Condensation of oligomers

Scheme 3.2 Preparation of cyclic PBT oligomers via high-dilution condensation of oligomers [13]...

Furfural reacts with ketones to form strong, crosslinked resins of technical interest in the former Soviet Union the U.S. Air Force has also shown some interest (42,43). The so-called furfurylidene acetone monomer, a mixture of 2-furfurylidene methyl ketone [623-15-4] (1 )> bis-(2-furfurylidene) ketone [886-77-1] (14), mesityl oxide, and other oligomers, is obtained by condensation of furfural and acetone under basic conditions (44,45). Treatment of the "monomer" with an acidic catalyst leads initially to polymer of low molecular weight and ultimately to cross-linked, black, insoluble, heat-resistant resin (46). 

A newer process for preparation of mixtures of cycHc oligomers has been discovered (15,35). This process utilizes a triethylamine-catalyzed hydrolysis condensation of BPA bischloroformate in a pseudo-dilution reaction. This efficientiy provides 85—90% yield of a mixture of cycHc oligomers having a degree of polymeriza tion of 2 to about 12 (eq. 12). 

Condensation of these monomers to form colloidlike oligomers (sol formation). 

The common synthetic route to bismaleimides or maleimide functionalized oligomers is the condensation of diamines or amino-terminated oligomers with maleic anhydride. Another possibiUty is the use of an AB-type monomer of the following general formula to build the polymaleimide, where X represents a functional group that can be employed in condensation reactions. 

Isohutyhdene Diurea., This is the condensation product of urea and isobutyraldehyde. Unlike the condensation of urea with formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single oligomer. Although similar in chemical stmcture to methylene diurea (MDU), its physical properties are quite different (Table 4). 

Lipase-catalyzed intermolecular condensation of diacids with diols results in a mixture of macrocycUc lactones and liuear oligomers. Interestingly, the reaction temperature has a strong effect on the product distribution. The condensation of a,(D-diacids with a,(D-dialcohols catalyzed by Candida glindracea or Pseudomonas sp. Upases leads to macrocycUc lactones at temperatures between 55 and 75°C (91), but at lower temperatures (<45°C) the formation of oligomeric esters predorninates. Optically active trimers and pentamers can be produced at room temperature by PPL or Chromobacterium viscosum Upase-catalyzed condensation of bis (2,2,2-trichloroethyl) (+)-3-meth5ladipate and 1,6-hexanediol (92). 

The UF-resin itself is formed in the acid condensation step, where the same high molar ratio as in the alkaline methylolation step is used (F/U = 1.8 to 2.5) the methylolureas, urea and the residual free formaldehyde react to form linear and partly branched molecules with medium and even higher molar masses, forming polydispersed UF-resins composed of oligomers and polymers of different molar m.asses. Molar ratios lower than approx. 1.7-1.8 during this acid condensation step might cause resin precipitation. 

The condensation reaction and the increase of the molar mass can also be monitored by GPC [25], With longer duration of the acid condensation step, oligomers of higher molar masses are progressively formed. 

A model similar to that of the iron complex 31 was proposed for the cobalt species synthesized as a result of co-condensation of cobalt vapors with pyrrole in vacuum. A frozen matrix formed is subsequently warmed to room temperature (89JA3881). An oligomer or a polymer results, in which a- and ir-donor functions are realized simultaneously. The model proposed differs from that for the iron pyrrolyl complex by inclusion of the Co—Co bonds to attain the 18-electron configuration. 

The first series of soluble oligo(/ ara-phenylene)s OPVs 24 were generated by Kern and Wirth [48] and shortly after by Heitz and Ulrich [49]. They introduced alkyl substituents (methyls) in each repeat unit and synthesized oligomers 24 up to the hexamer. Various synthetic methods, like the copper-catalyzed Ullmann coupling, the copper-catalyzed condensation of lithium aryls, and the twofold addition of organomelallic species to cyclohexane-1,4-dione, have been thereby investigated. 

By Naylor and Gilliam62, d (pT)6 was condensed in the presence of Poly A with carbodiimide to give d-(pT)12 with a yield of 5%. In the experiment of Shabarova and Prokofiev67, d-(pA)2 preactivated in the form of an amino add amidate was condensed on Poly U to give products with a yield of 10%. Uesugi and Ts o68 studied the condensation of (2 MeIp)6 or (2 MeIp)s in the presence of a Poly C template. The relative overall yield of the oligomer products was 43 to 71%. 

Simidzu et al.70) carried out template-directed synthesis using various types of templates, e. g neutral ones, 34 (PSt TCT) and PSt TCTC, and cationic ones, such as APVP, 35 (APVP PSt), UPVP PSt, ARPVP, APEI, and 36 (ARPEI). They obtained a large amount of purine-containing oligomers (dimer or trimer) as compared to pyrimidine ones by condensation of nucleotide mixture in the presence of the neutral templates containing pyrimidine bases, such as PSt TCT and PSt TCTC. The authors stated from the results that the existence of the... 

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. 

The condensation of arylsulfonyl acetonitriles 369a-c with 22a proceeds via addition of the in-situ formed anion 370 to the arylsulfonyl acetonitriles 369 to afford the dimers 371, in 69-94% yield, and hexamethyldisiloxane 7 [136]. Furthermore, y9-dicarbonyl compounds such as ethyl acetoacetate 372 a or ethyl benzoyl-acetate 372b are O-silylated by 22 a or 22 c to rather stable alkyl 3-O-trimethylsilyl-oxycrotonoate 373a and alkyl 3-0-trimethylsilyloxy-3-phenyl acrylate 373b [130]. Aliphatic nitro compounds such as nitromethane are O-trimethylsilylated and further transformed into oligomers [132] (cf Section 7.6) and are thus unsuitable reactants for silylation-C-substitutions (Scheme 4.50). 

In a few cases, the synthesis was directed towards well-defined oligomers (dimers, trimers, etc.). The synthesis of bis(5,7,3, 4 -tetra-0-benzyl)-EC 4/1,8-dimer from 5,7,3, 4 -tetra-0-benzyl-EC and 5,7,3, 4 -tetra-0-benzyl-4-(2-hydroxyethoxy)-EC was described by Kozikowski et al. [41]. This compound exhibited the ability to inhibit the growth of several breast cancer cell fines through the induction of cell cycle arrest in the Gq/Gi phase. Analogously, procyanidin-B3, a condensed catechin dimer, has been obtained through condensation of benzylated catechin with various 4-0-alkylated flavan-3,4-diol derivatives in the presence of a Lewis acid. This reaction led to protected procyanidin-B3 and its diastereomer. In particular, octa-O-benzylated procyanidin-B3 has been produced with high levels of stereoselectivity and in excellent isolation yields [42]. 

Condensation of the 5 -phosphoric imidazolide of adenosine (ImpA) with itself and P yP -diadenosine 5, 5 -phyrophosphate (AppA) in water in the presence of Na -mont-morillonite clay leads mainly to 3, 5 -linked oligomers. These results are discussed in the context of the potential importance of mineral catalysts to the formation of RNA on the... 

Experiments in which the condensation of AlBr was carried out in toluene/THF solution, where the THF fraction was very much reduced, ended not with the A1X oligomers described above, but with Al22Br2o units best described as Alj 2(AlBr2)10 12(THF) 6 (Fig. 14).52 In this compound, a distorted Al12 icosahedron is connected... 

The reaction mixture consisted only of glycine and water, without the addition of salts or other condensation catalysts. A stepwise, time-dependent synthesis of oligomers up to hexaglycine was observed. On addition of Ca2+ ions (pH 2.5), the oligomerisation continued further to give octaglycine. 

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