Direct condensation

The first route of producing PLA is generated using a polycondensation reaction of the hydroxyl and carboxylic acid group starting from lactic acid (shown in Fig. 23.2). Water produced during the polymerization has to be removed during 

The second route of producing PLA is to ring-open polymerize (ROP) lactide into high molecular weight (Mw 100,000) PLA. ROP are most commonly studied due to the possibility of an accurate control of the chemistry, and the properties of the resulting polymers in a more controllable manner. 

The ROP route includes polycondensation of lactic acid followed by a depolymerization into the dehydrated cyclic dimer, lactide [shown in Fig. 23.3). The depolymerization is conventionally done by increasing the polycondensation temperature and lowering the pressure and distilling off the produced lactide. Due to the two stereoforms of lactic acid, the corresponding optically active lactide can be found in two different versions. In addition, lactide can be formed from one D- and one L-lactic acid molecule yielding D,L-lactide[meso-lactide) [7]. 

Cargill Dow LLC has developed a patented, low-cost continuous process for the production of lactic acid-based polymers. The process combines the substantial environmental and economic benefits of synthesizing both lactide and PLA in the melt rather than in solution and, for the first time, provides a commercially viable biodegradable commodity polymer made from renewable 

Lactide has been used as receive and after recrystallization it from solutions in solvents like chloroform, ethyl acetate, acetone, etc. It has been reported that recrystallized L-lactide gives higher intrinsic viscosities than those obtained for the once-recrystallized monomer [10]. 

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Fructose (V) under similar conditions gives first the phenylhydrazonc (Va) by the direct condensation of the >C 0 group of carbon atom 2 with one molecule of phenylhydrazine. The second molecule of phenylhydrazine then oxidises the primary alcohol group of carbon atom 1 to the -CHO group by removal of two atoms of hydrogen, which as before serve to reduce the phenyl-hydrazine to aniline and ammonia. The compound (Vb) which is thus produced then undergoes direct condensation with the third molecule of phenylhydrazine, giving the osazone of fructose, or fructosazone (Vc). 

When diazoaminobenzene is added to a warm aqueous solution of hydrochloric acid, it tends to break up into its original components, i.e., to benzene-diazonium chloride and aniline, and an equilibrium is thus established. The diazonium chloride and the aniline, however, in addition to recombining to form diazoaminobenzene. also undergo direct condensation at the p-hydro-... 

Aldehydes and ketones may frequently be identified by their semicarbazones, obtained by direct condensation with semicarbazide (or amino-urea), NH,NHCONH a compound which is a monacidic base and usually available as its monohydrochloride, NHjCONHNH, HCl. Semicarbazones are particularly useful for identification of con jounds (such as acetophenone) of which the oxime is too soluble to be readily isolated and the phenylhydrazone is unstable moreover, the high nitrogen content of semicarbazones enables very small quantities to be accurately analysed and so identified. The general conditions for the formation of semicarbazones are very similar to those for oximes and phenylhydrazones (pp. 93, 229) the free base must of course be liberated from its salts by the addition of sodium acetate. 

The primary dihydro product (3) can be obtained by reduction of the quinoid (4). Compound 3, in which Ri = CeH= -CH= and Rj CgH can also be obtained by direct condensation of benzylidene selenosemicar-bazone with 2.3-dibromo-l,4-diphenyl-l,4-butanedione (Scheme 49). 

Mesityl oxide can also be produced by the direct condensation of acetone at higher temperatures. This reaction can be operated ia the vapor phase over 2iac oxide (182), or 2iac oxide—2irconium oxide (183), or ia the Hquid phase over cation-exchange resia (184) or 2irconium phosphate (185). Other catalysts are known (186). 

More conveniently, compound (13) was directly condensed with barbituric acid (14) in acetic acid (28) or in the presence of an acid catalyst in an organic solvent (29). The same a2o dye intermediate (13) and alloxantin give riboflavin in the presence of palladium on charcoal in alcohoHc hydrochloric acid under nitrogen. This reaction may involve the reduction of the a2o group to the (9-phenylenediamine by the alloxantin, which is dehydrogenated to alloxan (see Urea) (30). 

A Japanese patent has claimed improvements in the direct condensation of menadione with phytyl chloride in the presence of a reducing metal such as 2inc or iron powder (30). Tin chloride has been reported to be a useful catalyst for this condensation (31,32). 

Vinyl esters are prepared by the reaction of a fatty acid with either acetjfene in direct condensation or vinyl acetate by acidolysis. 

Polyesters. Polyesters (qv) are widely used as the matrix for conventional composites. Two resins of particular importance because of the large amounts used are (poly(ethylene terephthalate) [25038-59-9] (PET) and poly(butylene terephthalate) [24968-12-5] (PBT). Although polyesters can be made from diacids and diols by direct condensation. 

