Polymer Science and Engineering: Polymerization Techniques, Slides of Engineering

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POLYMER SCEINCE AND ENGINEERING

Polymer Synthesis: Polymerization Techniques

Dr. Samrina Sahir

Week 6

Polymerization techniques

✓ Kinetic / mechanistic factors related to chain length, chain

composition

✓ Technological factors e.g. heat removal, reaction rate,

viscosity of the reaction mixture, morphology of the product

✓ Economic factors ; production costs, environmental aspects,

purification steps etc.

Sometimes for one monomer several techniques of polymerizing are available. Choice of a specific technique depends on a number of factors:

✓ In case of homogeneous bulk polymerization, the feed is a gas , liquid or solid monomer. ✓ No initiators or additives are used. ✓ For homogeneous Solution polymerization, the monomer is completely dissolved in a solvent.

Homogeneous Polymerization:

Heterogeneous Polymerization:

✓ In heterogeneous Emulsion polymerization, the monomer molecules are emulsified in aqueous media in the form of micelles. ✓ For heterogeneous Suspension polymerization, the monomer is suspended in a non aqueous or other type of media as large droplets.

✓ In mass polymerization, the whole system remains in a homogeneous phase , because the monomer is taken in the liquid state, the initiator is dissolved in the monomer and the chain transfer agent (if used to control the molecular weight) is also dissolved in the monomer liquid. ✓ The reaction mixture is heated or exposed to a UV radiation source for initiating the polymerization and is kept on a rotatory shaker for proper mixing the mass and uniform heat transfer. ✓ This mode of polymerization may be employed to obtain the purest form of polymer Only the initiator and the chain transfer agent is used therefore, the product formed has a high degree of purity, i.e., without any contamination of reactant components

i .Bulk Polymerization:

Advantages

✓ It is quite simple and requires simple equipment. ✓ Direct utility of the polymer with no isolation/separation requirement. ✓ Polymers are of high purity obtained. Minimum chances of contamination. ✓ No requirement of additives other than the initiators and the chain transfer agent. ✓ Excellent colour and clarity (glass polymers) can be obtained ✓ Large castings may be prepared directly. Casting of objects of various shapes may be accomplished using the Batch Bulk Polymerization.

Disadvantages

✓ Viscosity of the reaction medium increases and its mixing becomes very difficult. Diffusibility of the growing polymer chain gets restricted. ✓ Excessive heat due to exothermic reaction may lead to an explosion or Trommsdorff-Norrish effect (Auto-acceleration). ✓ Non-uniformity in composition: Probability of chain collision becomes less and termination causes difficulty. Broad molecular weight distribution ✓ Conversion rate is only 80%

✓ The whole system is in a homogeneous phase , because the monomer, chain transfer agent (if used) and initiator are dissolved in an inert suitable solvent , while the ionic or coordination catalysts can either be dissolved or suspended in the solvent medium. ✓ The solvent enhances the heat capacity, thereby reducing the viscosity and facilitating heat transfer. The temperature is easily controlled because of the fact that the monomers are dissolved in solvent and the upper limit of the whole system depends upon the boiling point of the solvent. ✓ After the reaction is over the polymer is used as such in the form of polymer solution or isolated by evaporating the solvent. ✓ Compared to Bulk Polymerization, Solution Polymerization reduces both the reaction rate and the molecular weight of the compounds. No auto acceleration

ii .Solution Polymerization:

Disadvantages

✓ The method is costly since it uses costly solvents. ✓ The polymer produced is low average molecular weight (here solvents act as chain terminators) and is always contaminated with traces of the solvent. ✓ Polymer will also have to be isolated from the solution either by precipitation or by evaporation of the solvent. ✓ Environmental pollution due to solvent release. ✓ Purity of product is also not as high as that of bulk polymerization

The requirements for the selection of the solvent are:

✓ Both the initiator and monomer be soluble in it.

✓ Have acceptable chain-transfer characteristics.

✓ Suitable melting and boiling points for the conditions of the

polymerization and any subsequent solvent-removal step.

✓ ➤ Examples of suitable organic solvents include aliphatic and

aromatic hydrocarbons, esters, ethers, and alcohols.

These methods are carried out with controlled thermal and

catalyst conditions.

They may be further classified as –

Polymerization Methods

✓ This is the most widely used method of polymerization. ✓ This process overcomes the difficulty of heat control and low degree of polymerization. ✓ The organic reaction mass is emulsified with soap in a continuous aqueous phase. ✓ The dispersed particles are smaller in size than in Suspension Polymerization ( ≤ 0. μm). In addition, due to stabilizing action of soap, the emulsion obtained is stable and agitation may not be necessary. ✓ In some methods, a water – soluble initiator may be used. ✓ The main product of Emulsion Polymerization is latex, which forms the basis of the popular latex paints. By coagulating the latex with ionic salts and acids, solid rubber may also be obtained.

iii .Emulsion Polymerization:

An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation.