Polymer Preparation Techniques

Four general techniques are used for preparation of polymers by free-radical mechanism: polymerization in bulk, in solution, in suspension and in emulsion. The bulk or mass polymerization is probably the simplest of the four methods. Only the monomer and the initiator are present in the reaction mixture. It makes the reaction simple to carry out, though the exotherm of the reaction might be hard to control, particularly if it is done on a large scale. Also there is a chance that local hot spots might develop. Once bulk polymerization of vinyl monomers is initiated, there can be two types of results, depending upon the solubility of the polymer. If it is soluble in the monomer, the reaction may go to completions with the polymer remaining soluble throughout all stages of conversion. As the polymerization progresses, the viscosity of the reaction mixture increases markedly. The propagation proceeds in a medium of associated polymeric chains dissolved in or swollen by the monomer until all the monomer is consumed.

If the polymer is insoluble, it precipitates out without any noticeable increase in solution viscosity. Examples of this type of a reaction can be polymerizations of acrylonitrile or vinylidene chloride. The activation energy is still similar to most of the polymerizations of soluble polymers and the initial rates are proportional to the square root of initiator concentration. Also, the molecular weights of the polymerization products are inversely proportional to the polymerization temperatures and to initiator concentrations. Furthermore, the molecular weights of the resultant polymers far exceed the solubility limits of the polymers in the monomers. The limit of acrylonitrile solubility in the monomer is at a molecular weight of 10,000. Yet, polymers with molecular weights as high as 1,000,000 are obtained by this process. This means that the polymerizations must proceed in the precipitated polymer particles, swollen and surrounded by monomer molecules. The kinetic picture of free-radical polymerization applies best to bulk polymerizations at low points of conversion. As the conversion progresses, however, the reaction becomes complicated by chain transferring to the polymer and by gel effect. The amount of chain transferring varies, of course, with the reactivity of the polymer radical.

Bulk polymerization is employed when some special properties are required, such as high molecular weight or maximum clarity, or convenience in handling. Industrially, bulk polymerization in special equipment can have economic advantages, as with bulk polymerization of styrene. This is discussed in Chap. 6. Solution polymerization differs from bulk polymerization because a solvent is present in the reaction mixture. The monomer may be fully or only partially soluble in the solvent. This, the polymer may be (1) completely soluble in the solvent, (2) only partially soluble in the solvent, and (3) insoluble in the solvent. When the monomer and the polymer are both soluble in the solvent, initiation and propagation occur in a homogeneous environment of the solvent. The rate of the polymerization is lower, however, than in bulk. In addition, the higher the dilution of the reactants the lower is the rate and the lower is the molecular weight of the product. This is due to chain transferring to the solvent. In addition, any solvent that can react to form telomers will also combine with the growing chains. If the monomer is soluble in the solvent, but the polymer is only partially soluble or insoluble, the initiation still takes place in a homogeneous medium. As the chains grow, there is some increase in viscosity that is followed byprecipitation. The polymer precipitates ina swollenstate and remainsswollen by the diffused and adsorbed monomer. Further propagation takes place in these swollen particles. Because propagation continues in the precipitated swollen polymer, the precipitation does not exert a strong effect on the molecular weight of the product. This was demonstrated on polymerization of styrene in benzene (where the polymer is soluble) and in ethyl alcohol (where the polymer is insoluble). The average molecular weight obtained in benzene at 100C was 53,000 while in ethyl alcohol at the same temperature it was 51,000 [280]. When the monomers are only partially soluble and the polymers are insoluble in the solvents the products might still be close in molecular weights to those obtained with soluble monomers and polymers. Polymerization of acrylonitrile in water can serve as an example. The monomer is only soluble to the extent of 5–7% and the polymer is effectively insoluble. When aqueous saturated solutions of acrylonitrile are polymerized with water-soluble initiators, the systems behave initially as typical solution polymerizations. The polymers, however, precipitate out rather quickly as they form. Yet, molecular weights over 50,000 are readily obtainable under these conditions.

