2. Acrylamide Polymerization
Poly(acrylamide) is made by the free-radical polymerization of acrylamide, which is derived from acrylonitrile by either catalytic hydrolysis or bioconversion. AAm can be polymerized in solution, bulk, inverse emulsion, suspension or as precipitation polymerization.
Acrylamide (2-propenamide, C3H5ON) readily undergoes free-radical polymerization to high molecular weight poly(acrylamide). Free-radical initiation can be accomplished using organic peroxides, azo compounds, inorganic peroxides including persulfates, redox pairs, photoinduction, radiation-induction, electroinitiation, or ultrasonication.
Several reasons account for the ultrahigh molecular weights achievable. First, preparations of polyacrylamides are usually conducted in water, and the chain transfer constant to monomer and polymer appears to be zero in water. Second, the value of (kp/kt )1/2, about 4.2, is unusually high and is independent of the pH of the media.
The rate of polymerization is proportional to the 1.2–1.5 power of the monomer concentration and to the square root of the initiator concentration. All this results in a high rate of propagation. Chain termination is primarily by disproportionation.
The large amount of heat (82.8 kJ/mol) that evolves during polymerization can result in a rapid temperature rise. One way in which this exotherm problem has been addressed in commercial high-solids and high-molecular-weight processes has been through the use of an adiabatic gel process in which the initiation temperature is 0◦C. In another approach, controllable-rate redox polymerization of aqueous acrylamide-in-oil emulsions can be carried out at moderate temperatures of 40–60◦C in order to accommodate the exotherm and to achieve very high molecular weights. At 70–100◦C, a persulfate initiator can give a grafted or branched polymer. Additives greatly affect the rate and the kinetics of polymerization. These additives include metal ions, surfactants, chelating agents, and organic solvents. The high chain-transfer constant of compounds such as 2-propanol, bisulfite ion, or persulfate ion to active polymer has been reported. Chain-transfer agents have been used purposely to control molecular weight, minimize insoluble polymer, and control cross-linking and the degree of branching in commercial preparations. There are numerous laboratory methods to prepare polyacrylamides. However, there are only a few viable commercial processes used to manufacture materials that meet the necessary performance standards. There are many requirements for commercial materials: very low to very high molecular weights, low insolubles content, low residual monomer content, fast dissolution rate, ease of handling, minimal dusting (for dry solids), product uniformity, long-term storage stability (to ensure performance consistency), high solids (to reduce shipping costs), and consistent performance characteristics. Several common commercial processes are summarized below.
2.1 Solution polymerization
The solution polymerization is the oldest and most common method for production of high molecular weight polyacrylamide and takes place as batch and continuous process. A 10 to 70% solution of deoxygenated monomer in water polymerizes rapidly at low temperatures with all common radical initiators. The polymerization is started by increasing the temperature to 40–80 C, depending on the initiating system. The monomer concentration is limited by the polymerization enthalpy, the rapid kinetic and the molecular mass of the desired polymer. Therefore transfer agents like isopropanol are often used to reduce molecular weight. Many authors have shown that polymerization of polyacrylamide is strongly influenced by temperature, solvent, concentration of monomer and initiator, additives (inorganic salts, Lewis acids) and pH value. It could be shown that propagation rate increases with rising temperature. A maximum velocity of polymerization is reached at 50 to 60 C. At higher temperatures the propagation rate decreases because of side reactions (intermolecular imidization) and higher rates of termination.
Commercial production of polyacrylamides by solution polymerization is conducted in aqueous solution, either adiabatically or isothermally. Process development is directed at molecular weight control, exotherm control, producing low levels of residual monomer, and control of the polymer solids to ensure that the final product is fluid and pumpable. A generic example of a solution polymerization follows. An acrylamide monomer solution (2–30 wt% in water) is typically prepared, and deaerated by sparging it with an inert gas (eg, nitrogen) to reduce the oxygen content in solution. Stainless steel batch reactors or glass continuous stirred tank reactors are often used for solution polymerizations. A chelating agent is added to complex autopolymerization inhibitors such as copper or other metals, if they are present. The polymerization is then initiated using one of several free-radical initiator systems (azo, peroxy, persulfate, redox, or combinations) at concentrations ranging from 0.001 to 10 wt% on monomer. The rate of polymerization depends on reaction conditions, but it typically depends on the 1.2–1.6 order of monomer concentration and 0.5 order of initiator concentration. The heat evolved during polymerization (82.8 kJ/mol) can be removed by an external cooling system. For adiabatic processes, the temperature rise needs to be estimated and great care needs to be exercised to avoid exceeding the reflux temperature. Chain-transfer agents and inorganic salts can be added to improve processing and to reduce insolubles.
