Step-growth condensation polymerisation

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. 

Heck-type step-growth condensation polymerisation involves mainly palladium-based catalysts, although nickel-based catalysts are also effective. It is worth noting that this polycondensation requires a change in the oxidation state of the metal (e.g. Pd) [schemes (30) and (31)] [71], which is in contrast to chain growth polymerisation, such as ethylene/carbon monoxide alternating copolymerisation promoted by Pd-based catalysts [schemes (82) and (83) in Chapter 3], for which the preservation of the oxidation state of palladium, Pd(II), is typical [83-85] ... 

This is the usual method for step-growth (condensation) polymerisation. The reaction is often carried out at a high temperature, but there are no real problems with heat transfer out of the reaction vessel (i.e. temperature build-up). The degree of polymerisation increases linearly with time, so that the viscosity of the reaction mixture only increases relatively slowly this allows for efficient gas (e.g. water vapour) bubble transfer out of the system as well. 

Step-growth condensation copolymerisations give rise to additional experimental difficulties, with respect to the former reactions studied, due to the continuous release of e.g. water. Indeed, the evaporation of water produced by the reaction may obscure the detection of the cure process and prohibit a reliable quantification of the reaction heat and the reaction conversion. To illustrate how condensation polymerisations can be studied by MTDSC, the post-cure condensation reactions of melamine-formaldehyde (MF) resins will be discussed [91]. 

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

In the condensation polymerisation or step growth polymerisation, the polymer molecules are built up through many separate reaction of functional groups. 

The process of condensation takes place in a stepwise manner, ultimately resulting in the formation of polymer. This process is also known as step growth polymerisation. 

The polymerisation reaction is a step growth polymerisation similar to a condensation polymerisation of amides or esters. The reaction starts with monomers, which dimerise, trimerise, etc. continuously maintaining a Flory-Schulz distribution. 

There are two main chemical mechanisms by which a synthetic polymer may be produced namely by either a condensation (step growth) polymerisation or addition (chain) polymerisation. 

The addition and condensation polymers are nowadays also referred as ehaln growth polymers and step growth polymers depending on the type of polymerisation mechanism they rmdergo during their formation. 

IPolymerisation chain growth polymerisation and condensation or step growth... 

In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on. Since, each step produces a distinct functionalised species and is independent of each other, this process Is also called as step growth polymerisation. 

More complex polymer structures can be made by step-growth polymerisation usually two monomer structures alternate in the chain. The alternative name is condensation polymerisation, since a by-product of water or other small molecule is often produced. Each monomer molecule has reactive groups at both ends. For example a diol can react with a dibasic acid. 

These polymers are formed by rearrangement of the monomer(s) or reactant(s) in an incremental manner, without elimination of any byproducts. Though they do not fall into either of the previous two classes, they exhibit some characteristics of both for example, polyurethane, which is formed by a step growth polymerisation mechanism. It is not formed by condensation (as no by-product is formed), nor is it an addition polymer, as it is not formed by chain growth mechanism. 

The process by which bi-or poly-functional reactants are condensed to form polymer chains with the elimination of small molecules in each condensing step, is known as condensation polymerisation. As the polymer chains are grown incrementally, it may also be called step growth polymerisation. Esterification (direct or ester exchange), amidation, nucleophilic and electrophilic substitution are the general reactions for this polymerisation process. The formation of vegetable oil-based polyester is an example of this type of polymerisation (Fig. 1.1). 

Some examples of commercially important step-growth polymerisations together with non condensation step-growth polymerisations are illustrated in Figure 6.1. 

Figure 6.1 Examples of important condensation (a, b, c and d) and non-condensation (e) step-growth polymerisations, (a) Polyesterification, (b) Ester-interchange polymerisation, (c) Polyamidation. (d) Self-condensation of an A-B monomer.

The industrial process (Figure 1) consists of a reactor (acting as the reboiler), a packed column, a total condenser and two distillate vessels. The polymer is manufactured through reversible linear polycondensation or step-growth polymerisation. The overall reaction can be characterised by the following scheme ... 

In case of condensation or step-growth polymerisation (p. 169), the growing polymer chain continues to have one active group at each end and the polymer does not die . Cliain lengthening, however, may stop due to decreased activity as a result of increase in the size and molecular weight of the polymer and due to cyclization. 

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