Bulk polymerisation

Heat transfer problems are greatly diminished, compared to an actual bulk polymerisation, because the aqueous phase can conduct away most of the heat generated. The size distribution of the final particles should closely follow that of the initial monomer emulsion droplets (provided coalescence is avoided). 

In bulk polymerisation, the polymer is produced in a reactor where only the monomer and a small amoxmt of an initiator are present. Bulk polymerisation processes are characterised by high product purity, high reactor performances and low separation costs, but also by high viscosities in the reactors. Bulk processes cause reactor fouling, and in the case of polycondensation products, a high vacuum is required. 

This is the usual method for step-growth (condensation) polymerisation. The reaction is often carried out at a high temperature, but there are no real problems with heat transfer out of the reaction vessel (i.e. temperature build-up). The degree of polymerisation increases linearly with time, so that the viscosity of the reaction mixture only increases relatively slowly this allows for efficient gas (e.g. water vapour) bubble transfer out of the system as well. 

This method can be used for chain-growth polymerisation, but only on a small scale, preferably at low temperature. Heat and bubble transfer may give problems, since the degree of polymerisation (and hence, also the viscosity of the reaction mixture) increases very rapidly from the beginning of the reaction. 

For certain monomers (e g. vinyl chloride), the polymer is insoluble in its own monomer (above some critical molar mass). Hence, in these cases, the polymer precipitates (as aggregated, swollen particles) from the monomer after a while. Eventually all the monomer is converted to pol5mier. 

For these reasons, despite the apparent advantages and also despite the fact that bulk polymerisation is so often the method of choice for the laboratory preparation of vinyl polymers, this technique is not widely used in industry. Only three polymers are produced in this way, namely poly(ethylene), poly(styrene), and poly(methyl methacrylate). 

Of these polymers, poly(ethylene) is produced from a gaseous monomer under pressure, either high or low, and thus some of the disadvantages 

At the bottom of the tower, the high molar mass poly(styrene) is extruded, granulated, and cooled prior to packaging. 

This involves heating monomer mixed with initiator. The problems associated with heat transfer through a viscous reaction mass, result in poor temperature control. This means that long retention times are employed to reduce the chai of localised overheating. Bulk polymerisation can be carried out in disposable containers which are completely filled with the reactants, and held at constant temperature in an oven or wat bath. When polymerisation is complete, the container is cut away and the product crushed and dissolved in solvent or other medium for use as a surface coating. Alternatively, a continuous or senu-continuous method of polymerisation can be used. 

Polymers made in this way have a wide molecular weight distribution. Control of the reaction conditions is minimal, and batch to batch reproducibility is poor. This technique is rarely used for polymers intended for coating applications. Normally they are confined to polymers used in the packaging and moulding industries. 

Bulk polymerisation techniques are usually carried out as continuous or semi-continuous processes. Polymerisation is restricted to a low degree of conversion and is stopped while the viscosity of the reaction mass is still low enough to allow effective agitation. Ibe urueacted monomer is removed and recycled. 

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Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

Films from prepolymer solutions can be cured by heating at 150°C. Heating the prepolymer in molds gives clear, insoluble moldings (38). The bulk polymerisation of DAP at 80°C has been studied (35). In conversions to ca 25% soluble prepolymer, rates were nearly linear with time and concentrations of bensoyl peroxide. A higher initiator concentration is required than in typical vinyl-type polymerisations. 

Favorable rates and yields of DAP prepolymer are obtained by solution polymerisation in CCl —bensene mixtures (68). Bulk polymerisation at 80°C with bensoyl peroxide is advanced to a certain viscosity before addition of ethanol to precipitate the prepolymer that is then dried (69). 

Monofilament Synthetic Absorbable Sutures. Ethicon iatroduced the first monofilament synthetic absorbable suture ia 1984 when it marketed PDS polydioxanone (4) sutures. The polymer is produced by the bulk polymerisation of 2,5- -dioxanone. The suture is distributed under the trade name PDS 11. It is claimed to retain approximately 50% of its strength four weeks after implantation, 25% at six weeks, and to be absorbed within six months. 

Acrylate esters can be polymerised in a variety of ways. Among these is ionic polymerisation, which although possible (6—9), has not found industrial apphcation, and practically all commercial acryUc elastomers are produced by free-radical polymerisation. Of the four methods available, ie, bulk, solution, suspension, and emulsion polymerisation, only aqueous suspension and emulsion polymerisation are used to produce the ACMs present in the market. Bulk polymerisation of acrylate monomers is hasardous because it does not allow efficient heat exchange, requited by the extremely exothermic reaction. 

Bulk polymerisation processes have been known for many years but until the mid-1960s the only commercial process was one operated by Pechiney-St Gobain in France. This process was a one-stage process and according to one patent example vinyl chloride was polymerised with 0.8%% of its own weight of benzoyl peroxide in a rotating cylinder containing steel balls for 17 hours at 58°C. 

