Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.
Free radical polymerization is a key synthesis route for obtaining a wide variety of different polymers and material composites. The relatively non-specific nature of free radical chemical interactions makes this one of the most versatile forms of polymerization available and allows facile reactions of polymeric free radical chain ends and other chemicals or substrates. In 2001, 40 billion of the 110 billion pounds of polymers produced in the United States were produced by free radical polymerization.
Initiation is the first step of the polymerization process. During initiation, an active center is created from which a polymer chain is generated. Not all monomers are susceptible to all types of initiators. Radical initiation works best on the carbon-carbon double bond of vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones. Initiation has two steps. In the first step, one or two radicals are created from the initiating molecules. In the second step, radicals are transferred from the initiator molecules to the monomer units present. Several choices are available for these initiators.
Types of initiation and the initiators
- Thermal decomposition: The initiator is heated until a bond is homolytically cleaved, producing two radicals (Figure 1). This method is used most often with organic peroxides or azo compounds.
- Photolysis: Radiation cleaves a bond homolytically, producing two radicals (Figure 2). This method is used most often with metal iodides, metal alkyls, and azo compounds. An acceptable photoinitiator system should fulfill the following requirements:
- High absorptivity in the 300-400 nm range.
- Efficient generation of radicals capable of attacking the olefinic double bond of vinyl monomers.
- Adequate solubility in the binder system (prepolymer + monomer).
- Should not impart yellowing or unpleasant odors to the cured material.
- The photoinitiator and any byproducts resulting from its use should be non-toxic.
- Redox reactions: Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron (Figure 3). Other reductants such as Cr2+, V2+, Ti3+, Co2+, and Cu+ can be employed in place of ferrous ion in many instances.
- Persulfates: The dissociation of a persulfate in the aqueous phase (Figure 4). This method is useful in emulsion polymerizations in which the radical diffuses into a hydrophobic monomer-containing droplet.
- Ionizing radiation: α-, β-, γ-, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical (Figure 5).
- Electrochemical: Electrolysis of a solution containing both monomer and electrolyte. A monomer molecule will receive an electron at the cathode to become a radical anion, and a monomer molecule will give up an electron at the anode to form a radical cation (Figure 6). The radical ions then initiate free radical (and/or ionic) polymerization. This type of initiation of especially useful for coating metal surfaces with polymer films.
- Plasma: A gaseous monomer is placed in an electric discharge at low pressures under conditions where a plasma (ionized gaseous molecules) is created. In some cases, the system is heated and/or placed in a radiofrequency field to assist in creating the plasma.
- Sonication: High-intensity ultrasound at frequencies beyond the range of human hearing (16 kHz) can be applied to a monomer. Initiation results from the effects of cavitation (the formation and collapse of cavities in the liquid). The collapse of the cavities generates very high local temperatures and pressures. This results in the formation of excited electronic states which in turn lead to bond breakage and radical formation.
- Ternary Initiators: A ternary initiator is the combination of several types of initiators into one initiating system. The types of initiators are chosen based on the properties they are known to induce in the polymers they produce. For example, poly(methyl methacrylate) has been synthesized by the ternary system benzoyl peroxide-3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole-di-η5-indenylzicronium dichloride (Figure 7).metallocene, an initiator, and a heteroaromatic diketo carboxylic acid. Metallocenes in combination with initiators accelerate polymerization of poly(methyl methacrylate) and produce a polymer with a narrower molecular weight distribution. The example shown here consists of indenylzirconium (a metallocene) and benzoyl peroxide (an initiator). Also, initiating systems containing heteroaromatic diketo carboxylic acids, such as 3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole in this example, are known to catalyze the decomposition of benzoyl peroxide. Initiating systems with this particular heteroaromatic diket carboxylic acid are also known to have effects on the microstructure of the polymer. The combination of all of these components—a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid—yields a ternary initiating system that was shown to accelerate the polymerization and produce polymers with enhanced heat resistance and regular microstructure.
Due to side reactions and inefficient synthesis of the radical species, chain initiation is not 100%. The efficiency factor, f, is used to describe the effective radical concentration. The maximum value of f is 1.0, but values typically range from 0.3-0.8. The following is a list of reactions that decrease the efficiency of the initiator.
