Vinylic monomers polymerisation

Coacrete can also be made water-repeUent by the polymerisation of vinyl monomers on the surface (85). Polymerisation can be iaitiated with peroxides, and polyfunctional methacrjiates can be used as crosslinking agents. These treatments have a tendency to produce changes ia color and gloss. 

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. 

Most vinyl monomers will polymerise by free-radical initiation over a wide range of monomer concentration. Methyl methacrylate can even be polymerised... 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

The polymer may be prepared readily in bulk, emulsion and suspension, the latter technique apparently being preferred on an industrial scale. The monomer must be free from oxygen and metallic impurities. Peroxide such as benzoyl peroxide are used in suspension polymerisations which may be carried out at room temperature or at slightly elevated temperatures. Persulphate initiators and the conventional emulsifying soaps may be used in emulsion polymerisation. The polymerisation rate for vinylidene chloride-vinyl chloride copolymers is markedly less than for either monomer polymerised alone. 

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. 

The monomers used in chain polymerisations are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerisations a small trace of an initiator material is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The two most commonly used free radical initiators for these reactions are benzoyl peroxide and azobisisobutyronitrile (usually abbreviated to AIBN). They react as indicated in Reactions 2.1 and 2.2. 

However, other molecules exist which form free radicals of such high stability that they effectively stop the chain process. These molecules are called retarders or inhibitors the difference is one of degree, retarders merely slowing down the polymerisation reaction while inhibitors stop it completely. In practice vinyl monomers such as styrene and methyl methacrylate are stored with a trace of inhibitor in them to prevent any uncontrolled polymerisation before use. Prior to polymerisation these liquids must be freed from this inhibitor, often by aqueous extraction and/or distillation. 

Anionic polymerisation involves the development of a negative charge on the growing polymer molecule. This is achieved through the use of a catalyst that can readily form anions which themselves react with the vinyl monomer. An example is shown in Reaction 2.14. Here the initiator is sodium and the reaction is carried out in liquid ammonia at -75 °C. 

Polymerisation of vinyl monomers with electron-donating substituents can proceed by a cationic mechanism, while monomers with electron-withdrawing group can polymerise by an anionic pathway. 

The methodology employed, however, is applicable to many other free radical polymers generated from vinyl monomers (such as, e.g., polystyrene). It should be noted that this methodology is also equally applicable to many polymers generated by condensation and ionic polymerisation routes. 

An initiator is required in this type of polymerisation. The polymerisation pro-ceeds through the intera-ction of the initiator with the p electron system of the vinyl monomers. 

This technique is extensively used for the free radical polymerisation of vinyl monomers containing water soluble initiators. The monomers like vinyl chloride, butadiene, chloroprene, vinyl acetate, acrylates and methacrylates are polymerised by this technique. 

In 1866 AD a polymeric product was formed from styrene and sulphuric acid. Another breakthrough was the production of synthetic rubber from butadiene by using metallic sodium or potassium by German scientists during 1911 -22. In 1929, Ziegler reported polymerisation of vinyl monomers using butyllithium. 

Typical monomers which polymerise through cationic mechanisms are isobutene, styrene, a-methylstyrene, vinyl ethers and vinyl carbazoles. At present, about 100 vinyl monomers are known that can be polymerised by the cationic initiators. 

Explain the role of the photochemical reactions of carbonyl compounds in the photoinitiated polymerisation of vinyl monomers and cross-linking in polymers. 

The polymerisation of vinyl monomers can be initiated by radicals formed by a-cleavage or hydrogen abstraction of a carbonyl compound. 

Mansour, O.Y., Nagaty, A. and Beshay, A-F. (1982). Grafting of some vinyl monomers onto ligno-cellulose and cellulose in the presence of lignin. In Graft Polymerisation of Lignocellulosic Fibers, Hon, D.N.S. (Ed.). ACS Symposium, 187, pp. 253-268. 

Sonopolymerisation is not only restricted to the polymerisation of vinyl monomers. The use of ultrasound in ring opening polymerisation, condensation polymerisation and polymerisation by coupling reactions have all been reported. 

In real industrial polymerisation, important physical events must be taken into account. For most vinyl monomers, the enthalpy change which accompanies polymerisation is Izirge and isothermal conditions cannot always be guaranteed. The viscosity of polymerising fluids can be very high so that complete mixing is not achieved in a stirred reactor. 

Atom Transfer Radical Polymerisation (ATRP) was discovered independently by Wang and Matyjaszewski, and Sawamoto s group in 1995. Since then, this field has become a hot topic in synthetic polymer chemistry, with over 1000 papers published worldwide and more than 100 patent applications filed to date. ATRP is based on Kharasch chemistry overall it involves the insertion of vinyl monomers between the R-X bond of an alkyl halide-based initiator. At any given time in the reaction, most of the polymer chains are capped with halogen atoms (Cl or Br), and are therefore dormant and do not propagate see Figure 1. 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

The idea of the preparation of porous polymers from high internal phase emulsions had been reported prior to the publication of the PolyHIPE patent [128]. About twenty years previously, Bartl and von Bonin [148,149] described the polymerisation of water-insoluble vinyl monomers, such as styrene and methyl methacrylate, in w/o HIPEs, stabilised by styrene-ethyleneoxide graft copolymers. In this way, HIPEs of approximately 85% internal phase volume could be prepared. On polymerisation, solid, closed-cell monolithic polymers were obtained. Similarly, Riess and coworkers [150] had described the preparation of closed-cell porous polystyrene from HIPEs of water in styrene, stabilised by poly(styrene-ethyleneoxide) block copolymer surfactants, with internal phase volumes of up to 80%. 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

Most of the salts capable of polymerising the vinyl monomers already described are also efficient initiators of cyclic monomer polymerisations, but in addition appropriate salts of more stable species such as oxonium, sulphonium and diazonium ions... 

In the hrst step, a redox reaction occurs between Ce(IV) and the -CH2OH end group of PEO, generating a free radical in a-position of the -OH group of PEO. In a consequent step, the radical is transferred from the PEO chain to the vinyl monomer. The radicals formed initiate the actual polymerisation reaction (propagation) ... 

A typical recipe for the polymerisation of a vinyl monomer would be to form an oil-in-water emulsion from ... 

Organolead compounds have been recently widely used. They are used in agriculture (as pesticides), in medicine, as well as in various chemical processes as catalysts for the polymerisation of vinyl monomers, chlorination of hydrocarbons, etc. Lead tetraalkyl derivatives are used as additives for engine fuels due to their antiknock properties. 

Neutral organolanthanide metallocenes [532 536] and cationic zirconocene organic derivatives [537] appear to be effective catalysts for the polymerisation of polar vinyl monomers in which the heteroatom is conjugated with the double bond, such as acrylates [CH2=CH—C(0R)=0] and methacrylates [CH2= C(Me)-C(0R)=0]. 

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