Composites Copolymerisation

Rea.CtlVltyRa.tlO Scheme. The composition of a copolymer at any point in time depends on the relative rates that each monomer can add to a chain end. If it is assumed that the chemical reactivity of a propagating chain depends only on the terminal unit and is not affected by any penultimate units, then four possible propagation steps in the copolymerisation of two monomers, and M2, with two growing chain ends, M and M2, can be written as follows ... 

An alternative approach is to copolymerise only up to a limited degree of conversion, say 40%. In such cases although there will be some variation in composition it will be far less than would occur if the reaction is taken to completion. 

Whereas the glass transition of a copolymer is usually intermediate between those of the corresponding homopolymers this is not commonly the case with the melting points. Figure 4.12 shows the effect of copolymerising hexamethylenesebacamide with hexamethyleneterephthalamide. Only when the monomer units are isomorphous, so that the molecules can take up the same structure, is there a linear relationship between melting point and composition (as with hexamethyleneadipamide and hexamethyleneterephthalamide). 

The value of the reachvity rahos is crihcal in determining the composition of the copolymer. If the reactivity raho is greater than 1, the radical prefers to react with chains having the same kind of terminal unit, e.g. A- with A. On the other hand, if the reactivity ratio is less than 1, the monomer prefers to react with chains which end in the other kind of monomer. In the special case that r r2 = 1, the reaction is described as ideal copolymerisation because it results in a truly random copolymer whose composition is the same as the composition of the reaction mixture from which polymerisation took place. 

Copolymerisation is also possible (Fig. 4). Dimethyldisilane reacts with diphenylsilane with formation of a copolymer with the composition H[(MeSiHx)(PhSiHy)]nH. This copolymer is a viscous liquid and is spinnable. By heating to 180° C the polymerization continues and a solid results [23]. The presence of branched structures, which were not found with the polymerization of monosilanes, the very rapid polymerization rate achievable, and the observable SiSi cleavage points to another mechanism, as was postulated for monosilanes. 

The monomers are randomly distributed in the Polymer chain in most of cases but in case of copolymerisation of styrene and maleic anhydride, there is perfect alternate arrangement of monomers in the chain regardless of initial composition of monomers. 

Most copolymers are heterogeneous in both molecular weight and composition. The latter of these arises from the mechanism of the copolymerisation (particularly at high conversion) and individual copolymer molecules differ slightly in their value of WA. Solutions of heterogeneous copolymers constitute multicomponent systems... 

In the work of Belov et al. the kinetic model that has been developed quantitatively describes the initial rate of copolymerisation, the kinetic curves of the consumption of the two monomers and the molecular weight characteristics of the resulting copolymers and their composition as mixture of ketoesters, diesters, diketones as function of the total pressure up to 40 bar,... 

Fig. 6.19 shows the effect of ultrasound (25 kHz cleaning bath) upon the electroxi-datively initiated copolymerisation of isoprene with a-methyl styrene, in CH2CI2/ BU4NBF4 at — 30 °C using platinum electrodes in a divided cell [81]. With ultrasound the copolymerisation proceeds effectively at lower potentials, with some curvature in the dependence of composition upon potential. Fig. 6.20 gives the overall conversion of the system in the absence and presence of ultrasound. 

As a necessary preliminary to the study of how compositional heterogeneity affects the properties of the polymers, compositionally heterogeneous and homogeneous co- and terpolymers had to be synthesised. It Is common in copolymerisations for the relative reactivity of the co-monomers to be different (5) so that during polymerisations carried out to high conversion In a free-running batch reactor, the Initially formed polymer Is richer In the more reactive monomer, whereas, at the end of the reaction the polymer produced contains a greater proportion of the less reactive monomer. In such circumstances, compositional... 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Another approach is based on the copolymerisation of a mixture of two acrylic monomers. One is of the anionic type (or cationic) and the other one is poly-hydroxylated (Fig. 4.3). The latter is used to ensure the hydrophilic character necessary for the stationary phase. A limitation of these resins is their variable swelling, which depends on the composition of the mobile phase. They are normally used for medium pressure chromatography and certain biochemical applications. 

