POLYMERISATION
IN
HETEROGENEOUS
SYSTEMS
-Dhruv Sapra , B.Tech, Delhi Technological university
POLYMERISATION IN HETEROGENEOUS SYSTEMS
Introduction
Polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains. There are many forms of polymerization and different systems exist to categorize them. During the lectures in the class we discussed the various forms of polymerization and studied it's kinetics, with the assumption of pure monomers or we considered just simple solutions of monomer and polymer in a solvent. But certain other types of polymerizing systems are of great interest interests because they offer practical advantages in industrial applications. The polymer systems are broadly divided as Homogeneous and Heterogeneous for the purpose of their study on the basis of the phase in which the monomer and the polymer exists.
The homogeneous polymerization techniques involve pure monomer or homogeneous solutions of monomer and polymer is a solvent. There are two such systems:
· Bulk system:
Polymerization in bulk, perhaps the most obvious method of synthesis of polymers, is widely practiced in the manufacture of condensation polymers, where the reactions are only mildly exothermic, and most of the reaction occurs when the viscosity of the mixture is still low enough to allow ready mixing, heat transfer, and bubble elimination. Control of such polymerizations is relatively easy.
Bulk polymerization of vinyl monomers is more difficult, since the reactions are highly exothermic and, with the usual thermally decomposed initiators, proceed at a rate that is strongly dependent on temperature This, coupled with the problem in heat transfer incurred because viscosity increases early in the reaction, leads to
difficulty in control and a tendency to the development of localized "hot spots" and "runaways" " Except in the preparation of castings, for example, of poly(methy1 methacrylate), bulk polymerization is seldom used commercially for the manufacture of vinyl polymers.
Some of polymers prepared using Bulk polymerization are:
Polycaproamide (Nylon 6), LDPE, PET
· Solution system:
Polymerization of vinyl monomers in solution is advantageous from the standpoint of' heat removal (e..g., by allowing the solvent to reflux) and control, but has two potential disadvantages. First, the solvent must be selected with care to avoid chain transfer and, second, the polymer should preferably be utilized in solution, as in the case of poly(viny1 acetate) to be converted to poly(viny1 alcohol) and some acrylic ester finishes, since the complete removal of solvent from a polymer is often difficult to the point of impracticality, and is indeed a very expensive affair commercially.
Some of polymers prepared using Solution polymerization are:
HDPE, Polypropylene, Polystyrene, poly (acrylic acid), polyacrylamide
Heterogeneous polymerization techniques involve heterogeneous solutions of monomer and polymer in a solvent. These systems can be either emulsions, dispersions, suspensions or interfacial. Let us now discuss each of these polymerizations in detail.
Polymerization from Gaseous Monomers
The polymerization of gaseous monomers can take place with the formation of a liquid phase (polymer melt) or a solid polymer. In each case the polymerization of ethylene provides the most important industrial example.
The high-pressure polymerization of ethylene to branched polyethylene takes place by a free-radical mechanism in the presence of a liquid phase, at temperatures above the melting point of the polymer.. The polymerization is carried only to low conversion, with the remaining ethylene recovered when the pressure is lowered and recycled.
The low-pressure polymerization of' ethylene to linear polyethylene takes place by coordination polymerization using a catalyst suspended in gaseous ethylene in a fluid-bed reactor. The ethylene polymerizes to the solid phase, with the small amount of the catalyst required remaining in the solid polymer.
If the polymerization takes place with the formation of the solid polymer, then due to the solid-gas interface, it becomes an example of Interfacial polymerization.
Some of the polymers that undergo this polymerization include:
Polyethylene, Polypropylene
Emulsion Polymerization
Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating solvent(usually water), monomer, initiator, and surfactant. These latex particles are typically 100 nm in size, and are made of many individual polymer chains.
Process
· the monomer has only a very limited (but finite) solubility in the
solvent (e.g. styrene in water). Most of it is present initially in
dispersed droplets (hence the term “emulsion” polymerisation); one
role of the (anionic) surfactant is to help stabilise these droplets, by
adsorbing at the droplet / water interface. However, some of the
monomer is present in the water phase – this is important !
Schematic diagram of emulsion polymerization
· the surfactant is present at a concentration > its c.m.c. that is critical micelles concentration(at the reaction temperature). Therefore, most of it is present as micelles,
again in the water phase.