- Primary recombination: Two radicals re-combine before initiating a chain (Figure 8). This occurs within the solvent cage, meaning that no solvent has yet come between the new radicals.
- Other recombination pathways: Two radical initiators re-combine before initiating a chain but not in the solvent cage (Figure 9).
- Side reactions: One radical is produced instead of the three radicals that could be produced (Figure 10).
During polymerization, a polymer spends most of its time in increasing its chain length, or propagating. After the radical initiator is formed, it attacks a monomer (Figure 11). In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule into another radical. This begins the polymer chain. Figure 12 shows how the orbitals of an ethylene monomer interact with a radical initiator.
Once a chain has been initiated, the chain propagates (Figure 13) until there is no more monomer (living polymerization) or until termination occurs. There may be anywhere from a few to thousands of propagation steps depending on several factors such as radical and chain reactivity, the solvent, and temperature. The mechanism of chain propagation is as follows:
Chain termination will occur unless the reaction is completely free of contaminants. In this case, the polymerization is considered to be a living polymerization because propagation can continue if more monomer is added to the reaction. Living polymerizations are most common in ionic polymerization, however, due to the high reactivity of radicals. Termination can occur by several different mechanisms. If longer chains are desired, the initiator concentration should be kept low; otherwise, many shorter chains will result.
- Combination of two active chain ends: one or both of the following processes may occur.
- Combination: two chain ends simply couple together to form one long chain (Figure 14). One can determine if this mode of termination is occurring by monitoring the molecular weight of the propagating species: combination will result in doubling of molecular weight. Also, combination will result in a polymer that is C2 symmetric about the point of the combination.
- Radical disproportionation: a hydrogen atom from one chain end is abstracted to another, producing a polymer with a terminal unsaturated group and a polymer with a terminal saturated group (Figure 15).
- Combination of an active chain end with an initiator radical (Figure 16).
- Interaction with impurities or inhibitors. Oxygen is the common inhibitor. The growing chain will react with molecular oxygen, producing an oxygen radical, which is much less reactive (Figure 17). This significantly slows down the rate of propagation. Nitrobenzene, butylated hydroxyl toluene, and diphenyl picryl hydrazyl (DPPH, Figure 18) are a few other inhibitors. The latter is an especially effective inhibitor because of the resonance stabilization of the radical.
Contrary to the other modes of termination, chain transfer results in the destruction of only one radical, but also the creation of another radical. Often, however, this newly created radical is not capable of further propagation. Similar to disproportionation, all chain transfer mechanisms also involve the abstraction of a hydrogen atom. There are several types of chain transfer mechanisms.
- To solvent: a hydrogen atom is abstracted from a solvent molecule, resulting in the formation of radical on the solvent molecules, which will not propagate further (Figure 19). Halogens, except fluorine, are easily transferred.
- To monomer: a hydrogen atom is abstracted from a monomer. While this does create a radical on the affected monomer, resonance stabilization of this radical discourages further propagation (Figure 20).
- To initiator: a polymer chain reacts with an initiator, which terminates that polymer chain, but creates a new radical initiator (Figure 21). This initiator can then begin new polymer chains. Therefore, contrary to the other forms of chain transfer, chain transfer to the initiator does allow for further propagation. Peroxide initiators are especially sensitive to chain transfer.
- To polymer: the radical of a polymer chain abstracts a hydrogen atom from somewhere on another polymer chain (Figure 22). This terminates one of the polymer chains, but allows the other to branch. When this occurs, the average molar mass remains relatively unaffected.
Effects of chain transfer: The most obvious effect of chain transfer is a decrease in the polymer chain length. If the rate of termination is much larger than the rate of propagation, then very small polymers are formed with chain lengths of 2-5 repeating units (telomerization). The Mayo–Lewis equation estimates the influence of chain transfer on chain length (xn): . Where ktr is the rate constant for chain transfer and kp is the rate constant for propagation. The Mayo-Lewis equation assumes that transfer to solvent is the major termination pathway.