Early studies involved the development of a process which could treat all types of papers of varying ages and composition. Graft copolymerisation seemed to offer this potential. The basic concept of graft copolymerisation can be seen in Figure 1. Radical sites are created on the cellulose backbone and these sites allow the... 

Chemical modification of polymers continues to be an active field of research [1-5]. It is a common means of changing and optimising the physical, mechanical and technological properties of polymers [5-7]. It is also a unique route to produce polymers with unusual chemical structure and composition that are otherwise inaccessible or very difficult to prepare by conventional polymerisation methods. For example, hydrogenated nitrile rubber (HNBR) which has a structure which resembles that of the copolymer ethylene and acrylonitrile, is very difficult to prepare by conventional copolymerisation of the monomers. Polyvinyl alcohol can only be prepared by hydrolysis of polyvinyl acetate. Most of the rubbers or rubbery materials have unsaturation in their main chain and/or in their pendent groups. So these materials are very susceptible towards chemical reactions compared to their saturated counterparts. 

Although both Phillips and Ziegler Natta catalysts copolymerise ethylene and propylene, the latter catalysts are more widely used because they can be more readily tailored to produce a narrow distribution of compositions and molecular weight [43]. 

In the copolymerisation, each monomer competes for the available catalytic active species, and the composition, structure and molecular weight of the copolymer produced reflect this competition. The mechanistic features of the copolymerisation are in principle similar to those of the homopolymerisation. In the copolymerisation, however, the effect of the kind of last inserted monomeric unit and the kind of comonomer undergoing insertion (Mi or M2) should be taken into consideration the propagation step can proceed in at least four ways [448] ... 

In the copolymerisation of butadiene and isoprene with Ti-based catalysts, both monomeric units of the copolymers obtained are essentially of a ciy-1,4 structure the microstructure of monomeric units in the copolymers does not differ substantially from that in the homopolymers [196-198], Nd-based catalysts provide butadiene/isoprene copolymers with more than 95% cis-1,4 monomeric units [89,199,200], On the other hand, Co-based catalysts give copolymers in which the structure of the monomeric units depends markedly on copolymer composition [19,201,202], Similarly, the structure of the monomeric units depends on copolymer composition in copolymers of butadiene and 2,3-dimethylbutadiene obtained by copolymerisation with Co-based catalysts [201,203],... 

Examples of the relationship da/dN versus /4Kj are given in Fig. 4.3 for PMMA and PVC In addition to the chemical composition of the polymer, the configuration of the macromolecules plays an important role as can be seen from the effect of molecular weight on the fatigue crack resistance Copolymerisation and... 

In contrast to the requirements for homopolymerisation processes, the parameters needed to fully describe copolymerisation processes are more numerous. Molecular features such as the copolymer composition, composition distribution and chain sequence structure and their variation with conversion are compounded with those of copolymer MW and MWD. To understand copolymerisation processes, it is desirable to decouple as many of these molecular parameters as possible and study the influence of polymerisation reactor conditions on each. As yet there have been relatively few reports on the detailed behaviour of copolymerisation reactors (6-9 ). This work forms part of a wider range of investigations which are being carried out in our laboratories of control methods for the production of speciality polymers. 

It is common for the monomers taking part in copolymerisation reactions to be of different reactivities which leads to a drift in copolymer composition with conversion. The instantaneous copolymer composition can be related to the instantaneous composition of the monomer feed through r5 and r2, the monomer reactivity ratios (10) as shown in Equation (1). 

Methods of achieving uniform composition copolymers from various polymerisation processes have been described. Hanson and Zimmerman (11) used a continuous recycle reactor to produce copolymers of a known and predictable homogeneous composition at relatively high percentage conversion. Hatate et al (12) studied a continuous copolymerisation in stirred tank reactors and considered the effect of micro-mixing on the copolymer... 

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