Step growth polymerisation

Addition and condensation polymerisation belong to this class of polymerisation mechanisms. The specific feature is that all species with reactive groups can take part in the curing process (also called polymer coupling ). 

Epoxy-amine systems follow an addition step-growth polymerisation mechanism. The two principal reactions of primary and secondary amines with epoxy oligomers are shown in Reaction scheme 1 [30]. These reactions are catalysed by acids, phenols and alcohols (e.g. impurities in commercial epoxy resins). The presence of water causes a tremendous acceleration, but does not alter the network structure. The hydroxyl groups formed by the amine-epoxy addition steps are also active catalysts, so that the curing reaction usually shows an accelerating effect in its early stage (autocatalysis). 

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The differences between chain growth and step growth polymerisations are shown in Table 1. 

Table 1 Chain growth vs. step growth polymerisations...

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. 

Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 - butadiene and styrene can form a copolymer. 

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. 

Independently of each other, Lambert et al. [69] and Suzuki et al. [70] both gained access to low-generation silane dendrimers (Gl, G2) in 1995. The latter prepared a first-generation polysilane dendrimer 3 by a stepped growth polymerisation technique. Coupling of methyl[tris(chlorodimethylsilyl)]silane (1) to tris-(trimethylsilyl)silyllithiurn (2) led to the first-generation branched dendrimer 3 (Fig. 4.38). 

P. R. Dvomic, Polymer chemistry 1930-1960 the founding of a new science Part I free radical and step growth polymerisation reactions , Hem. Pregl., 1992, 33, 117-125 [in Serbo-Croat],... 

Conjugated polymers may be made by a variety of techniques, including cationic, anionic, radical chain growth, coordination polymerisation, step growth polymerisation or electrochemical polymerisation. Electrochemical polymerisation occurs by suitable monomers which are electrochemically oxidised to create an active monomeric and dimeric species which react to form a conjugated polymer backbone. The main problem with electrically conductive... 

Step-growth polymers, such as polyamides and polyesters, are prepared by reactions between difunctiunal molecules. Polyamides (nylons are formed by step-growth polymerisation between a diacid and a diamine polyestera arc formed from a diacid and a dtol. 

Scheme 6.6 (a) Step-growth polymerisation. Short chains can link to give a variety of sequences, (h) Chain-growth polymerisation involves the addition of new molecules to one end of a growing chain. A represents a free radical or similar active centre where addition of new material can occur... 

The main types of polymerisation reaction are addition and step-growth polymerisations. Commodity plastics are all made by addition polymerisation, in which a vinyl monomer (one containing a C=C double bond) is converted into the polymer by the opening of the double bond. For example, the polymerisation of ethylene can be written... 

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. 

The theoretical form of the MWD can be calculated for step-growth polymerisations, since the reaction is at equilibrium. The number fraction fi of molecules with i repeat units is equal to the probability that a molecule chosen at random has i repeat units. Since each step in forming a chain is mutually exclusive, we can multiply the probabilities that the first unit is... 

Polyester thermosets are based on partly unsaturated linear polyester from the step-growth polymerisation of propylene glycol, phthalic anhydride and maleic anhydride. 

Verify Eq. (2.8) for the molecular mass averages of a step-growth polymerisation. Smn the series obtained when Eq. (2.7) is substituted in the formulae for Mn and Mw. 

More generally the repeat unit is not the same as the monomer or monomers but, as already indicated, it is nevertheless sometimes called the monomer . Some of the simpler, classical processes by which many of the bulk commercial polymers are made are described below. These fall into two main types, addition polymerisation and step-growth polymerisation. 

An important example of a reaction employed in step-growth polymerisation that does not involve the elimination of a small molecule is the reaction of an isocyanate and an alcohol... 

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). 

While for step-growth polymerisations (section 2.2.1) and chain-growth polymerisations without termination (section 2.2.2) an overall distribution of reacting species or one type of reacting species (the monomer) is mechanistically characterising the observed reaction rate, the balance between two distinct and mechanistically different species (the monomer and a macro-molecular radical) is determining the observed rate of chain-growth polymerisations with termination. 

Note that although the results reported by Yu and von Meerwall [65] and by Deng and Martin [66,67] showed a dependency of diffusion coefficient on molecular weight, the diffusion of reactive groups toward each other concerned with epoxy cure reactions (step-growth polymerisation reactions) is only controlled by segmental diffusion irrespective of the size of molecules to which they are attached. Neither molecular weight nor symmetry or composition play any role within overall diffusion control. 

The different conversion-dependence of rj is related to the molecular weight evolution and network development. For addition step-growth polymerisation systems, the molecular weight of the polymer chains gradually increases, while for (linear) free radical chain-growth polymerisations the... 

It also should be pointed out that the auto-acceleration at high conversion closely before the onset of vitrification is not observed for step-growth polymerisation thermosetting systems [42,68,80,83-85]. 

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