Note: some of the monomer will be solubilized in the micelles – also important !
thus the monomer is actually distributed in 3 locations :
droplets > aqueous solution (small amount) > micelles [1]
· the initiator is soluble (and therefore present) in the water phase. The initial locus of polymerisation is, therefore, again in the aqueous solution (as in dispersion polymerisation), i.e. that is the first monomer to polymerise.
"Redox" Initiation
The decomposition of peroxide-type initiators in aqueous systems is greatly accelerated by the presence of a reducing agent. This acceleration allows the attainment of high rates of radical formation at low temperatures in emulsion systems.
A typical redox system is that of ferrous iron and hydrogen peroxide In the absence of a polymerizable monomer the peroxide decomposes to free radicals as follows:
Another widely used peroxide-type initiator is the persulfate ion. With a reducing agent R the reaction is
The reducing agent is often the thiosulfate ion,
or the bisulfite ion,
Many other redox initiator systems have been used.
· The growing, oligomeric free-radical chains, e.g. 2-SO4Am ·, beyond a certain value of m (typically 5 to 10), will co-micellise in with the existing micelles from the added anionic surfactant. The primary locus of polymerisation now switches to the micelles, where the
solubilised monomer can now begin to polymerise.
· as polymerisation (in the micelles) continues, particles form, as in dispersion polymerisation, and the distribution of monomer, as represented by the sequence labelled [1] above, is gradually pulled to the right. Polymerisation continues in the growing particles until all the monomer in the droplets and free solution is exhausted.
· the size of the final particles is controlled by the number of micelles present (i.e. the initial surfactant concentration); this is much greater than in dispersion polymerisation – hence the smaller particles.
Smith-Ewart Kinetics
(Smith 1948a,b).. In an ideal emulsion system, free radicals are generated in the aqueous phase at a rate of about lO-I3 per cubic centimeter per second. There are about lOI4 polymer particles per cubic centimeter. Simple calculations show that termination of the free radicals in the aqueous phase is negligible and that diffusion currents are adequate for the rapid diffusion of free radicals into the polymer particles-on the average, about one per particle every 10 sec It can also be calculated from the known termination rate constants that two free radicals within the same polymer particle would mutually terminate within a few thousandths of a second. Therefore each polymer particle must contain most of the time either one or no free radicals. At any time half of the particles (on the average) contain one free radical, the other half none.. The rate of polymerization per cubic centimeter of emulsion is
where N is the number of polymer particles per cubic centimeter. Since the monomer concentration is approximately constant, the rate depends principally on the number of particles present and not on the rate of generation of radicals.
The degree of polymerization also depends upon the number of' particles:
where p is the rate of generation of radicals. Unlike Vp, Xn is a function of the rate of free-radical formation. In bulk polymerization rate can be increased only by increasing the rate of initiation; this, however, causes a decrease in the degree of polymerization In emulsion polymerization the rate may be increased by increasing the number of polymer particles If the rate of initiation is kept constant, the degree of polymerization increases rather than decreases as the rate rises Since the number of polymer particles is determined by the number of soap micelles initially present, both rate and molecular weight increase with increasing soap concentration. The Smith-Ewart kinetics require that
where [E] is the soap or emulsifier concentration.
Deviations from Smith - Ewart kinetics
The above kinetic scheme is highly idealized, though valuable for its simplicity. It explains adequately only a small portion of the vast literature on emulsion polymerization, though it works well for monomers such as styrene, butadiene, and isoprene, whose water solubility is very low, less than 0.1%. Among the circumstances under which the Smith-Ewart kinetics fails to apply quantitatively are the following:
a. Larger particles (>O 1 - 0 15 km in diameter), which can accommodate more than one growing chain simultaneously
b. Monomers with higher water solubility (1--lo%), such as vinyl chloride, methyl methacrylate, vinyl acetate, and methyl acrylate Here initiation in the aqueous phase, followed by precipitation of polymer, becomes important. These particles may absorb emulsifier, decreasing [E] and thus N, or they may serve as sites for polymerization, increasing N.
c. Chain transfer to emulsifier This often takes place in large enough amount to suggest that growing chains are localized near the surfaces of the particles, where the soap exists.