Monomer to polymer conversions of 99.5% are achievable in 2–6 h of polymerization time. The products can have a molecular weight ranging from one thousand to four million. Polymer solids can be 2–30%. The process can be used in conjunction with thermal drying or precipitation methods in order to obtain products in either powder or granular form. Short residence times in drum drying have been used to avoid chain degradation and formation of insolubles. Precipitation in C1- C4 alcohols can be done to obtain nonsticky rubbery polymer gel that can be further extruded and then dried with hot air. The resulting granules can be milled and sieved to produce a uniform product. Care is taken to avoid very finely divided material that can cause dusting problems.
Some commercial low molecular weight polyacrylamides (LMPAM) are manufactured in solution and sold at 10–50% solids. For example, LMPAM containing DADMAC comonomer is made at 40% solids and can be reacted with glyoxal to produce a strengthening resin for paper. Furthermore, LMPAM hydrolyzed with sodium hydroxide to polyacrylate is manufactured at 30% solids and is used as an antiscalant. High molecular weight polyacrylamide is also prepared in solution at 2–6 wt% solids and is often further modified using, for example, the Mannich reaction.
2.2 Bulk polymerization
Bulk polymerization can be divided into two types: polymerization in the solid phase and in the molten phase. Bulk polymerization is interesting for the following reasons: (1) polymerization of crystalline monomer may lead to crystalline and stereoregular polymers, and (2) impurities, such as solvent, catalyst, and initiator, may be avoided. However, only the second reason is realistic since polymer obtained by solid-state polymerization is amorphous and shows no tendency to crystallize. The crystalline matrix is unable to exert any appreciable steric control. Further investigations have shown that propagation takes place at the polymer–monomer interface, controlled by local strains and defects in the crystal. Polymerization in the molten monomer soon becomes heterogeneous because of insolubility of polymer in its own monomer. AAm can be polymerized by ionizing radiation. Crystals are irradiated continuously during polymerization at temperatures between 0 and 60 C.
Monomer can also be exposed to g-rays at about 80 C, then removed from the radiation source and allowed to polymerize at higher temperature with a lower propagation rate. If a limiting conversion is reached at one temperature, chain ends are still reactive.
Polymerization can be continued by warming up to higher temperatures. Molecular weight increases with time, a transfer reaction to monomer occurs only to a very limited extend, and reaction with oxygen can be neglected.
2.3 Dispersion Polymerization
Water-in-oil emulsions contain at least 30 wt% of a petroleum-based hydrocarbon that is a valuable natural resource. By using such formulations, oils are consumed unnecessarily and can enter the world’s waterways as a source of secondary pollution. An aqueous polymer dispersion is one environmentally responsible formulation that contains no oil or surfactant, and near-zero amounts of volatile organic compounds. Dispersion polymerization can be used to prepare cationic, anionic, and nonionic polyacrylamides.
A method of avoiding the high solution viscosities of high molecular weight water-soluble polymers comprises emulsifying the aqueous monomer solution in an oil containing surfactants, homogenizing the mixture to form a water-in-oil (inverse) emulsion, and then polymerizing the monomers in the emulsion. The resulting polymer latex can be inverted in water, releasing the polymer for use.
Inverse emulsion polymerization is used for the preparation of polymers with ultrahigh molecular masses. For this type of polymerization, the expression ‘dispersion polymerization’ is often used in the literature. A concentrated monomer solution (about 40% monomer in water) is dispersed under intensive stirring in aliphatic or aromatic hydrocarbons in the presence of additives (emulsifiers, protective colloids).
Polymerization can be initiated by either water-soluble or oil-soluble initiators. The advantage of this process is based on the constant viscosity of the reaction mixture, as the increase of viscosity takes place only in the dispersed phase. By the use of additives (tensides), the dispersion inverts when the emulsion is stirred into water. Precipitation from the aqueous solution yields a polymer with ultrahigh molar mass. The quality of polymer made by inverse emulsion polymerization is influenced by the following factors:
(1) species and concentration of initiator,
(2) species and concentration of additives (emulsifiers, protective colloids),
(3) type of oil phase, and
4) particle size of the dispersed water phase.