Methods of radical polymerization
There are four industrial methods of radical polymerization:
- Bulk polymerization: reaction mixture contains only initiator and monomer, no solvent.
- Solution polymerization: reaction mixture contains solvent, initiator, and monomer.
- Suspension polymerization: reaction mixture contains an aqueous phase, water-insoluble monomer, and initiator soluble in the monomer droplets (both the monomer and the initiator are hydrophobic).
- Emulsion polymerization: similar to suspension polymerization except that the initiator is soluble in the aqueous phase rather than in the monomer droplets (the monomer is hydrophobic, and the initiator is hydrophilic). An emulsifying agent is also needed.
Other methods of radical polymerization include the following:
- Template polymerization: In this process, polymer chains are allowed to grow along template macromolecules for the greater part of their lifetime. A well-chosen template can affect the rate of polymerization as well as the molar mass and microstructure of the daughter polymer. The molar mass of a daughter polymer can be up to 70 times greater than those of polymers produced in the absence of the template and can be higher in molar mass than the templates themselves. This is because of retardation of the termination for template-associated radicals and by hopping of a radical to the neighboring template after reaching the end of a template polymer.
- Plasma polymerization: The polymerization is initiated with plasma. A variety of organic molecules including alkenes, alkynes, and alkanes undergo polymerization to high molecular weight products under these conditions. The propagation mechanisms appear to involve both ionic and radical species. Plasma polymerization offers a potentially unique method of forming thin polymer films for uses such as thin-film capacitors, antireflection coatings, and various types of thin membranes.
- Sonication: The polymerization is initiated by high-intensity ultrasound. Polymerization to high molecular weight polymer is observed but the conversions are low (<15%). The polymerization is self-limiting because of the high viscosity produced even at low conversion. High viscosity hinders cavitation and radical production.
Controlled Radical Polymerization (CRP)
Also known as living radical polymerization, this method relies on completely pure reactions so that no termination caused by impurities occurs. Because these polymerizations stop only when there is no more monomer and not when termination occurs, the polymerization can continue upon the addition of more monomer. Block copolymers can be made this way. Complete living radical polymerization allows for control of molecular weight and dispersity; however, this is very difficult to achieve and instead a pseudo-living polymerization occurs in which there is only partial control of molecular weight and dispersity. There are several types of CRPs. ATRP and RAFT are the main types.
- Atom Transfer Radical Polymerization (ATRP): based on the formation of a carbon-carbon bond by atom transfer radical addition. This method, developed by Polish chemist Krzysztof Matyjaszewski, requires reversible activation of a dormant species (such as an alkyl halide) and a transition metal halide catalyst (to activate dormant species).
- Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT): requires a compound that can act as a reversible chain transfer agent, such as dithio compounds.
- Stable Free Radical Polymerization (SFRP): used to synthesize linear or branched polymers with narrow molecular weight distributions and reactive end groups on each polymer chain. The process has also been used to create block co-polymers with unique properties. Conversion rates are about 100% using this process but require temperatures of about 135 °C. This process is most commonly used with acrylates, styrenes, and dienes. The reaction scheme in Figure 23 illustrates the SFRP process.TEMPO molecule (Figure 24), premature termination by coupling is reduced. As with all living polymerizations, the polymer chain grows until all of the monomer is consumed.
In typical chain growth polymerization, the reaction rate for initiation, propagation and termination can be described as follows.
where f is the efficiency of the initiator and kd, kp, and kt are the constants for initiator dissociation, chain propagation and termination, respectively. [I], [M] and [M\cdot] is the concentration of the initiator, monomer and the active growing chain.
Under the steady state approximation, the concentration of the active growing chains remains constant, i.e. the rate of initiation and termination is the same. The concentration of active chain can be derived and expressed in terms of the other known species in the system.
In this case, the rate of chain propagation can be further described using a function of the initiator and monomer concentration
The kinetic chain length v is a measure of the average number of monomer units reacting with an active center during its lifetime and is related to the molecular weight through the mechanism of the termination. Without chain transfer, the dynamic chain length is only the function of propagation rate and initiation rate.