There are different techniques for carrying out solution polymerization reactions. Some can be as simple as combining the monomer and the initiator in a solvent and then applying agitation, heat and an inert atmosphere [292]. Others may consist of feeding into a stirred and heated solvent the monomer or the initiator, or both continuously, or at given intervals. It can be done throughout the course of the reaction or through part of it [293]. Such a set up can be applied to laboratory preparations or to large-scale commercial preparations. It allows a somewhat better control of the exotherm during the reaction. In both techniques the initiator concentration changes only a few percent during the early stages of the reaction, if the reaction temperature is not too high. The polymerization may, therefore, approach a steady state character during these early stages. After the initial stages, however, and at higher temperatures, the square root dependence of rates upon the initiator concentration no longer holds. This is a result of the initiator being depleted rapidly. The second technique, where the initiator, or the monomer and the initiator are added continuously was investigated at various temperatures and rates of addition [294–299]. If the initiator and monomer are replenished at such a rates that their ratios remains constant, steady state conditions might be extended beyond the early stages of the reactions. How long they can be maintained, however, is uncertain. Suspension polymerization [298] can be considered as a form of mass polymerization. It is carried out in small droplets of liquid monomer dispersed in water or some other media and caused to polymerize to solid spherical particles. The process generally involves dispersing the monomer in a non solvent liquid into small droplets. The agitated stabilized medium usually consists of nonsolvent (often water) containing small amounts of some suspending or dispersing agent. The initiator is dissolved in the monomer if it is a liquid or it is included in the reaction medium, if the monomer is a gas. To form a dispersion, the monomer must be quite insoluble in the suspension system. To decrease the solubility and to sometimes also increase the particle size of the resultant polymer bead, partially polymerized monomers or prepolymers may be used. Optimum results are obtained with initiators that are soluble in the monomer. Often, no differences in rates are observed between polymerization in bulk and suspension. Kinetic studies of styrene suspension polymerization have shown that all the reaction steps, initiation, propagation, and termination, occur inside the particles [299]. The main difficulty in suspension polymerization is in the forming and in the maintaining uniform suspensions. This is because the monomer droplets are slowly converted from thin immiscible liquids to sticky viscous materials that subsequently become rigid granules. The tendency is for the sticky particles to attach to each other and to form one big mass. The suspending agent’s sole function is to prevent coalescing of the sticky particles. Such agents are used in small quantities (0.01–0.5% by weight of the monomer). There are many different suspending agents, both organic and inorganic. The organic ones include methylcellulose, ethyl cellulose, poly(acrylic acid), poly(methacrylic acid), salts of these acids, poly(vinyl alcohol), gelatins, starches, gums, alginates, and some proteins, such as casein or zein. Among the inorganic suspending agents can be listed talc, magnesium carbonate, calcium carbonate, calcium phosphate, titanium and aluminum oxides, silicates, clays, such as bentonite, and others. The diameter of the resultant beads varies from 0.1 to 5 mm and often depends upon the rate of agitation. It is usually inversely proportional to the particle size. Suspension polymerization is used in many commercial preparations of polymers. Zhang, Fu, and Jiang, reported a study of factors influencing the size of polystyrene microspheres in dispersion polymerization [300]. The found that that the size of polystyrene microspheres decreased with an increasing amount of stabilizer and also increased with increasing the amount of monomer and initiator. The amount of stabilizer and monomer concentration were the major factors influencing the size distribution of polystyrene microspheres. The size of the microspheres decreased with an increase of the solvency of reaction media. The size distribution, however, hardly changed. The size of polystyrene microspheres increased with an increase in the reaction temperature. but the size distribution hardly changed.

