Second Step Polycondensation

Bis-(2-hydroxyethyl) terephthalate obtained during the first step is heated between 280 °C and 300 °C under vacuum (10-50 Pa). Ethylene glycol is formed and is eliminated as by-product. Polymers with polymerization degree closed to 100 are classically obtained. The reaction time including the two steps is long and usually varies from 5 to 10 h. Reaction time could be reduced by high temperature 

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In direct melt polymerization, PBS is prepared using a two-step process. In the first step, esterification takes place at a temperature ranging from 150 to 200°C under atmospheric pressure or in a low vacuum, hi the second step, polycondensation is followed under a high vacuum at a higher temperature, e.g., 220-240°C, for... 

Polyethylene terephthalate (PET) is a linear thermoplastic polymer, which was initially commercialized for packaging carbonated soft drinks due to its excellent gas barrier properties that allows it to retain CO. Raw materials used for production of PET are ethylene glycol and terephthalic acid (or dimethyl terephthalate). Production of PET is a two-step process in the first step trans-esterification or esterification takes place, depending on whether terephthalic acid or dimethyl terephthalate is used, and in the second step polycondensation of resulting oligomers produces PET (Massa et al.,2011). 

Starting with DMT, methanol is removed from the reaction starting with TA, water is removed. Catalysts ate used to transesterrfy DMT but not for direct esterification of TA. The second step is the polycondensation reaction which is driven by removing glycol. A polycondensation catalyst is used. 

A PEIT of 50/50 (molar ratio) composition is synthesized by a two-step reaction sequence as follows. In the first step, 97.10 g (0.5 mol) dimethyl terephthalate (DMT), 97.10 g (0.5 mol) dimethyl isophthalate (DMI), 136.55 g (2.2 mol) 1,2-ethanediol, and zinc acetate dihydrate ester interchange catalyst (2.7 x 10 4% mass of the total amount of DMI and DMT mixture) are weighed into a threenecked flask fitted with a mechanical stirrer, a nitrogen inlet, and a condenser. The medium is stirred for 2.0-2.5 h at 180-210°C under nitrogen. Ninety-two percent of the theoretical amount of methanol is removed by distillation. In the second step, antimony acetate polycondensation catalyst and trimethyl phosphate thermal stabilizer (9.9 x 10-4 and 1.5 x 10 3% mass of the total amount of DMI... 

Second step solid state post-polycondensation 260-280 °C, 0.7 mbar (4 h)... 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

The second step is the thermal conversion of borylaminoborazine into poly(borylaminoborazine). Continuous elimination of parent alkylamine is the main by-product dining the thermal polycondensation of 2,4,6-trialkylaminoborazine (see earlier). We expected the continuous elimination of the starting 5-alkylam i noborane during the thermal polycondensation of borylborazine. However, the alkylami noborane is a liquid, which requires that the thermal polycondensation must be performed in vacuo to continuously remove the evolving B(NHR)3 from the reaction mixture. This procedure also precludes any competing polycondensation reaction from B(NHR)3. 

The principle of the second synthetic approach to polycatenanes, i.e. stepwise polycondensation, has been proposed by Shaffer and Tsay, but not experimentally demonstrated [42, 43], This approach has the advantage over multifunctional polycondensation that a linear polymer is formed before cyclization (Scheme 7). However, the second step, which consists of the cyclization of n macrocycles along the polymer chain 19, is likely, again, to give rise to an undefined network, containing some rotaxane and catenane units 21, similar to the multifunctional polycondensation approach. 

This process involves two steps. In the first step, a,co-dihydroxy-terminated oligo(propylene succinate)s (SP) were prepared by the thermal polycondensation of excess diol and diacid, (see first part of Scheme 9). In the second step, the... 

This parallelism is reflected in the proposed mechanism for the ionization of methane which shows that (a) the second step of the scheme invoives attack of an ethyl cation on methane, but the reaction cannot stop there, and goes on to (b), the third step, which involves attack of a secondary-isopropyl cation on methane. The primary and secondary alkyl cations are very strongly acidic species and are unstable under the reaction conditions. The condensation reaction essentially terminates with the much more weakly acidic tertiary-butyl ion. Alkane polycondensation and olefin polymerization side reactions producing stable, less acidic, tertiary ions obscured the simple alkylation reactions of the primary and secondary alkyl cations. Implicit in this mechanism, however, is that it is possible to react an acidic energetic primary cation (such as the ethyl cation) with molecules as weakly basic as methane and thus, the door was opened to new chemistry through activation of the heretofore passive, weakly basic, "paraffins" (20-24). 