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. 

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. 

Bulk polymerisation is extensively used in the manufacture of the sheet and to a lesser extent rod and tube. In order to produce a marketable material it is important to take the following factors into account ... 

The average molecular weight of most bulk polymerised poly(methyl methacrylates) is too high to give a material which has adequate flow properties for injection moulding and extrusion. 

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. 

High molecular weight polymers are produced by an adiabatic bulk polymerisation process ° using di-tert-butyl peroxide (0.02%) and 2,2 -azo-bisdi-isobutyronitrile (0.01%) as initiators and pressurised with N2. Heating to 80-90°C causes an onset of polymerisation and a rapid increase in temperature. After the maximum temperature has been reached the mass is allowed to cool under pressure. A typical current commercial material (Luvican M.170) has a A -value of about 70 (as assessed in a 1% tetrahydrofuran solution). 

There are three main techniques of polymerisation bulk, solution and emulsion. In bulk or mass polymerisation, the catalyst is added directly to the monomer and heat may be applied to start the reaction. In solution polymerisation, the monomer is dissolved in an organic solvent. In emulsion polymerisation, the monomer or monomers are stirred up with water and an emulsifying agent to form a stable emulsion. Control of the reaction is obviously much easier with either solution or emulsion polymerisation than with bulk polymerisation. 

The product obtained by bulk polymerisation is of high purity because except initiator and chain transfer agent, no other additive which could contaminate the product is used. 

Method-II This method used the bulk polymerisation in a tower-type reactor. Ethylene having trace amount of oxygen is charged to the reactor at the pressure of 1500 atmosphere and 1900°C temperature. 

Bulk and suspension polymerisation are the most commonly used techniques. In bulk polymerisation styrene is heated to 80°C for about 2 days to get a viscous solution of polymer in styrene. The solution is then fed to a tower wherein polymerisation is completed at 100°C, 150C° and 180C° stagewise. In suspension process, styrene is suspended in dimineralised water in presence of suspending agent and initiator like benzoyl peroxide and heated to 20°C. The product is washed with acid, water and dried. 

Bulk Polymerisation In bulk polymerisation no initiator is required. Styrene gets partially polymerised batch-wise by heating the monomer in large vessels at 80°C for two days until there occurs about 35 per cent conversion. Then the syrupy mixture is allowed to fed continuously into the top of the tower which is twenty-five feet high. The top of the tower is kept at a temperature of about 100°C, the centre at 150° C and the bottom at 180°C. 

The polymerisation of methyl methacrylate can also be carried out by all the techniques. The sheet is made by bulk polymerisation and casting. The monomer is heated to 95°C with benzoyl peroxide at 90° C to form a syrup. Which is put in a casting cell that is held... 

Commercially, polyMethylmethacrylate is made by either suspension or bulk polymerisation using a peroxide free-radical initiator ... 

Industrial polymerisation of vinyl chloride is carried out either in suspension or emulsion. Limited quantities of PVC are also made by bulk polymerisation. 

Caprolactam can also be prepared by bulk polymerisation process using anionic catalysts like strong bases and metal hybrides. 

All the kp+ calculated by the original workers for bulk polymerisations and those obtained at high monomer concentrations are wrong because they are second-order rate-constants, calculated on the assumption that the polymerisations are second-order reactions. This is a considerable curiosity because all the kinetic curves published showed clearly that the polymerisations were of zero order with respect to the monomer concentration (m). A new set of kp+ values is given here. 

The symbol pl+B denotes the first-order rate-constant of the bulk polymerisations. If M is a mono-alkene, we have the simplest case, with no aromatic groups or hetero-atoms in monomer or polymer, so that... 

Table 1 Alleged rate-constants kp+ for bulk polymerisation and first-order k, +B calculated from them Pi ...

After Williams initial reports (Bonin etal., 1965 Hubman etal., 1966), the most detailed account of the methods used for determining kp+ is in the group of three papers resulting from the cooperation of Williams with Okamura s group in Kyoto (Ueno et al., 1967 Hayashi et al., 1967 Williams et al., 1967) in these the kp+ of styrene, a-methylstyrene and isobutyl vinyl ether were reported. The bulk polymerisation of styrene and of a-methylstyrene gave kp+ values of the same order of magnitude (ca. 106) and two orders of magnitude less than those for cyclopentadiene and isobutene the same is true for our kpi shown in Table 1. 

The bulk polymerisation of 4-methoxystyrene (Deffieux et al., 1980) also gave a kp of the same order of magnitude, but to calculate it from their less detailed measurements the authors had to assume values for the rates of initiation and termination in order to obtain [P+n] , and they chose to use those for styrene. It seems to us that it would have been more logical to take the rates of initiation and termination for another ether, e.g., IBVE, but in fact according to our calculations the resulting p+ is only ca. 50% greater, a difference which is hardly significant in the present context. 

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