Inverse emulsion systems
Emulsion polymerization can also be carried out in systems using an aqueous solution of a hydrophilic monomer, such as acrylic acid or acrylamide, emulsified in a continuous oil phase using an appropriate water-in-oil emulsifier Either oil- or water-soluble initiators can be used The mechanism seems to be that of normal emulsion polymerization, but the emulsions are often less stable.
Mini Emulsion systems
A miniemulsion is a special case of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases, one surfactant and one co-surfactant (typical examples are hexadecane or acetyl alcohol).
The shearing proceeds usually via ultrasonication of the mixture or with a high-pressure homogenizer, which are high-shearing processes.
Stable droplets are then obtained, which have typically a size between 50 and 500 nm. The miniemulsion process is therefore particularly adapted for the generation of nano-materials. There is a fundamental difference between traditional emulsion polymerisation and a miniemulsion polymerisation.
Advantages of emulsion polymerization include:
§ High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate.
§ The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate.
§ Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight.
§ The final product can be used as is and does not generally need to be altered or processed.
Disadvantages of emulsion polymerization include:
§ Surfactants and other polymerization adjuvant remain in the polymer or are difficult to remove
§ For dry (isolated) polymers, water removal is an energy-intensive process
§ Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.
§ Cannot be used for condensation, ionic or Ziegler-Natta polymerization usually.
Polymers produced by emulsion polymerization can be divided into three rough categories.
§ Synthetic rubber
§ Some grades of styrene-butadiene (SBR)
§ Some grades of Polybutadiene
§ Fluoroelastomer (FKM)
§ Plastics
§ Some grades of PVC
§ Some grades of polystyrene
§ Some grades of PMMA
§ Acrylonitrile-butadiene-styrene terpolymer (ABS)
§ PTFE
§ Dispersions (i.e. polymers sold as aqueous dispersions)
§ polyvinyl acetate copolymers
§ Styrene-butadiene
§ VAE (vinyl acetate - ethylene copolymers)
Suspension Polymerization
Suspension polymerization (also known as pearl polymerization, bead polymerization and granular polymerization) is a polymerization process that uses mechanical agitation to mix the monomer or mixture of monomers in a liquid phase such as water, polymerizing the monomer droplets while they are dispersed by continuous agitation. This process is used in the production of most PVC, a widely used plastic, as well as Sodium polyacrylate, a superabsorbent polymer used in disposable diapers.
Suspension polymerization consists of an aqueous system with monomer as a dispersed phase and results in polymer as a dispersed solid phase.
Method:
A reactor fitted with a mechanical agitator is charged with a water insoluble monomer and initiator (+ a chain-transfer agent to control molecular weight).
Droplets of monomer (containing the initiator and chain-transfer agent) are formed (50 – 200 µm). These sticky droplets are prevented by the addition of a protective colloid (PVA). Near the end of polymerization, the particles are hardened, are then recovered by filtration, and followed by washing step.
Advantages:
• Excellent heat transfer because of the presence of the solvent.
• Solvent cost and recovery operation are cheap.
Disadvantages:
• Contamination by the presence of suspension and other additives low polymer purity.
• Reactor cost may higher than the solution cost.
E.g. PVC, PSAN, Poly (vinylidene chloride –VC)
Precipitation Polymerization
In the preparation of a polymer insoluble in its monomer, or in polymerization in the presence of a non solvent for the polymer, marked deviations from the kinetics of homogeneous radical polymerization may occur. The normal bimolecular termination reaction is not effective, as the result of trapping or occlusion of radicals in the un swollen, tightly coiled precipitating polymer. The theory has been confirmed by the demonstration of the presence of radicals in the polymer both by chemical methods and by electron paramagnetic resonance. The lifetime of the trapped radicals is many hours at room temperature, and if polymer containing such radicals is heated in the presence of monomer to a temperature where the mobility of the radicals is increased, extremely rapid polymerization takes place
Kinetics
The polymerization of vinyl chloride in bulk or in the presence of a non solvent (Mickley 1962) follows a rate equation of the form
where the first term inside the braces arises from the normal rate equation for polymerization in the homogeneous liquid phase, and the second term represents the increment in rate due to polymerization in the precipitated polymer particles.