Emulsion polymerization is used widely in commercial processes [300, 301]. The success of this technique is due in part to the fact that this method yields high-molecular-weight polymers. In addition, the polymerization rates are usually high. Water is the continuous phase and it allows efficient removal of the heat of polymerization. Also, the product from the reaction, the latex, is relatively low in viscosity, in spite of the high molecular weight of the polymer. A disadvantage of the process is that water-soluble emulsifiers are used. These are hard to remove completely from the polymers and may leave some degree of water sensitivity. The reaction is commonly carried out in water containing the monomer, an emulsifier or a surface active agent, and a water-soluble initiator. Initiation may be accomplished through thermal decom position of the initiator or through a redox reaction. The polymer forms as a colloidal dispersion of f ine particles and polymer recovery requires breaking up the emulsion. The full mechanism of emulsion polymerization is still not completely worked out. It is still not clear why a simultaneous increase in the polymerization rate and in the molecular weight of the product is often observed. Also, in emulsion polymerization, at the outset of the reaction the monomer is in a form of finely dispersed droplets. These droplets are about 1 m in diameter. Yet, during the process of a typical polymerization, they are converted into polymer particles that are submicro scopic, e.g., 1,000 A ˚ in diameter. At the start of the reaction the emulsifier exists simultaneously in three loci: (a) as a solute in water; (b) as micelles; (c) and as a stabilizing emulsifier at the interface between the monomer droplets and the water. The bulk of the emulsifier, however, is in the micelles. The monomer is also present in three loci: (a) in the monomer droplets that are emulsified and perhaps 1–10 m in diameter; (b) it is solubilized in the micelles, perhaps 50–100 A ˚ in diameter; (c) and it is present as individual molecules dissolved in the water. The bulk of the monomer is in the droplets. There are on the average 1018/mL of monomer-swollen micelles in the reaction mixture at the outset of the reaction [302]. At the start of the reaction there are also on the average 1012/mL monomer droplets that act as reservoirs. The monomeris supplied from the droplets to radical-containing micelles when the reaction progresses by a process of diffusion through the aqueous phase (Fig. 3.3). The first hypothesis of the mechanism of emulsion polymerization was formulated by Harkins [305]. According to this hypothesis, the water-soluble initiator decomposes in the aqueous phase. This results in formation of primary radicals. The primary radicals in turn react with the monomer molecules dissolved in the water (though their number may be quite small). Additional monomer molecules may add to the growing radicals in the water until the growing and propagating chains of free radicals acquire surface-active properties. At that stage, the growing radicals consist of inorganic and organic portions: These growing radical-ions tend to diffuse into the monomer-water interfaces. The probability that the diffusion takes place into monomer-swollen micelles rather than into monomer droplets is backed by the considerations of the relative surface areas of the two. There are on the average 1018 micelles in each milliliter of water. These are approximately 75 A ¯ in diameter and each swollen micelle contains on the average 30 molecules of the monomer. At the same time, the diameters of the monomer droplets are approximately 1 m. and it is estimated that there are only approximately 1012 such droplets per milliliter of water. Thus, the micelles offer 60 times more surfaces for penetration than do the droplets. The initiating radicals are almost always generated in the water phase. After formation in the water phase, a number of free radicals may be lost due to recombination. Termination is also possible after reaction of free radicals with some of the monomers dissolved in the water. Several theories tried to explain the entry process. Thus, a “diffusion control” model [307, 308] supposes that diffusion of aqueous-phase radicals into the particle surface is the rate-controlling step for entry. Another theory suggests that displacement of surfactant from the particle surface is the rate determining step [309]. A third one assumes that the entry can be thought of as a colloidal interaction between a latex particle and primary phase oligomeric aqueous-phase radical. These are the radicals formed through reactions of initiating radicals and monomer molecules dissolved in water [310]. The most accepted entry model appears to be the “control by aqueous-phase growth” model of Maxwells et al. [311]. This theory postulates that free radicals generated in the aqueous phase propagate until they reach a critical degree of polymerization (let us call it z), at which point they become surface active and their only fate is irreversible entry into a latex particle; the rate of entry of z-mers into a particle is assumed to be so fast as not to be rate-determining. An efficiency of less than 100% arises if there is significant aqueous-phase termination of the propagating radicals. The entry model of Maxwells et al. was derived from and/or supported by data on the influence of particle surface characteristics (charge, size) on the entry rate coefficient [312]. It was assumed that the aqueous radicals became surface active when the degree of polymerization reached 2–3. This was based on thermodynamic considerations of the entering species. Further data on the Maxwell et al. entry model was obtained by Gilbert and coworkers [313] who studied the effects of initiator and particle surface charges. They obtained kinetic data for radical entry in the emulsion polymerization of styrene and concluded that their data further supports the Maxwell et al. entry model and refutes the alternative models mentioned above. Once the radicals penetrate the micelles, polymerization continues by adding monomers that are inside. The equilibrium is disturbed and the propagation process proceeds at a high rate due to the concentration and crowding of the stabilized monomers. This rapidly transforms the monomer swollen micelles into polymer particles. The changes result in disruptions of the micelles by growths from within. The amount of emulsifier present in such changing micelles is insufficient to stabilize the polymer particles. In trying to restore the equilibrium, some of the micelles, where there is no polymer growth, disintegrate and supply the growing polymer particles with emulsifier. In the process many micelles disappear per each polymer particle that forms. The final latex usually ends up containing about 1015 polymer particles per milliliter of water. By the time conversions reach 10–20% there are no more micelles present in the reaction mixtures. All the emulsifier is now adsorbed on the surface of the polymer particles. This means that no new polymer particles are formed. All further reactions are sustained by diffusion of monomer molecules from the monomer droplets into the growing polymer particles. The amount of monomer diffusing into the particles is always in excess of the amount that is consumed by the polymerization reaction due to osmotic forces [297].