The free selenazole hydrazines are solids, sometimes well crystallized compounds. They show the typical properties of hydrazines. Thus they reduce Fehling s solution on warming and liberate silver, even in the cold, from ammoniacal silver nitrate solution. Further, they react with carbonyl compounds for example, benzylidene hydrazones are formed with benzaldehyde. These are identical with the hydrazones formed by direct condensation from benzaldehyde selenosemicarbazone and the corresponding a-halogenocarbonyl compound. 2-Hydrazino-4-phenylselenazole has also been reacted with acetophenone. The 2-a-methylbenzylidenehydrazone of 4-phenyl-selenazole (2, K = CJl, R" = H, R" = NH—N CMe-aH ) forms golden yellow plates mp 171°C. ... 

This sequence is equally applicable to keto esters. Thus, condensation of guanidine with ethyl acetoacetate gives the pyrimidone, 134. Elaboration as above gives the pyrimidine, IJ5 acylation with the sulfonyl chloride (88) followed by hydrolysis yields sulfamerazine (107). Reaction of guanidine with beta dicarbonyl compounds gives the pyrimidine directly. Condensation of the base with acetonyl acetone affords the starting amine for sulfadimidine (108). ... 

Furthermore, macro-azo-initiators have also been synthesized by direct condensation of AIBN with diols (Pinner reaction, [42,43]) or formaldehyde [44,45]. The... 

Two isomeric pyrimido[4,5-6]diazepinones, 13 and 15, are obtained from pyrimidine-4,5-diamine and ethyl acetoacetate.297 Compound 13 is obtained by direct condensation of the components. 

Benzimidazoles have been prepared by direct condensation of o-phenylene-diamine 190 with a carboxyhc acid at 110 °C in water and under microwave irradiation (Scheme 69) [124]. Differently functionahzed benzimidazoles 192 were obtained with this method especially regarding the nature of the substituent on the imidazole ring. 

The presence of PSCs also leads to the removal of nitrogen oxides (NO and NO2) from the gas phase. As long as there are significant amounts of NO2 it will react with chlorine monoxide (CIO) to produce chlorine nitrate (reaction 11). This species subsequently reacts with HQ on PSC surfaces to produce nitric acid (reaction 13), which remains in the condensed phase. Also, nitric acid directly condenses with water to form nitric acid trihydrate particles, hence it is not available to regenerate NO2 by photochemical processes, as it does when it is in the gas phase. 

In the solid state, metal clusters can connect to each other through outer and/ or inner ligands, or by direct condensation of the metal octahedra (Fig. 6.2), to form a variety of frameworks. 

This direct condensation of alcohols has limited use as a route to polymers because many alcohols undergo other reactions under the conditions required for condensation. Nature forges ether linkages in a more roundabout way than by eliminating water directly. The most important pol Tners containing ether linkages are two biochemical macromolecules, starch and cellulose. 

By direct condensation of suitable tetraaldehydes with pyrolle, workers 236) have succeeded in synthesizing capped porphyrins a particular one is shown in Fig. 16. 

Since the amine by-product formation was essentially derived from the reaction of an enamine or a ketone with iodoaniline, the direct use of a ketone as the substrate instead of an amine, would also be expected to yield the indole (Scheme 4.21). Indeed, we were gratified to find that direct condensation of o-iodoaniline 24 (77, R, = H) with cyclohexanone (in the presence of 5mol% Pd(OAc)2 and 3 equiv DAB CO as a base at 0.3 M and 105 °C afforded the tetrahydrocarbazole 81a in 77% yield with no other major impurities (Figure 4.4) [5], The use of DMF as a solvent is crucial to the success of this reaction other solvents such as acetonitrile and toluene were ineffective. 

For the elongation of the chain starting from 11 toward the N-terminal group, direct condensation of the activated p-nitrophenyl esters of such amino acids as N-protected L-cysteine, L-glutamic acid, glycine, L-leu-cine, L-proline, L-serine, L-tyrosine, andL-valine with 2-acetamido-3,4,6-... 

The macrocyclic tetraene 2,3,9,10-Me4[14]tetraene (tmt) may be synthesized by a metal-directed condensation between trimethylenediamine (tn) and 2,3-butanedione in the presence of Co(OAc)2, ultimately resulting in the trans-dichloro Co111 complex [Co(tmt)Cl2]+ (59).302... 

Modro has synthesized a series of phosphoric triamides from P(0)C13 as outlined in Scheme 33 <2001PJC1089>. This approach was not successful in forming the thio analogue but direct condensation of P(S)C13 with the triamine 322 yielded the thiophosphoryl analogue compound 323 (Scheme 34). 

The simultaneous protection of two hydroxyl groups by condensation of sugars with carbonyl compounds to give 1,3-dioxolanes or 1,3-dioxanes is also applicable to D-glucuronic acid. Principally, for the preparation of O-alkylidene-a-D-glucofuranuronic acid derivatives, two methods are available 1, direct condensation of 2 or 4 with... 

These reactions are catalyzed by ammonium acetate, the function of which is to generate protonated imines (322). Under mild reaction conditions, condensation of a- hydroxyamino-oximes with acetone dialkylketals takes place. The procedure can be successfully applied in cases where direct condensation with acetone... 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

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