Because of the easy modification of all these parameters, much attention has been given in recent years to water-in-oil emulsion polymerization of acrylamide. For suspension polymerization the initial system is obtained by dispersion of an aqueous monomer solution in an organic liquid by mechanical stirring in the presence of stabilizers. The dispersion medium may be represented by aromatic and aliphatic saturated hydrocarbons. The polymerization is initiated by water-soluble initiators, UV or g-radiation. The process occurs in droplets of an aqueous monomer solution (diameters 0.1–5.0 mm) that act as microreactors.
b. Inverse Emulsions with Biodegradable Oils
Some examples of inverse emulsion polymerization processes employing biodegradable oils include materials with aqueous phase monomer mixtures, such as AMD and AETAC or AMD and MAETAC, dispersed in a biodegradable oil, such as bis- (2-ethylhexyl)adipate (187), containing a polymeric emulsifier that is a copolymer of dimethylaminoethylmethacrylate and mixtures of methacrylates. A buffering acid, such as a dicarboxylic acid, is used to stabilize cationic copolymers. Aliphatic dialkylethers are also used as biodegradable oils, in conjunction with SMO as an emulsifier, to produce high-molecular-weight cationic copolymers.
c. Inverse Emulsion Polymerization Acrylamide in Near-Critical and Supercritical Fluid Conditions
Supercritical fluids exhibit both liquid-like properties (eg, solubilizing power), and gas-like properties. Aqueous AMD has been dispersed and even microemulsified in near-supercritical ethane–propane mixtures using nonionic surfactants such as ethoxylated alcohols. Emulsion polymerization of AMD was then conducted at 60◦C for 5 h and 379 bar, at the near-supercritical condition of certain ethane–propane mixtures. 2,2_-Azobis(isobutyronitrile) (AIBN) was used as the initiator.
The resulting polyacrylamide had a low molecular weight in the range of (2.7–5.8) × 105 Da. The ethane and propane can be easily recovered and recycled in a production plant. Emulsion polymerization of AMD was also conducted at 60 C for 1 h and 345 bar in near-supercritical CO2. AIBN was the initiator. An amide end-capped hexafluoropropylene oxide oligomer that has high solubility in the near-supercritical CO2 was found to stabilize the dispersed particles (190–192). Only a few classes of polymers have good solubility in near-supercritical CO2. The advantages of using carbon dioxide include very low viscosities during polymerization and ease of recovery.
d. Microemulsion Polymerization
One inherent problem with water-in oil emulsions of acrylamide-based polymers is the potential formation of unstable lattices both during production and in finished products. The coagulum that can form in the reactor can result in a time-consuming cleanout. Technology has continuously improved reactor configuration, types of agitation, proper cooling and a proper balance of aqueous, oil, and emulsifier ratios.
Microemulsion polymerization can provide improvements to address these problems. Monomer microemulsions are thermodynamically stable systems comprising two liquids, insoluble in each other, and a surfactant. They form spontaneously without homogenization. The resulting polymer microlattices are typically nonsettling, transparent, and about 100 nm in diameter. These systems can have high emulsifier levels: more than 8 wt%, which is about 4–5 times more than emulsifier levels in conventional inverse emulsions. Consequently, the cost of producing microemulsions becomes less attractive. However, further refinements to technology lead to the development of cost-effective microemulsified Mannich acrylamide polymers. This technology was used to develop functionalized polyacrylamides. In one case, a polyacrylamide microlatex reacted with formaldehyde and dimethylamine (Mannich reaction), and then quaternized with methyl chloride to yield a very highly charged cationic carbamoyl polymer. These commercial products are widely used in many applications for solid–liquid separation. These products have been improved by treating them with buffer acid, a formaldehyde scavenger, and heat to produce a high performance cationic polymer.
Precipitation polymerization takes place in organic solvents or in aqueous organic mixtures, that serve as solvents for the monomer but as precipitates for the polymer. During the process the precipitation of the polymer takes places and polymerization proceeds under heterogeneous conditions. The advantage of precipitation polymerization is that the medium never gets viscous and the polymer is easy to isolate and dry.