Polymerisation of monomer reactants

With respect to the effect of environments, carbon fibres can be largely considered to be inert since they are stable to higher temperatures than the resin matrices can withstand. At temperatures above 300 °C, the fibres begin to degrade in oxidising atmospheres, but most polymers have a lower maximum service temperature. The advanced polyimides and PMR (polymerisation of monomer reactants) systems can survive temperatures up to 450 °C, but these are usually in short-term applications [27]. It is also unlikely that composites will be in contact with damaging solvents such as concentrated oxidising acids such as sulphuric and/or nitric acids. 

Figure 18 Chemical sequences involved in the polymerisation of monomer reactants (PMR) process. The average molecular weight of intermediate oligomer 43 is controlled by the relative amounts of monoester 41, diester 42 and diamine 34. Chain extension of bisnadimide 43 occurs at high temperature via the reverse Diels-Alder polymerisation process.

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

This section is devoted to a systematic analysis of publications concerned in a direct or indirect way with the mechanism of initiation in the cationic polymerisation of alkenyl monomers by Br nsted acids. Included in it are many examples of failures, i.e., systems in which no polymerisation was observed. Such experiments are important because the lack of production of active species can give considerable information a-bout the alternative processes taking place vdien the two reactants are mixed. We have also included all the information we could gather about the state and properties of the initiators in the media relevant to the general context of this review. 

From these new conditions the reaction in the batch reactor can be advanced to time t = 2At. At this time the entire procedure for computing the new feed flow-rate has to be repeated. Since no correction for the consumption of B has been made, the amount of B present must decrease as the reaction proceeds. As a result, progressively less of monomer A has to be added to keep p constant. When A is feed to the reactor not as pure monomer, but in solution the increase in volume caused by the added solvent will reduce the concentrations of the reactants and have a consequential effect on the overall rate of polymerisation. 

An important technique is that in which it is the precursor of the final colloidal particle that is reduced to a colloidal size. Thus a liquid reactant may be emulsified and then caused to react to form a colloidal dispersion of solid particles whose particle size distribution is related to that of the emulsion precursor. The commonest application of this method is in suspension polymerisation, in which an emulsion of monomer droplets, stabilised by a surfactant, is polymerised by adding an initiator which is soluble in the monomer. Polymerisation occurs within the monomer droplet, leading to the formation of a polymer latex. 

The first element of this definition requires that the substance consist of a minimum of three repeat units of monomer(s) in reacted form bonded to another monomer or other reactant [9]. Other reactants, in this case, may not necessarily be monomers, but are incorporated into the polymer to provide a particular function. Other reactants may include a free-radical initiator used to initiate a vinyl-free radical polymerisation (e.g., peroxide used to make polyethylene or polyacrylic acid), a chain transfer agent used to control molecular weight, e.g., mercaptan used in free-radical polymerisations, or crosslinkers used to build molecular weight, e.g., divinyl benzene. 

Figure 1.1 demonstrates the diffusion model-based fields of temperature, as well as the monomer and catalyst concentrations during the cationic polymerisation of isobutylene. It is clear that the process and experimental behaviour are close, mainly in the catalyst input areas where it is mixed with the monomer solution. Isobutylene polymerisation is similar to the behaviour of fast chemical processes the temperature and reaction rate in a reaction zone depend on the initial concentration of reactants, the value and the factor K, which is the heat transfer through the reactor wall Kjjt. Although the rate of isobutylene polymerisation is maximal within the catalyst input areas, the reaction occurs sufficiently far in the axial direction to result in a change of output characteristics and polymer properties (molecular characteristics) when moving away from catalyst input area. 

To overcome the major drawbacks of the mass polymerisation technique, monomers or reactants are dissolved in a suitable inert solvent or mixture of solvents, where the polymers are also expected to be soluble. The localised heat accumulation or gel effect is not observed in this case so heat dissipation is not a problem. However, the choice of solvent is critical in obtaining a high molecular weight with a controlled polymer structure. Desirable characteristics of solvents include non-interaction with the components of the medium, moderate volatility, non-toxicity and good solvating power, both for monomer and polymer. 