Metal-2,9,16,23-tetraaminophthalocyanines have been employed in the synthesis of polyimides and as curing agents for epoxy resins [133-135] Variables such as molar concentrations of the reagents, solvents and temperature were investigated to improve the conditions of the polycondensation. Solutions of the polyamide-acid copolymers can be used to fabricate films or fibers. The polyimide copolymers obtained in the second step of the reactions are insoluble. Excellent thermal stabilities up to 500 °C in air and 600 °C under vacuum were reported. 

The formation of a microcapsule wall through interfacial polycondensation/addition takes place in two steps. First step is the deposit of the oligomer (initial wall) at the oil droplet, and the second step is the wall thickness builds up. As described earlier, the polymerization occurs in oil phase, and the formed initial wall can limit the diffusion of the reactants. This reduces the polymerization rate that has great impact on the surface morphology and thickness of the microcapsule wall. - - Polycondensation by which polyamide, polyester, and polycarbonate microcapsules are prepared can generate acid byproduct during the process therefore, a base is needed to neutralize the acid and drive the reaction to complete. ... 

PET and PBT can be eonveniently synthesized, both at laboratory and industrial scale, in the presenee of metal eatalysts in a two-step-polyesterification of the diols, ethylene glycol or 1,4-butanediol respectively, and dimethyl terephthalate (DMT) or terephthalic acid (TPA). The first step is basically a transesterification or esterification process depending on the use of DMT or TPA respectively, while the second step is the polycondensation of the resulting oligomers. The polycondensation step occurs at higher temperature and at reduced pressure to facilitate the distillation of the diols and consequently the polymerisation of the oligomers. 

The first is the formation of 2,6-bis-(hydroxyethyl) arylate (BHEA) and bis-(hydroxymethylcyclohex-ane)-arylate (BHCA), respectively, from the transesterification of 2,6-dimethyl naphthalate or dimethyl terephthalate (DMT) with ethylene glycol or CHDM. The second step is the formation of PECA from polycondensation of the BHEA and BHCA mixture at elevated temperatures and reduced pressure [35]. 

Reactions (4.6)-(4.9) represent the typical esterification reactions, while reaction (4.10) is the polycondensation reaction, occurring mainly in the second step of polyester formation. Finally, reaction (4.11) is a side reaction resulting in diglycol repeating units, with ether Hnkages in the oligomeric chain. ki(i = 1,6) and k. i = 1,5) representing the kinetic rate constants of the six elementary reactions (lmol min ). 

In this section, the kinetics of the second step, that is, the polycondensation reaction of aliphatic polyesters, is investigated and a simple theoretical model is proposed to simulate both esterification and transesterification reactions taking place during polycondensation. 

An example of such a random reaction is the polycondensation of a,co-di-carboxylic acids, HOOC—R —COOH (S), with glycols, HO—R—OH (G). In the first reaction step, the monoester GS forms from G and S, with the elimination of water, and this has a degree of polymerization of 2. In the second step, this semiester can add on either another glycol molecule or another acid molecule, etc., as follows ... 

The resorcinol-formaldehyde polymer is classified as a phenolic resin. In fact, phenol-formaldehyde condensates are the first synthetic polymers introduced commercially (Bakelite) in the beginning of the twentieth century. Thus, the phenolic resin chemistry, on which there is a vast amotmt of information, can be used to understand the chemistry of the RF aerogel synthesis. In the first step, resorcinol reacts with formaldehyde to form hydro-xymethylated resorcinol in the second step, the hydroxymethyl groups condense with each other to form nanometer-sized clusters, which then crosslink by the same chemistry to produce a gel. The formation of clusters is influenced by typical sol-gel parameters such as the temperature, pH, and concentration of reactants. Though the first RF aerogel is synthesized by the aqueous polycondensation of resorcinol with formaldehyde with the use of sodium carbonate as catalyst [6, 12, 13], now several literature reports exist for aerogels made from phenol and formaldehyde [14—16]. 

Polybutylene terphthalate (PBT) is a semicrystalline thermoplastic polyester considered as a medium performance engineering polymer. It is produced industrially in a two-step batch or continuous process. The first step involves the transesterification of dimethyl terephthalate (DMT) with 1,4-butanediol (BDO) to produce hydrobutyl terephtlate (bis-HBT) at a temperature of 200 C. The second step consists to the polycondensation of bis-HBT at 250 C to yield PBT. It exhibits both excellent electrical properties and chemical resistance. When reinforced with glass fibers, it has improved stiffness and mechanical strength. Typical uses include connectors, capacitors and cable enclosures. PBT is also used in hot appliances such as iron and kettles. 

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