The function f (P) is proportional to the polymer concentration [PI at low conversion and to [P]2/3 at later times. Radical occlusion occurs, but the depth of radical penetration into the polymer particles is small, limiting radical activity to thin surface layers in larger particles present at higher conversions Termination occurs primarily in the liquid phase, probably as a result of transfer to monomer within the polymer particles, and subsequent diffusion of the monomer radicals to the liquid phase In contrast, transfer to monomer in the polymerization of acrylonitrile is so slow that permanent radical occlusion occurs.
Advantages
1) It requires more temperature and heat control.
2) They have low viscosity because of which they can dilute slurry.
3) No surfactant is required.
Some of the polymers going with precipitation polymerization include:
Polyvinyl chloride,
Dispersion Polymerization
It produces colloidal “latex” particles in the size range : 0.1 to 1mm
Ingredients- monomer, initiator and solvent.
a. The polymer is insoluble in the solvent, although the monomer is (just) soluble, e.g. styrene and water/methanol mixed solvent.
b. The initiator (e.g. K2S2O8 → 2 SO4 - , ~ 70°C) is soluble in the solvent and must possess a charge (when the solvent is aqueous or polar),e.g.
NOTE:
These “living”, low molar mass polymer chains (“oligomers”) closely resemble anionic surfactant molecules, and will therefore micellise, above their critical micelle concentration (c.m.c.) – at the temperature of the reaction , i.e. ~ 70°C.
These micelles will swell with monomer, and so the locus of polymerisation now switches to the micelles.
c. If electrolyte is added (e.g. NaCl) then this reduces the electrostatic repulsion between the particles, and some coagulation of the particles takes place. Whilst the particles are still “soft” (i.e. swollen with monomer), then coalescence of these coagulated particles occurs. This leads finally to particles with a greater average size.
d. In non-polar or semi-polar solvents, charge-stabilization of the growing particles is likely to be ineffective, and an alternative mechanism is required to achieve stabilisation of these particles to coagulation, and hence to gross precipitation. In this case “steric stabilisation” is employed, in which the surfaces of the particles are covered in a layer of attached, solvent-soluble polymer chains.
Polymer that is produced by dispersion polymerization include Polystyrene
Difference between dispersion and precipitation polymerisation:
A distinction should be made between precipitation and dispersion polymerization, due to the similarities. A dispersion polymerization is actually a type of precipitation polymerization, but the difference lies in the fact that precipitation polymerizations give larger and less regular particles, as a result of little or no stabilizer present.
The main difference between dispersion and precipitation polymerisation is the locus of polymerisation. In dispersion polymerisation the particles are the main locus of polymerisation, whereas in precipitation polymerisation the continuous phase is the locus of polymerisation because the medium or the monomer does not swell the precipitated polymer.
This also means that there is a continuous nucleation and the particles are irregularly shaped. A typical example is the polymerisation of acrylonitrile in bulk.
During precipitation the environment of the polymer chain changes dramatically, and if the radical on the chain is still active, termination can be delayed, which means that precipitation polymerisation leads to higher molecular masses as compared to dispersion polymerisation. Precipitation polymerisation can be used to obtain particles measuring 0.5-5um in size.
Solid Phase Polymerization
A large number of olefin and cyclic monomers can be polymerized from the crystalline solid state. Among those that react rapidly under these conditions are styrene, acrylonitrile, methacrylonitrile, formaldehyde, trioxane, 0-propiolactone, diketene, vinyl stearate and vinyl carbazole.
Polymerization always appears to be associated with defects in the monomer crystals, most likely line defects; otherwise, it would be required that the monomer and the polymer be isomorphous, that is, that they have the same crystal structure, lattice parameters, and so on This seems extremely unlikely and has not been observed.
References:
Ø Articles on the different types of polymerization available on wikipedia dated 1/11/11.
Ø http://www.chembbs.com.cn/bbs/File/UserFiles/UpLoad/20090914033604sw.pdf
Ø http://repository.ui.ac.id/contents/koleksi/11/e6780dd3999c39fd995a386e9cddd71cd3a1abe8.pdf
Ø Book on Polymer Chemistry by Odian pg 350
Ø http://www.scribd.com/doc/19063903/14/Suspension-polymerisation?query=suspension
Ø Textbook of polymer chemistry By Fred W.Billmeyer, third edition, chapter six.
Ø http://www.cheric.org/PDF/MMR/MR12/MR12-2-0233.pdf - for the section of Precipitation polymerisation.
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