37.1. Classification According To Preparation Methods :
Preparation methods for polymers may be roughly divided into two
(A). Condensation Polymerization Methods.
(B). Addition Polymerization Methods.
(A). Condensation Polymerization Methods : These methods are usually employed for low molecular weight functional group reactions, where the stoichiometric proportions of the reactions are fixed for the desired final products. During processing, solvent addition may or may not be important. These reactants are usually mixed in a batch reactor with controlled temperature and addition of catalysts. Vacuum processing or AzeotropicDistillation may be employed to remove condensation products such as H2O to obtain a higher degree of polymerization.
(B). Addition Polymerization Methods : These methods are carried out with controlled thermal and catalyst conditions. They may be further classified as –
i .Bulk Polymerization : This mode of polymerization may be employed to obtain the purest form of polymer, and the greatest yield of polymer per unit volume may be obtained using this method. This method involves only the monomer molecule, an initiator and a chain transfer agent ( if necessary). In a large scale batch form, the process is to be run slowly or in continuous mode with large heat transfer area. Casting of objects of various shapes may be accomplished using the Batch Bulk Polymerization. Using continuous bulk polymerization processes, polystyrene and other thermoplastic compounds may be moulded.
ii .Solution Polymerization : An inert solvent is added to the reacting components in the reaction vessel in this process. The solvent enhances the heat capacity, thereby reducing the viscosity and facilitating heat transfer. Some of the solvent may be refluxed to remove heat from the reaction vessel. But , much of the reactor space is taken up by the solvent. As such , compared to Bulk Polymerization, Solution Polymerization reduces both the reaction rate and the molecular weight of the compounds. Additional batch and continuous processes are used to separate the final polymer product and to recover and store the solvent used.
iii .Suspension Polymerization : Inorder to control the enormous amount of heat release in Bulk Polymerization, Suspension Polymerization method was developed. The reaction mass is dispersed as minute droplets of size 0.01 – 1 mm in diameter in a continuous aqueous phase. Each of these droplets act as tiny bulk reactors. Heat transfer occurs from the droplets to the water having large heat capacity and low viscosity. Cooling jackets are used to facilitate heat removal. Agitators are used along with suspending agents in the aqueous phase inorder to maintain a specific droplet size and
The Suspension Polymerization process cannot be run in a continuous mode , since, such a system has stagnant corners where polymer accumulation may occur. On a commercial scale, Suspension Polymerization is carried out in jacketed, stainless steel or glass – lined stirred tanks, which may have a capacity of 20,000 gal or 75.5 m3.
Suspension Polymerization produces small, uniform polymer spheres. These are used directly, or may be extruded and chopped to form larger, moulded
iv .Emulsion Polymerization : 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 ( ≤ 0.1 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.
v .Homogeneous Polymerization: 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.
vi .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 n aqueous or other type of media as large droplets.
37.2.Classification According to Physical
Polymers can also be classified according to physical properties as –
i. Thermoplastic : The polymers in this category are composed of monomers which are linear or have moderate branching. They can be melted repeatedly and casted into various shapes and structures. They are soluble in solvents, but do not have appreciable thermal resistance properties. Vinyls, cellulose derivatives, polythene and polypropylene fall into the category of thermoplastic polymers.
ii. Thermosetting : There are some polymers which, when heated, decompose, and hence, cannot be reshaped. Such polymers have a complex 3-D network (cross-linked or branched) and are called Thermosetting Polymers. They are generally insoluble in solvents and have good heat resistance quality. Thermosetting polymers include phenol-formaldehyde, urea-aldehyde, silicones and allyls.
iii. Elastomers : These are resistant solids which have considerable flexibility. They are composed of polymers with glass transition temperature
below room temperature. One major difference between elastomers and plastics is that the elastomer is in a liquid state, while plastics are in the glassy state. Examples of elastomers are Butadiene, Butadiene co – polymers and their derivatives,polycondensationproducts , silicones and thiokols.
iv .Fibers : These are solids which can form thread – like structures and
have high tensile strength. Examples of fibers are Polyamides, Polyesters, Polyurethanes, Protein
37.3.Classification According To Applications
On the basis of applications, polymers can be further classified as –
i. Adhesives: Some polymers can be used for bonding . They are usually of the resin type and require some water resistance . Some common adhesives
Cellulose adhesives ; which consist of cellulose derivatives dissolved in a
solvent. Eg.Duco cement.
Vinyls ; these are rubber base type water-emulsified latex adhesives. Apart from these, some cheap natural products such as starch, dextrins, proteins and natural rubber may also be used for adhesive formulations.
ii. Coatings and films : A large bulk of the polymers produced are used for manufacturing coatings and films. Free films of polyethylene and cellulosic types, protective and decorative coatings are the products of the polymer
Coatings can be manufactured by solvent evaporation followed by polymerization. Emulsion and casting or extrusion of free films by mechanical methods can also produce coatings and films.
iii. Fibers: These are formed by extrusion or spinning of linear monomer molecules into thin sections of diameter in the range of 10 – 50 microns. Fibers have excellent tensile strength , creep and resilience.
Fibers are extensively used in the textile industry . Cotton, wool etc. are the examples of some natural fibers.
Q. 1. Mention the merits and demerits of Emulsion Polymerization w.r.t. other polymerization processes.
Ans. 1.One of the major advantages of Emulsion Polymerization is that it has good heat control over the entire polymerization process. The heat control occurs by transfer to the aqueous phase, and this enables little change in the viscosity of the emulsion medium. Secondly, in this process, the reaction mass is emulsified with soap in a continuous aqueous medium. Soap has a stabilizing action, and hence, the emulsion obtained is stable and no agitation is necessary. Due to this, extra cost incurred in employing agitators is avoided.
Moreover, Emulsion Polymerization overcomes the difficulty of low degree of polymerization, as encountered in Solution and Bulk Polymerization. Since the degree of polymerization directly depends upon the droplet sizes, and in this process, spherical monomer particles (micelles) in the range of 1 – 10 µ, are yielded ; hence, the degree of polymerization is sufficiently higher than in the other two processes.
However, Emulsion Polymerization has certain disadvantages. It blocks the emulsifiers used in the process. In addition, the process has poor capacity and low electrical resistance , which sometimes create major problems.
Q.2. What are stereospecific Polymers ? How are they achieved?
Ans .2. Stereospecific Polymer are specially oriented polymers which have certain properties that are completely different from the usual polymers. These special properties include high density and melting points ,crystallinity and improved mechanical properties.
The reason for having these special property in the polymers is because of tailored atomic arrangements, which are achieved by the addition of catalysts like Ziegler catalysts or supported metal oxide catalysts. Even, Xray and ¥ - ray radiations can bring about stereospecificity in the polymers.
The stereospecific polymers can be realized by examining the spatial arrangement of atoms on the main chain .