Assuming no chain transfer effect occurs in the reaction, the number average degree of polymerization Pn can be correlated with the kinetic chain length. In the case of termination by disproportionation, one polymer molecule is produced per every kinetic chain:
Termination by combination leads to one polymer molecule per two kinetic chains:
Any mixture of these both mechanisms can be described by using the value δ, and the contribution of disproportionation to the overall termination process:
If chain transfer is considered, then there are other pathways to terminate the growing chain. The equation for dynamic chain length will be modified as the following.
If chain transfer is considered, the kinetic chain length is not affected by the transfer process because the growing free-radical center generated by the initiation step stays alive after any chain transfer event, although multiple polymer chains are produced. However, the number average degree of polymerization decreases as the chain transfers, since the growing chains are terminated by the chain transfer events. Taking into account the chain transfer reaction towards solvent S, initiator I, polymer P, and added chain transfer agent T. The equation of Pn can be expanded:
It is usual to define chain transfer constants C for the different molecules
- , , , ,
In chain growth polymerization, the position of the equilibrium between polymer and monomers can be determined by the thermodynamics of the polymerization. The Gibbs free energy (ΔGp) of the polymerization is commonly used to quantify the tendency of a polymeric reaction. The polymerization will be favored if ΔGp < 0; if ΔGp > 0, the polymer will undergo depolymerization. According to the thermodynamic equation ΔG = ΔH - TΔS, a negative enthalpy and an increasing entropy will shift the equilibrium towards polymerization.
In general, the polymerization is an exothermic process, i.e. negative enthalpy change, since addition of a monomer to the growing polymer chain involves the conversion of π bonds into σ bonds or an ring opening reaction that releases the ring tension in a cyclic monomer. Meanwhile, during polymerization, a large amount of small molecules are associated, losing rotation and translational degrees of freedom. As a result, the entropy decreases in the system, ΔSp < 0 for nearly all polymerization processes. Since depolymerization is almost always entropically favored, the ΔHp must then be sufficiently negative to composite for the unfavorable entropic term. Only then will polymerization be thermodynamically favored by the resulting negative ΔGp.
In practice, polymerization is favored at low temperatures: TΔSp is small. Depolymerization is favored at high temperatures: TΔSp is large. As the temperature increases, ΔGp become less negative. At certain temperature, the polymerization reaches equilibrium (rate of polymerization = rate of depolymerization). This temperature is called the ceiling temperature (Tc). ΔGp = 0
Stereochemistry of polymerization
The stereochemistry of polymerization is concerned with the difference in atom connectivity and spatial orientation in polymers that has the same chemical composition. Staudinger studied the stereoisomerism in chain polymerization of vinyl monomers in late 1920s, and it took another two decades for people to fully appreciate the idea that each of the propagation steps in the polymer growth could give rise to stereoisomerism. The major milestone in the stereochemistry was established by Ziegler and Natta and their coworkers in 1950s, as they developed metal based catalyst to synthesize stereoregular polymers. The reason why the stereochemistry of the polymer is of particular interest is because the physical behavior of a polymer depends not only on the general chemical composition but also on the more subtle differences in microstructure.Atactic polymers consist of a random arrangement of stereochemistry and are amorphous (noncrystalline), soft materials with lower physical strength. The corresponding isotactic (like substituents all on the same side) and syndiotactic (like substituents of alternate repeating units on the same side) polymers are usually obtained as highly crystalline materials. It is easier for the stereoregular polymers to pack into a crystal lattice since they are more ordered and the resulting crystallinity leads to higher physical strength and increased solvent and chemical resistance as well as differences in other properties that depend on crystallinity. The prime example of the industrial utility of stereoregular polymers is polypropene. Isotactic polypropene is a high-melting (165 °C), strong, crystalline polymer, which is used as both a plastic and fiber. Atactic polypropene is an amorphous material with an oily to waxy soft appearance that finds use in asphalt blends and formulations for lubricants, sealants, and adhesives, but the volumes are minuscule compared to that of isotactic polypropene.
