Polycondensation step growth reaction

Polycondensation is considered to be a step growth reaction . The process often (but not always) needs a catalyst which is usually a metal salt or a combination of metal salts. 

The degree of polymerisation is generally lower than in the case of chain polymerisation (between 1000 and 10000) due to inherent process characteristics. The molecule grows step by step at a relatively slow rate. The growth proceeds slowly from monomer to dimer, trimer, etc. until full sized macromolecules are formed only at very high conversion rates towards the end of the reaction time as illustrated by the Table 2.1  

Generally, polycondensation reactions are carried out either in bulk or in organic solvents. 

The control of oxygen is important not only for safety reasons, but also for product quality. Oxygen causes side reactions resulting in products which discolour the end-product and increase the concentration of low molecular weight products. These parts either remain in the product or have to be removed and sent for waste treatment, for instance incineration. The high reaction temperature at the end of the reaction may also lead to degradation products, which also cause discoloration. Localised heat spots have to be avoided. 

The build-up of solid layers in the inside of the reactors or heat exchangers also occurs in these reactions (see Section 2.3.2.1). 

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In step growth reactions, on the other hand, neither are specific activated centers present to force the connection of the monomers, nor does the process occur as a cascade reaction. Instead, the monomers are tied together in discreet, independent steps via conventional organic reactions such as ester-, ether-, amide-, or urethane formation. Depending on whether small molecules are set free in the connection step, one distinguishes between polycondensations (2.9) and polyadditions (2.10) ... 

W.H. Carothers, the pioneer of step-growth reactions, proposed a simple equation relating to a quantity p describing the extent of the reaction for linear polycondensations or polyadditions. 

HMF is a suitable precursor for the synthesis of bifunctional furan monomers as summarized in Scheme 4. All these monomers can be used to prepare polycondensates by step-growth reactions with the other corresponding bifunc-tional monomers derived either from petrochemical precursors or from renewable resources. The polycondensates obtained, such as polyesters, polyamides, and polyurethanes, etc. have been characterized [107-113]. Scheme 5 shows another approach for the synthesis of bifunctional monomers through acid-catalyzed condensation of the corresponding mono-functional furan derivatives with an aldehyde... 

HMF on the other hand, is ideally suited as a precursor to bifunctional furan monomers to be used in step growth reactions as shown in Scheme 6.12 which includes FDCA and FCDA. Again, all these compounds wctc synthesized and used to prepare the corresponding polycondensates, like polyesters, polyamides, polyurethanes, etc [4]. 

Polycondensation Step growth polymerization in which small molecules on each reaction... 

Two different types of step-growth reactions exist polycondensation reactions, where during the polymerization a low molecular by-product is formed (e.g., water or a lower alcohol), and polyaddition reactions, where... 

The chemistry of main chain elastomers is limited to step-growth reactions, i.e., polycondensation and polyaddition reactions, which demand the highest purity of the starting materials and experimental conditions which exclude side reactions. 

There are several approaches to the synthesis of heterochain PCSs, which can be divided into three large groups chain ring opening and vinyl polymerizations, step-growth reactions (polymerization and polycondensation), and polymer-analogous transformations. 

Alternatively, some native terpenes like bile acids, which carry hydroxyl and carboxyl functionalities in the same molecule, can undergo step-growth reactions, usually polycondensations, which will be described in Sect. 2.2. 

Two cautions are in order about the preceding example. First, by writing an irreversible rate expression, we have assumed that any molecule of condensation is being continuously and efficiently removed from the reaction mass so that there is no depolymerization. Second, not all step-growth reactions are of second order. Some polyesterifications, for example, are catalyzed by their own acid groups and are, therefore, first order in hydroxyl concentration, second order in acid, and third order overall. The rate may also be proportional to the concentration of an added catalyst (usually acids or bases for polycondensations), if used. 

Metal catalyzed polymerizations are chain reactions necessitating a constant valence of the metal. In contrast to that metal catalyzed polycondensations are step growth reactions involving a change of the valence of the metal. Tuning of the reaction by structural variations of the metal catalyst are demonstrated with the Pd-catalyzed vinyl polymerization of norbomene, the alternating copolymerization of ethylene with carbon monoxide and the Heck reaction as examples. One and two electron processes can be involved in metal catalyzed polycondensations. The Heck reactions, the Ni-catalyzed synthesis of polyphenylenes, and Ru-catalyzed ArH-insertion reactions are discuss. ... 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

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. 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

Acyclic diene metathesis (ADMET) is a step-growth polycondensation reaction for the polymerization of o -dienes 729 The process is catalyzed by the same metal alkylidene initiators used for ROMP, and is driven by the removal of ethylene from the system (Scheme 13). Both molybdenum and ruthenium-based initiators have been used to prepare a variety of materials including functionalized polyethy-... 

Step-growth condensation polymers, such as polyesters and polyamides, are formed by reversible reactions. In the case of PET, the commercial synthesis is essentially carried out by two reactions. The first is the formation of bishydroxyethyl terephthalate by esterification of a diacid with a glycol or by transesterification of a diester with a glycol. The second is the formation of the polymer by a polycondensation reaction. 

The new polymers 9-28 were synthesized by the Pd-catalyzed reaction of (4,4 - or 5,5 -dibromo-2,2 -bipyridine)-bis(4,4 -terf-butyl-2,2 -bipyridine or 2,2 -bipyridine)ruthenium(II) complexes 5-8 and diethynylarenes (Table 2) in a step-growth polycondensation mechanism (Scheme 4). The typical reaction conditions used for the synthesis of the polymers involved stirring the argon-... 

Let us emphasise that the driving force for acyclic diene metathesis, which is a step-growth condensation polymerisation, is the release and removal of a small condensate molecule. The polycondensation is performed preferably under bulk conditions (no solvent used), since acyclic diene metathesis is thermally neutral and there is no need to remove the heat of the reaction, in contrast to exothermic cyclic olefin ring-opening metathesis polymerisation. 

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