This extra monomer suppliedis sufficient for equilibrium swelling of the particles [298]. As a result, the rate of polymerization becomes zero order with respect to time. When conversion reaches about 70%, all the remaining monomer is absorbed in the polymer particles and there are no more monomer droplets left. At this point the reaction rate becomes first order with respect to time. The qualitative approach of Harkins was put on a quantitative basis by Smith and Ewart [314–316]. Because 1013 radicals are produced per second and can enter between 1014 and 1015 particles, Smith felt that a free radical can enter a particle once every 10–100 s. It can cause the polymerization to occur for 10–100 s before another free radical would enter and terminate chain growth [317]. A period of inactivity would follow that would last 10–100 s and then the process would repeat itself. Such a “stop and go” mechanism implies that a particle contains a free radical approximately half of the time. It can also be said that the average number of radicals per particle is 0.5. This is predicted on conditions that (a) the rate of chain transfer out of the particle is negligible and (b) the rate of termination is very rapid compared with the rate of radical entry into the particle. The kinetic relationships derived by Smith and Ewart for the system are as follows: The rate of primary radical entering a particle = r_i = Ri/N

Rate of polymerization = RP = kP(M)N/2 Average degree of polymerization = DP = Nk_P (M)Ri where, kP is the constant for propagation, [M] is the concentration of monomer, N is the number of particles containing n radicals (~0.5) and the expression for the number of particles formed:

where, m is the volume increase of the particles, AS is the area occupied by one emulsifier molecule. S is the amount of emulsifier present. K is a constant ¼ 0.37 (based on the assumption that the micelles and polymer particles compete for free radicals in proportion to their respective total surface areas). K can also be equal to 0.53 (based on the assumption that the primary radicals enter only micelles, as long as there remain micelles in the reaction mixture). r is the rate of entry into the particles. The kinetic chain length can be written as:

The Smith-Ewart mechanism does not take into account any polymerization in the aqueous phase. This may be true for monomers that are quite insoluble in water, such as styrene, but appears unlikely for more hydrophilic ones such as methyl methacrylate or vinyl acetate. In addition, it was calculated by Flory that there is insufficient time for a typical cation-radical (such as a sulfate ion radical) to add to a dissolved molecule of monomer such as styrene before it becomes captured by a micelle [317]. This was argued against, however, on the ground that Flory’s calculations fail to consider the potential energy barrier at the micelle surfaces from the electrical double layer. This barrier would reduce the rate of diffusion of the radical-ions into the micelles [316]. Considerably different mechanisms were proposed by several groups [317, 318]. They are based on a concept that most polymerizations must take place at the surface of the particles or in their outer “shell” and not within the particles. It is claimed that the interiors of the particles are too viscous for free radicals to diffuse inside at a sufficiently fast rate. Two different mechanisms were proposed to explain why polymerization takes place preferentially in the shell layer. One of them suggests that the monomer is distributed nonuniformly in the polymer particles. The outer shell is rich in monomer molecules, while the inside is rich in polymer molecules [319]. The other explanation is that the radical ions that form from the water-soluble initiator are too hydrophilic to be able to penetrate the polymer particles [320]. Surfactant-free emulsion polymerization are carried out in the absence of a surfactant [321]. The technique requires the use of initiators that yields initiating species with surface-active properties and impartsthemtothepolymerparticles.Examplesofsuchinitiatorsarepersulfates.Thelatticesthatform are stabilized by chemically bound sulfate groups that are derived from persulfate ions. Because the surface-active groups are chemically bound, the lattices are easier to purify and free the product from unreacted monomer and initiator. Generally, the particle number per milliliter from a surfactant-free emulsion polymerization is smaller than the particle number from typical emulsion polymerization. In an inverse emulsion polymerization an aqueous solution of a hydrophilic monomer is emulsified in an organic solvent and the polymerization is initiated with a solvent soluble initiator. This type of emulsion polymerizations is referred to as water in oil polymerization. Inverse emulsion polymerization is used in various commercial polymerizations and copolymerization of water-soluble monomers. Often nonionic emulsifiers are utilized. The product emulsions are often less stable than the oil in water emulsions. A special approach to emulsion polymerization is called miniemulsion polymerization [322]. These reactions contain both micelles and monomer droplets, but the monomer droplets are smaller than in macrosystems. Usually, a water-soluble surfactant is used for emulsification. An example of such a surfactant can be sodium dodecyl sulfate. In addition, a highly water-insoluble costabilizer is added, such as hexadecanol. Thus, miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water, a surfactant and a hydrophobic material. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets. An appropriate formulation of a miniemulsion suppresses coalescence of droplets. The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials or with respect to the stability of the formed latexes. Although labeled “emulsion,” it appears that some may involve a combination of emulsion and suspension polymerizations. It was reported [323] that by using a difunctional alkoxyamine as an initiator for the homopolymerization of butyl acrylate in miniemulsion, to increase the achievable molar mass and to use the polymer as a difunctional macroinitiator for the synthesis of triblock copolymers in aqueous dispersed systems. Well-defined polymers with one alkoxyamine functionality at each end were obtained, providing that monomer conversion was kept below 70%. Beyond this conversion, extensive broadening of the molar mass distribution was evidenced, as the consequence of termination and transfer to polymer. Tsavalas et al. [324] reported that a phenomenon seemingly unique to hybrid miniemulsion polymerization was observed by them, where monomer conversion would either plateau at a limiting value or quickly switch to a dramatically lesser rate. They attributed this phenomenon to a combina tion of three factors. The first one is the degree to which the monomer and resinous component are compatible. The second is the resultant particle morphology after approximately 80% monomer conversion, which roughly corresponds to the portion of reaction where this morphology is established. The third factor is the degree of interaction between the growing polymer and the resin (a grafting reaction). Of these three, the first two factors were found by them to be much more significant in contributing to the limiting conversion. RAFT emulsion polymerization is a new development that has attracted considerable attention. It be carried out in a regular emulsion polymerization [325] and in a reverse emulsion polymerization [326]. Also, recently, several reports in the literature have described miniemulsion RAFT polymerizations. In some instances, use is made of water-soluble RAFT agents to control polymer molecular weight [327]. Also, Hawkett and coworkers reported using surface active RAFT agents to emulsify the dispersed phase, stabilize the particles and also control the molecular weight. This yielded polymer latexes that were free from surfactant and costablilizer [328]. One of these special RAFT agents was illustrated as follows:

