Natural polymerisation

Styrene. Styrene is readily polymerised to a glass-clear resin, polystyrene, but the exact nature of the polymer is influenced by the nature of the catalyst, the temperature, solvent, etc. 

Emulsion polymerisation of a mixture of butadiene and styrene gives a synthetic rubber (Buna S GBS rubber), which is used either alone or blended with natural rubber for automobile tyres and a variety of other articles. 

The ionic nature of the radicals generated, by whatever technique, can contribute to the stabilisation of latex particles. Soapless emulsion polymerisations can be carried out usiag potassium persulfate as initiator (62). It is often important to control pH with buffets dutiag soapless emulsion p olymerisation. 

Eree-radical initiation of emulsion copolymers produces a random polymerisation in which the trans/cis ratio caimot be controlled. The nature of ESBR free-radical polymerisation results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstmcture is not amenable to manipulation, although the temperature of the polymerisation affects the ratio of trans to cis somewhat. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

One other variant in double-bond polymerisations may be mentioned here. Many conjugated dienes may be polymerised in such a way as to generate long chain molecules with residual double bonds in the chain. Well-known examples of such dienes are buta-1,3-diene and isoprene, which yield 1,4-polybutadiene and 1,4-polyisoprene respectively (Figure 2.4). Natural rubber has a formula corresponding to the 1,4-polyisoprene. 

A number of olefins may be polymerised using certain metal oxides supported on the surface of an inert solid particle. The mechanism of these polymerisation reactions is little understood but is believed to be ionic in nature. 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

It was found that the amount of chlorine that could be removed (84-87%) was in close agreement to that predicted by Flory on statistical grounds for structure Figure 12.10(a). It is of interest to note that similar statistical calculations are of relevance in the cyclisation of natural rubber and in the formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol). Since the classical work of Marvel it has been shown by diverse techniques that head-to-tail structures are almost invariably formed in addition polymerisations. 

Free-radical polymerisation techniques involving peroxides or azodi-isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas. 

By rolling on a two-roll mill the molecular weight of the polymer can be greatly reduced by mechanical scission, analogous to that involved in the mastication of natural rubber, and so mouldable materials may be obtained. However, bulk polymerisation is expensive and the additional milling and grinding processes necessary make this process uneconomic in addition to increasing the risk of contamination. 

The ease with which acrylic monomers may polymerise with each other and with other monomers has led to a host of compositions, frequently of undisclosed nature, being offered for use as moulding materials, casting resins, coating resins, finishing agents and in other applications. 

The silicones have established their value as water-repellent finishes for a range of natural and synthetic textiles. A number of techniques have been devised which result in the pick-up of 1-3% of silicone resin on the cloth. The polymer may be added as a solution, an emulsion or by spraying a fine mist alternatively, intermediates may be added which either polymerise in situ or attach themselves to the fibre molecules. 

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

This pump is the same in principle as the piston type but differs in that the gland is at one end of the cylinder making its replacement easier than with the standard piston type. The sealing of piston and ram pumps has been much improved but, because of the nature of the fluids frequently used, care in selecting and maintaining the seal is very important. The piston or ram pump may be used for injections of small quantities of inhibitors to polymerisation units or of corrosion inhibitors to high pressure systems, and also for boiler feed water applications. 

Another classification system, first suggested by Carothers in 1929, is based on the nature of the chemical reactions employed in the polymerisation. Here the two major groups are the condensation and the addition polymers. Condensation polymers are those prepared from monomers where reaction is accompanied by the loss of a small molecule, usually of water, for example polyesters which are formed by the condensation shown in Reaction 1.1. 

The name Nylon was given by the Du Pont company of America to their first synthetic condensation polymer formed by the reaction of difuncfional acids with difuncfional amines, ft had been made as part of the fundamental programme of W. H. Carothers to investigate the whole topic of polymerisation. The term has gradually been extended to other related polymers. These materials are strictly polyamides, but this term includes that otherwise distinct class of natural macromolecules, the proteins. The term nylon is retained for its usefulness in distinguishing synthetic polyamides from the broader class of such polymers. 

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

In the natural system the sites of spore wall formation, i.e. the sporan-gial loculus, act as mini-reactor vessels in which the above interactions can occur. If a polymerisation occurs within one such structure, the resulting (polymer) architectures will probably closely resemble the self-assembled ones formed in our artificial sporangia. 

The remainder of the work on Ni(II) complexes involves the use of chelating ligands in which the carbene is functionalised with pendant heteroatom donor(s). The picolyl-functionalised NHC dicationic complex 29 (Fig. 4.11) was tested for ethylene polymerisation after treatment with MAO [34]. This complex was found to be highly active in a preliminary test (330 kg moF bar h" ), giving predominantly linear polyethylene. Unfortunately this work does not seem to have been followed up. The same system was active for norbomene polymerisation (TOF = 24 400 h" over 1 h). Maximum activity was achieved at 80°C whereafter thermal deactivation became significant, although the nature of this deactivation was not studied. The phenoxide-functionalised carbene complex 30 (Fig. 4.11) was also... 

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