed a relationship between the expected vapor... 

Xerogels Ag, Au Two steps (1) gamma radiation initiation of metal-particle growth in a solution containing metal ions and multifunctional silanes (2) xerogels with metal clusters grafted on an oxide network prepared via hydrolysis and condensation 76... 

In consideration of the kinetic law obtained, Rp i0 of magnitude range, one can conclude that the common polymerization mechanism, based on bimolecular termination reactions, is no longer valid for these multifunctional systems when irradiated in condensed phase. Indeed, for conventional radical-induced polymerizations, the termination step consists of the interaction of a growing polymer radical with another radical from the initiator (R), monomer (M) or polymer (P) through recombination or disproportionation reactions ... 

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