The other two RAFT agents used by them had similar structures. A surface active iniferter was also reported by Choe and coworkers [330, 331]:

This RAFT agent allowed polymerization of methyl methacrylate initiated by ultraviolet light irradiation in the absence of added surfactant or initiator. Rieger and coworkers [332] reported a surfactant free RAFT emulsion polymerization of butyl acrylate and styrene using poly(N,N-dimethylacrylamide) trithiocarbonate macromolecular transfer agent. They observed that the polymerizations were fast and controlled with molar masses that matched well the theoretical values and low polydispersity indexes. Monomer conversions close to 100% were reached and the polymerizations behaved as controlled systems, even at 40% solids contents. The products were poly(N,N-dimethyl acrylamide)-b-poly(n-butyl acrylate) and poly(N,N dimethylacrylamide)-b-polystyrene amphiphilic diblock copolymers formed in situ.

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بذكرى شهادته.. جموع الزائرين تشيع النعش الرمزي للإمام الكاظم (عليه السلام) عند مرقده الطاهر في بغداد

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الآخبار الصحية

الخبز الأسمر أم الأبيض.. أيهما أفضل لضبط سكر الدم؟

عقاقير التخسيس و"الانتحار".. الحكومة الأميركية تحسم الجدل

الهرم الغذائي الجديد.. لماذا يعد "انقلابا" في خارطة الطعام؟

لحماية ظهرك وقلبك في آن واحد.. ما وضعية النوم الصحيحة؟

المزيد

الآخبار التكنلوجية

كيف يعزز الضوء الطبيعي تفكيرك وانتباهك خلال اليوم

سابقة.. طاقم من محطة الفضاء الدولية يعود إلى الأرض مبكرا

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ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

المزيد

مواضيع ذات صلة

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Lewis Acids in Cationic Initiations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Special Types of Controlled/Living Polymerizations

Combinations of Click Chemistry and ATP as Well as ATP and RAFT Polymerizations

Reversible Addition-Fragmentation Chain Transfer Polymerization

Nitroxide-Mediated Radical Polymerizations

Atom Transfer Radical Polymerizations

Controlled/“Living” Free-Radical Polymerization