Reactions polycondensation

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. 

New heat-resistant polymers containing -iiitrophenyl-substituted quinoxaline units and imide rings as well as flexible amide groups have been synthesi2ed by polycondensation reaction of a dianainoquinoxaline derivative with diacid dichlorides (80). These polymers are easily soluble in polar aprotic solvents with inherent viscosities in the range of 0.3—0.9 dL/g in NMP at 20°C. AH polymers begin to decompose above 370°C. 

The mechanism of the polycondensation reaction remains unclear. A vanety of possible reactive intermediates have been suggested, including sdyl radicals and sdyl anions. An anionic propagation mechanism (100,101,103) has been strongly suggested, although the case is by no means setded (104). Other Synthetic Methods. 

The second largest use at 21% is for unsaturated polyester resins, which are the products of polycondensation reactions between molar equivalents of certain dicarboxyhc acids or thek anhydrides and glycols. One component, usually the diacid or anhydride, must be unsaturated. A vinyl monomer, usually styrene, is a diluent which later serves to fully cross-link the unsaturated portion of the polycondensate when a catalyst, usually a peroxide, is added. The diacids or anhydrides are usually phthahc anhydride, isophthahc acid, and maleic anhydride. Maleic anhydride provides the unsaturated bonds. The exact composition is adjusted to obtain the requked performance. Resins based on phthahc anhydride are used in boat hulls, tubs and spas, constmction, and synthetic marble surfaces. In most cases, the resins contain mineral or glass fibers that provide the requked stmctural strength. The market for the resins tends to be cychcal because products made from them sell far better in good economic times (see Polyesters,unsaturated). 

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

Tankage of additional =Si — OH tetrahedra occurs as a polycondensation reaction (eq. 4) and eventually results in a Si02 network. The H2O and alcohol expelled from the reaction remain in the pores of the network. 

The hydrolysis and polycondensation reactions initiate at numerous sites within the TMOS/H2O solution as mixing occurs. When sufficient intercoimected Si—O—Si bonds are formed in a region, the material responds cooperatively as colloidal (submicrometer) particles or a sol. The size of the sol particles and the cross-linking within the particles, ie, the density, depends on the pH and R ratio, where R = [H2 0]/[Si(0R)4]. 

Polycondensation reactions (eqs. 3 and 4), continue to occur within the gel network as long as neighboring silanols are close enough to react. This increases the connectivity of the network and its fractal dimension. Syneresis is the spontaneous shrinkage of the gel and resulting expulsion of Hquid from the pores. Coarsening is the irreversible decrease in surface area through dissolution and reprecipitation processes. 

The addition of an alkanolamine, such as diethanolamine, to TYZOR TBT, as well as the use of a less moisture-sensitive alkanolamine titanate complex such as TYZOR TE, has been reported to prolong catalyst life and minimi2e ha2e formation in the polymer (475—476). Several excellent papers are available that discuss the kinetics and mechanism of titanate-cataly2ed esterification and polycondensation reactions (477—484). 

PolybenZimidaZoles. The polyben2imida2oles (PBIs) are generally produced by the high temperature, melt polycondensation reaction of aromatic bis-ortho-diamines and aromatic dicarboxylates (acids, esters, or amides) in a reaction such as that shown in equation 11 to form ben2imida2ole [51-17-2] as the repeating unit. 

Recently, the above mentioned model reaction has been extended to polycondensation reactions for synthesis of polyethers and polysulfides [7,81]. In recent reports crown ether catalysts have mostly been used in the reaction of a bifunctional nucleophile with a bifunctional electrophile, as well as in the monomer species carrying both types of functional groups [7]. Table 5 describes the syntheses of aromatic polyethers by the nucleophilic displacement polymerization using PTC. 

Interfacial polymerization is mainly used in polycondensation reactions with very reactive monomers. One of the reactants, usually an acid... 

Table 2 shows a list of collagen model peptides which have teen prepared. Many efforts have been made to prevent racemization. The polycondensation reaction seemed to be more sensitive to racemization than the coupling steps preparing the monomeric tripeptide. Therefore, the sequence of the monomer was selected with Gly or Pro at the C-terminal chain end, because racemization is mostly favored at the carboxy-activated amino acid, and these amino acids cannot racemize. 

Activating agents, such as trifluoroacetic anhydride 1,1 -carbonyldiimidazolc carbodiimides sulfonyl, tosyl, and picryl chlorides and a range of phosphorus derivatives can promote direct solution reactions between dicarboxylic acids and diols or diphenols in mild conditions. The activating agents are consumed during the reaction and, therefore, do not act as catalysts. These so-called direct polycondensation or activation polycondensation reactions proceed via the in situ transformation of one of the reactants, generally the carboxylic acid, into a more... 

As is die case for odier polycondensation reactions, internal interchange reactions are possible for ADMET, similar to diat of polyesters and polyamides.16 Interchange reactions involve a catalyst molecule on a polymer chain end reacting widi an internal double bond in another polymer chain. The result is two new polymer chains however, no change in the molecular weight distribution... 

Another factor in step-growth polymerizations is cyclization versus linear polymerization.1516 Since ADMET is a step-growth polymerization, most reactions are carried out in the bulk using high concentrations of the reactant in order to suppress most cyclic formation. A small percentage of cyclic species is always present but is dependent upon thermodynamic factors, typical of any polycondensation reaction. 

At the same time, ring-opening-polymerization (ROP) processes, which dominated the phosphazene field for decades [38], tend now to be substituted by polycondensation reactions. These seem to be more feasible and reproducible, easier to carry out, and able to guarantee predictable MWs and MW distributions for these materials [10]. 

A variety of synthetic procedures have been described based on the ringopening polymerization processes of (NPCl2)3 to (NPCl2)n followed by the nucleophilic replacement of the reactive chlorines with carefully selected nucleophiles, and on polycondensation reaction processes of new monomers and of substituted phosphoranimines. 

Furfuryl alcohol in an acid medium gives rise to reactions of polycondensation reactions of successive electrophilic substitutions involving furan molecules. This reaction is identical to the reaction described for benzyl alcohol on p.256 and represents the same dangers. It is carried out under the same conditions, ie in a sulphuric medium. The electrophilic species that comes into play is very similar to the benzyl cation. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

It was assumed that linear polymeric particles are formed in the low water content solutions which show spinnability on the way of progressing hydrolysis-polycondensation reaction. In order to confirm this, the molecular weights and intrinsic viscosities of the solutions listed in Table 1 have been measured (2.). Figure 2 shows the log Mjj versus log[ri] plots. The slope of the plot a is larger than 0.5, that is, 0.75 and 0.64 respectively for solutions 1 and 2 of... 

Table I. Polycondensation Reactions between 1,1 -Bis(6-amino-ethyl)ferrocene (1 and (6-hydroxyethyl)ferrocene (2) with Diacid Chlorides and Diisocyanates...

The synthesis of block polymers of diacetylene-silarylene and diacetylene-carboranylenesiloxane polymers (99a-e) (Fig. 61) by the polycondensation reaction of 1,4-dilithiobutadiyne with l,4-bis(dimethylchlorosilyl) benzene and/or l,7-bis(tetramethylchlorodisiloxane)-m-carborane have been reported by Sundar and Keller.129 These polymers are a hybrid between the carboranylenesiloxane and silarylene-siloxane polymers and have high char yields (up to 85%) at 1000°C in N2 and in air, reflecting the thermal stabilizing effects of the carborane and aromatic units in the polymeric backbone. 

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