RIM is a relatively new process. It can be used for processing of unfilled resin as well as fibre-reinforced composites. The process was discussed in Chapter 1. The process is similar to RTM (discussed previously) with some variation in mould release and reinforcement sizing to optimise resin chemistry with the process. The low viscosity reactant systems facilitate composite materials production, so-called structural RIM composites [19, 20] in which continuous fibre reinforcement mats are placed in mould cavities before injection. Capital investment and operational cost in RIM are therefore much less than those for conventional injection moulding. Polymerisation of a monomer is usually initiated by heat. However, in RIM, the polymerisation is initiated by impingement mixing (not by heat). Hence it is possible to activate polymerisation at relatively low temperature. Unlike RTM, in RIM the mould-fill times are very low ( 1 s) and a cycle time of <60 s is typical. The process is used for the rapid and automated production of large, thin and complex-shaped parts. 

Compared to BMI adhesives, the nadimide-terminated resins discussed in Section 4.3.4.2 exhibit a better thermal-oxidative resistance, but they require a complex cure schedule in autoclaves. Condensation and imidisation of the monomer reactants is performed at 205°C under partial vacuum to remove methyl alcohol and water. Addition polymerisation of the norbomene rings is then conducted under full vacuum at 290°C with an applied air pressure of 0.7 MPa. PMR adhesives have been successfully tested for bonding parts of cruise missiles, space shuttle, and YF-12 aircraft. The lap shear strength data indicate an excellent adhesion after a long-term ageing at 232°C and approximately 200 h at 316°C. 

Chain growth may temporarily stop when all the monomer units are used up. This happens particularly in case of polymerisation of carefully purified reactants (say styrene) by anionic initiators represented as A H [ NH2 Li (Lithium amide), C4HgLi ( -butyl lithium), CjpHgNa (SrxJium... 

As initiation and propagation occurs, the temperature of the reactants is allowed to rise to the reflux temperature of the solvent present. The reactants are held at reflux temperature under total reflux conditions, to condense all vapours and return the liquid to the reaction vessel. The latent heat of vaporisation of solvent and monomer helps to dissipate the heat of reaction. The course of the polymerisation is monitored by the measurement of viscosity and non-volatile content. 

The course of the polymerisation is monitored by measurement of the solids content of the reactants or by analytical methods to detect the presence of free monomer. Additional initiator may be added if required, to complete the reaction. 

The reactants are held at 80°C for 30 minutes for the polymerisation of the vinyl acetate, then the initiator phase and monomer phase are fed separately to the reaction vessel over a two hour period, whilst maintaining the reaction temperature at 80°C. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

As explained earfier step polymerisations generally occur by condensation reactions between functionally substituted monomers. In order to obtain high molar mass products bifuncfional reactants are used monofunctional compounds are used to control the reaction while trifunctional species may be included in order to give branched or crosslinked polymers. A number of types of reaction may be involved, as described briefly in the following paragraphs. 

The properties of a polymer network depend not only on the molar masses, functionalities, chain structures, and proportions of reactants used to prepare the network but also on the conditions (concentration and temperature) of preparation. In the Gaussian sense, the perfect network can never be obtained in practice, but, through random or condensation polymerisations(T) of polyfunctional monomers and prepolymers, networks with imperfections which are to some extent quantifiable can be prepared, and the importance of such imperfections on network properties can be ascertained. In this context, the use of well-characterised random polymerisations for network preparation may be contrasted with the more traditional method of cross-linking polymer chains. With the latter, uncertainties can exist with regard to the... 

The chemistry of cyanoacrylate adhesives contains no co-reactants but can polymerise at room temperature on any substrate that is exposed to atmospheric moisture or alkaline surfaces. Synthesised cyanoacrylate esters can be methyl, ethyl, n-propyl, n-butyl, allyl, ethoxyethyl and methoxyethyl. The basic structure of the cyanoacrylate monomer is ... 

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