Step-addition polymerisation

As mentioned previously, the synthesis of polyurethanes, by the reaction of a diisocyanate (or polyisocyanate) with oligo-diols (or oligo-polyols), is a polyaddition reaction (or step-addition polymerisation), a particular type of polycondensation reaction. There is a great difference between the polycondensation and the polyaddition reactions and the classical radical polymerisation or ionic (living) polymerisation reactions. In radical polymerisations (typical chain reactions), the high MW polymer is formed at the beginning of polymerisation. The reaction system is constituted from monomer and high... 

In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high MW polymer is formed only in the last stages of the poly addition reaction, at high conversion rates. Chain transfer and termination reactions are absent. 

In Figure 2.6 one can compare, the MW growth of polymers in radical, living anionic and step-addition polymerisation reactions. 

Addition polymerisation is effected by the activation of the double bond of a vinyl monomer, thus enabling it to link up to other molecules. It has been shown that this reaction occurs in the form of a chain addition process with initiation, propagation and termination steps. 

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. 

Most polymerisation reactions can be assigned to two catagories usually described as addition and condensation polymerisation (sometimes called chain reaction polymerisation and step reaction polymerisation , respectively). In both types of reaction, some form of initiator or catalyst is usually required. The tranformation which occurs in the addition polymerisation of a single monomer species can be represented as... 

Stepwise addition polymerisation (polyaddition) or condensation polymerisation (polycondensation) are possible polyreactions for the first step. The two latter combinations attained interest in the technical synthesis of polyimides and polybenzimidazoles. 

The range of monomers which can be employed is largely dictated by the physical chemistry of the emulsion system. For instance, monomers must be sufficiently hydrophobic to allow the formation of stable w/o HIPEs. In addition, most systems which have been studied have used polymerisation methods which require either an initiation step, or addition of a catalyst. This is due to the fact that the first step in the preparation of the polymer is the preparation of HIPE this can only proceed satisfactorily in the absence of any significant degree of polymerisation. Thus, it can be seen that radical addition polymerisation is suitable for the synthesis of PolyHIPE polymers, whereas condensation polymerisation can be more problematical. Also, the latter reactions often generate water as the by-product, hence the aqueous component of the HIPE is inhibiting to the polycondensation. 

The most common way of creating polymers is through addition polymerisation - a process which involves three steps, namely polymer initiation, addition and termination. 

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 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. 

A development of particular importance for the controlled production of block copolymers is the perfection of various so-called living polymerisation techniques. In the classical addition polymerisations there was always a termination stage, leading to the production of chains with non-reactive groups at both ends of the polymer chain. Polymerisation could therefore stop before all monomer had been exhausted, although ideally the termination step was of much lower probability than the propagation step. In living polymerisations there is no termination step and the reaction proceeds in the ideal case until all monomer has been exhausted. The chains still have reactive ends and a second type of monomer can then be added to the reaction to produce a block of a different type of polymer. 

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. 

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). 

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... 

Only a few studies on oleochemistry have been reported until now. Similar to the effects seen with the thiol-ene reaction, one can take advantage of this interesting tool for the synthesis of novel monomers to be polymerised further by classical methods. Also, the possibility of two thiol groups reacting with one ethynyl group via two-step addition makes thiol-yne click polymerisation an interesting method for the preparation of hyperbranched polymers. 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

The radical trapping technique can be used to study the initiation step in polymerisation where the initiating species is a phosphorus-centred radical. Addition of phosphorus-centred radicals to alkenes was found to be competitive with direct trapping by the aminoxyl. 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

As outlined in Chapter 1, polymerisation reactions can be classified as either condensation or addihon processes, the basis of the classification suggested by W. H. Carothers in 1929. More useful, however, is the classification based on reaction kinetics, in which polymerisation reactions are divided into step and chain processes. These latter categories approximate to Carothers condensation and addition reactions but are not completely synonymous with them. 

Initiation. Initiation in radical polymerisation consists of two steps the dissociation of the initiator to form two radical species, followed by addition of a single molecule to the initiating radical (Figure 18). Initiators include any organic compound with a labile group, such as an azo (-N = N-), disulfide (—S—S—) or peroxide (-0-0-) compound. The labile bond can be broken by various ways like heat, UV light, /-irradiation or by a redox reaction. 

Making use of the higher reactivity of butadiene in anionic polymerisation y <1, 2 > 1) to get the triblock SBS copolymer in two steps. The first step is the synthesis of a PS sequence with a living end, then, upon addition of a mixture of styrene and butadiene, butadiene will add first, building a "pure" PB sequence, and styrene will finally build the third sequence (two steps). 

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

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