POLYMERIZATION PROCESSES

Industrial radical initiated polymerizations can be carried out in one of four different ways:

1. With monomer only — bulk 2. In a solvent — solution

3. With monomer dispersed in an aqueous phase — suspension 4. As an emulsion

Bulk polymerization is used in the production of polystyrene, poly(methyl-methacrylate), and poly(vinyl chloride). The reaction mixture contains only mono-mer and initiator, but because the reaction is exothermic, hot spots tend to develop when heat removal is inefficient. Auto-acceleration occurs in the highly viscous medium, making control difficult and impeding efficient monomer conversion. To overcome some of the disadvantages, low conversions are used, after which the

9813_C003.fm Page 76 Friday, June 15, 2007 10:53 AM

Free-Radical Addition Polymerization 77

unreacted monomer is stripped off and recycled. The main advantages of the tech-nique lie in the optical clarity of the product and its freedom from contaminations.

Quiescent mass polymerization is an unstirred reaction used for casting sheets of poly(methyl methacrylate). A low-molar-mass prepolymer is prepared, and then the main polymerization is carried out in situ making use of the Trommsdorff effect to obtain high-molar-mass material and tougher sheets. The two-stage approach helps to control the heat evolved.

In solution polymerization, the presence of the solvent facilitates heat transfer and reduces the viscosity of the medium. Unfortunately, the additional complication of chain transfer arises and solvents must be selected with care.

Ethylene, vinyl acetate, and acrylonitrile are polymerized in this way. The redox initiated polymerization of acrylonitrile is an example of precipitation polymeriza-tion where the polyacrylonitrile formed is insoluble in water and separates as a powder. This can lead to undesirable side reactions known as popcorn polymeriza-tions when tough cross-linked nodules of polymer grow rapidly and foul the feed lines in industrial plants.

Suspension polymerization counteracts the heat problem by suspending droplets of water-insoluble monomer in an aqueous phase. The droplets are obtained by vigorous agitation of the system and are in the size range 0.01 to 0.5 cm diameter.

The method is, in effect, a bulk polymerization, which avoids the complications of heat and viscosity build up.

Emulsion polymerization is an important technological process widely used to prepare acrylic polymers, poly(vinyl chloride), poly(vinyl acetate), and a large num-ber of copolymers. The technique differs from the suspension method in that the particles in the system are much smaller, 0.05 to 5 µm diameter, and the initiator is soluble in the aqueous phase rather than in the monomer droplets. The process offers the unique opportunity of being able to increase the polymer chain length without altering the reaction rate. This can be achieved by changing either the temperature or the initiator concentration, and the reasons for this will become more obvious when we examine the technique more closely.

The essential ingredients are monomer, emulsifying agent, water, and a water-soluble initiator. The surfactant is normally an amphipathic long chain fatty acid salt with a hydrophilic “head” and a hydrophobic “tail.” In aqueous solutions these form aggregates or micelles (0.1 to 0.3 µm long), consisting of 50 to 100 molecules oriented with the tails inward, thereby creating an interior hydrocarbon environment and a hydrophilic surface of heads in contact with the water. The micelles exist in equilibrium with free molecules in the aqueous phase, and the concentration must exceed the critical micelle concentration of the emulsifier.

When monomer is added to the dispersion, the bulk of it remains in the aqueous phase as droplets, but some dissolves in the micelles, swelling them. Free radicals are generated from a water-soluble redox system such as persulfate + ferrous ions

at a rate of 10¹⁶ dm⁻³ s⁻¹. The radicals diffuse through the aqueous phase and penetrate both the micelles and droplets, but as the concentration of micelles (about 10²¹ dm⁻³)

S₂O₈^2–+ Fe²⁺ Fe³⁺+ SO₄^2–+SO₄^–

9813_C003.fm Page 77 Friday, June 15, 2007 10:53 AM

78 Polymers: Chemistry and Physics of Modern Materials

far exceeds that of the droplets (10¹³ to 10¹⁴ dm⁻³), polymerization is centered almost exclusively in the micelle interior. After only 2 to 10% conversion, the character of the system has changed markedly. Constant replenishment of the polymer swollen micelles takes place by diffusion from the droplets, which decrease steadily in size until at about 50 to 80% conversion they have been totally consumed. Polymerization then continues at a steadily decreasing rate until all the remaining monomer in the micelle is converted into polymer.

A schematic diagram of a typical emulsion system is shown in Figure 3.6. The polymerization process can be described using the model developed by Smith and Ewart. This assumes that a micelle-containing monomer is penetrated by a radical diffusing in through the aqueous phase, and this initiates a chain propagation reaction with rate vp. The chain will continue to grow until another radical reaches the micelle and enters, where it will encounter the growing chain end and terminate it. The micelle will then remain quiescent until another chain is initiated by the entry of another radical, and the process of chain growth continues until the next radical arrives and once more terminates the chain, i.e., the model assumes that only one active radical can be tolerated in a micelle, and so this will contain either one or zero radicals at any one time. The result is that the polymerization process within any micelle in the system comprises a series of start–stop reactions, and the rate of the on–off switching is controlled by both the rate of radical production and the number of micelles in the reaction medium. As the entry of a radical into a micellar particle is random, the chances of a chain growing in a micelle at any particular time are 50:50. This means that if there are N* micellar particles containing monomer and polymer in the system, then on average only N*/2 will be active at any period during the course of the complete polymerization reaction, so the rate v_p will be proportional to the concentration of the monomer in the micelle [M*] and to the number of active micelles, (N*/2), i.e.,

vp = kp[M*][N*/2] (3.29)

FIGURE 3.6 Schematic representation of an emulsion polymerization system.

Monomer swollen polymer particle Emulsifier

ions

Micelle d = 5 – 10 nm Solubilized

monomer Monomer emulsion

droplet d = 1 – 10 μm R-R 2R

Continuous aqueous phase 9813_C003.fm Page 78 Friday, June 15, 2007 10:53 AM

Free-Radical Addition Polymerization 79

where k_p [M*] is the rate of polymerization within a single micelle. If the rate of radical production is v_i, then the rate at which they enter a micelle is (v_i/N*), which is the rate of initiation (or termination) in the micelle. The kinetic chain length in a micellar particle is then

(3.30)

The consequences of this analysis are (1) an increase in the initiator concentration decreases the polymer chain length while leaving the rate of polymerization unaf-fected and, more surprisingly, (2) for a fixed initiator concentration both v_p and the chain length are a function of the number of micelles in the system. Thus, an increase in the surfactant concentration alone is sufficient to increase the polymerization rate and the molar mass of the product. This can be understood on the basis of the model, as increasing the number of micelles while holding the rate of radical production constant means that the time between the penetration of the micelle by successive radicals will be increased, thereby allowing the chain propagation to continue for longer periods before termination.

A combination of high rates and large x_n can be obtained without temperature variation, and this provides the system with its particular appeal. Control of the chain length can be achieved, when desired, by adding a chain transfer agent such as dodecyl mercaptan.