Classical polymerisation processes

In polymerisation, monomer units react to give polymer molecules. In the simplest examples the chemical repeat unit contains the same group of atoms as the monomer (but differently bonded), e.g. ethylene polyethylene 

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. 

The simplest type of addition reaction is the formation of polyethylene from ethylene monomer  

Polyethylene is a special example of a generic class that includes many of the industrially important macromolecules, the vinyl and vinylidene polymers. The chemical repeat unit of a vinylidene polymer is -fCH2—CXY where X and Y represent single atoms or chemical groups. For a vinyl polymer Y is H and for polyethylene both X and Y are H. If X is —CH3, Cl, —CN, — or 0(C=0)CH3, where represents the mono-substituted benzene ring, or phenyl group, and Y is H, the well-known materials polypropylene, poly(vinyl chloride) (PVC), polyacrylonitrile, polystyrene and poly(vinyl acetate), respectively, are obtained. 

When Y is not H, X and Y may be the same type of atom or group, as with poly(vinylidene chloride) (X and Y are Cl), or they may differ, as in poly-(methyl methacrylate) (X is —CH3, Y is —COOCH3) and poly(a-methyl styrene) (X is —CH3, Y is — ). When the substituents are small, polymerisation of a tetra-substituted monomer is possible, to produce a polymer such as polytetrafluoroethylene (PTFE), with the repeat unit )CF2—CF2, but if large substituents are present on both carbon atoms of the double bond there is usually steric hindrance to polymerisation, i.e. the substituents would overlap each other if polymerisation took place. 

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With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

The polydiacetylene crystals (1-4) most strikingly corroborate these conjectures. Along this line of thought is also shown that this electron-phonon interaction is intimately interwoven with the polymerisation process in these materials and plays a profound role there. We make the conjecture that this occurs through the motion of an unpaired electron in a non-bonding p-orbital dressed with a bending mode and guided by a classical intermolecular mode. Such a polaron type diffusion combined with the theory of non radiative transitions explains the essentials of the spectral characteristics of the materials as well as their polymerisation dynamics. ... 

In MPPh, the polymerisation process proceeds in accordance with the classic kinetic scheme with quadratic chain termination ... 

The polymerisation processes described in the previous section are the classical processes used for producing the bulk commercial polymers. Newer processes have been and are being developed with a variety of aims in mind. These involve the production of novel polymer topologies (see box) precise control over chain length and over monomer sequences in copolymers control of isomerism (see section 4.1) production of polymers with special reactive end groups, the so-called telechelic polymers, production of specially designed thermally stable polymers and liquid-crystal polymers with a variety of different structures and properties. Other developments include the production of polymers with very precisely defined molar masses, and of networks with precisely defined chain lengths... 

Radical polymerisation (thermal or photochemical) is a fast process several minutes for water-in-oil microemulsions and an hour for oil-in-water microemulsions, compared to several hours for the classic emulsion process. This is not surprising given that, in disperse media, the polymerisation rate is proportional to the number of particles V oc N). In a microemulsion, there are far more droplets than in an ordinary emulsion. 

Oxidative degradation of polyethylene (PE) and polypropylene (PP) can occur at all stages of their lifecycle (polymerisation, storage, processing, fabrication and in-service). The auto-oxidation process of polyolefins is best described by the classical free-radical-initiated chain reaction outlined in Scheme 1 [1]. Impurities initially present in the polymers during polymerisation or melt processing, exert profound effects on the behaviour of the final polymer article in service. 

The low temperature polymerisation of isobutene by SnCl4 in ethyl chloride is one of the classical studies of the golden era of cationic polymerisation. Norrish and Russell " found that with no added water an extremely slow reaction period was followed by a sudden acceleration. A similar phenomentm was later reported by Polton and Sigwalt for the polymerisation of indene in a dry system. It seems reasonable to suppose that the slow initial process reflects direct initiation in both systems, and that the sudden accelemtion arises from the internal production of a cocatalyst, probably hydrogen chloride formed from the dehydrochlorination of active species. 

A very recent paper by Cerrai and coworkers came to our attention after most of this review had been written, but its importance calls for inclusion in this chapter. Indeed the authors question the very nature of the initiating process in the classical electrolytic polymerisation. They reached this conclusion after a very thorough study of the electrochemical polymerisation of cyclohexylvinyl ether in ethylene chloride with tetra-butylammonium tetrafluoroborate and perchlorate, having shown that initiation could not be attributed to the anodic oxidation of the electrolyte anion, of the solvent, or of the monomer. The acid formed at the anode compartment was therefore throught to originate from the electrolysis of residual moisture in the system. This conclusion was supported by the fact that under the most rigorous experimental conditions the rates of polymerisation were considerably lower than when the runs had been performed un-... 

Within the framework of this concept, initiation of polymerisation initially leads to accnmnlation of the number of propagating chains and makes the dependence of the nnmber of chains more profound. On the contrary, crosslinking reduces the number of chains, and, at the end of the process, the quantity dN/dt decreases to zero. This approach makes it possible to take into account the unsteady character of the polymerisation. However, becanse the mechanisms involved in the propagation of polylignol chains [3] are markedly different from the classical mechanism of free radical polymerisation, this variant hardly pertains to lignin formation. 

The cationic (electrophilic) polymerisation of isobutylene is the most attractive process, in a family of rapid liquid-phase chemical reactions, for theoretical study as it is both theoretically clear and practically important. This process can be considered to be a classical model of a rapid liquid-phase chemical reaction [2, 26]. 

In order to understand how synthetic polymers are prepared, the methods and processes of polymerisation are presented in this chapter and Chapter 4. It can be seen that some properties of polymers, especially surface properties, depend on the synthesis. The classical techniques are introduced first, but the more specific techniques needed to synthesise special polymers are emphasised. 

The C=C unsaturation in all these structures is 1,2-disubstituted, so initiation and propagation by free-radical and cationic mechanisms are sluggish. Hence, they elicit slow processes and poor yields of macromolecular structures though, ultimately, crosslinked materials are generated (as in the case of classical oxido-polymerisation discussed in Chapter 2). Preparation of blown oils by thermal treatment >300 C is also a poor approach because of substantial losses associated with thermal degradation. It follows that applications are not particularly attractive in practical terms except in the traditional realm of siccative paints and inks. 

The early work [1-6] established that fluorine substituted monomers could be polymerised by classical initiator systems derived from transition metal chlorides, however, the process was poorly characterised and gave largely atactic products with broad molecular weight distributions. 

To prepare stable and small microlatexes, difficult to obtain using emulsion polymerisation. In particular, the classic process in inverted emulsion leads to unstable latexes with a wide range of particle dimensions. 

Microemulsion processes may well find applications in areas which have traditionally used emulsion polymerisation. At the present stage of research, it is mainly water-in-oil microemulsion polymerisation which offers the most possibilities and several patents have been taken out [6.5]. This process minimises certain problems encountered in classic inverted emulsions, namely instability of the latexes they produce, large polydispersity of polymer particles, and the large quantity of coagulum which increases production costs. Water-soluble (co)polymers prepared in microemulsion polymerisation can be used in various ways ... 

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