When a monomer adds to a radical chain end, there are two factors to consider regarding its stereochemistry: 1) the interaction between the terminal chain carbon and the approaching monomer molecule and 2) the configuration of the penultimate repeating unit in the polymer chain. The terminal carbon atom has sp2 hybridization and is planar. Consider the polymerization of the monomer CH2=CXY. There are two ways that a monomer molecule can approach the terminal carbon: the mirror approach (with like substituents on the same side) or the non-mirror approach (like substituents on opposite sides). If free rotation does not occur before the next monomer adds, the mirror approach will always lead to an isotactic polymer and the non-mirror approach will always lead to a syndiotactic polymer (Figure 25).
However, if interactions between the substituents of the penultimate repeating unit and the terminal carbon atom are significant, then conformational factors could cause the monomer to add to the polymer in a way that minimizes steric or electrostatic interaction (Figure 26).
Traditionally, the reactivity of monomers and radicals are assessed by the means of copolymerization data. Q-e scheme, the most widely used tools for the semiquantitative prediction of monomer reactivity ratios, was first proposed by Alfrey and Price in 1940s. The scheme takes into account the intrinsic thermodynamic stability and polar effects in the transition state. A given radical and a monomer is considered to have an intrinsic reactivity of Q1 and Q2, respectively. The polar effects in the transition state, the supposed permanent electric charge carried by that entity (radical or molecule), is quantified by the factor e, which is a constant for a given monomer, and has the same value for the radical derived from that specific monomer. For reaction between a radical (species 1) and a monomer (species 2), the rate constant, k12, was postulated to be related to the four relevant reactivity parameters by
The monomer reactivity ratios for the copolymerization of monomers 1 and 2 can be given by
Free radical polymerization has found myriad applications including, but not limited to, the manufacture of polystyrene, thermoplastic block copolymer elastomers (which may be used for a wide variety of applications including adhesives, footwear, and toys), cardiovascular stents, chemical surfactants, and lubricants.
Free radical polymerization has many uses in research as well. One novel and particularly interesting application, one that exemplifies the power of the technique, is its use in the functionalization of carbon nanotubes. Carbon nanotubes, due to their intrinsic electronic properties, tend to form large aggregates in solution, precluding their use for useful applications. Adding small chemical groups to the walls of nanotubes can eliminate this propensity toward aggregation and can be used to tune the response of a nanotube to its surrounding environment; the use of polymers instead of smaller molecules can be used to drastically modify nanotube properties (and conversely, nanotubes can be used to modify polymer mechanical and electronic properties). For example, Lou et al. were able to demonstrate the coating of carbon nanotubes by polystyrene by first polymerizing polystyrene via chain radical polymerization and subsequently mixing it at 130 °C with carbon nanotubes to generate polystyrene radicals and graft them onto the walls of carbon nanotubes (Figure 27). The advantage of this approach lies in the order of chemical reaction – rather than growing a polymer off of a carbon nanotube (the “grafting from” approach), chain growth polymerization is used to first synthesize a polymer with predetermined properties. Purification of the polymer can be used to obtain a more uniform length distribution before grafting onto the nanotubes. Conversely, the “grafting from” approach, performed with radical polymerization techniques such as atom transfer radical polymerization (ATRP) or nitroxide-mediated polymerization (NMP) allows rapid growth of high molecular weight polymers (as opposed to the aforementioned “grafting to” approach where large, bulky polymers prohibitively slow the ability for free radical chain ends to find and couple with the nanotubes).
The power of free radical polymerization in polymerization from surfaces has also been exemplified in the synthesis of nanocomposite hydrogels. These gels are made of water-swellable nano-scale clay (especially those classed as smectites) enveloped by some network polymer and are often biocompatible and have mechanical properties (such as flexibility and strength) that make them promising candidates for applications such as synthetic tissue. Synthesis of these materials is currently possible only by the use of free radical polymerization. The general synthesis procedure is depicted in Figure 28. The clay is first dispersed in water where it forms very small, porous plates. Subsequent addition of the organic monomer, generally an acrylamide or acrylamide derivative, is immediately proceeded by addition of the initiator and a catalyst. The initiator is chosen to have stronger interaction with the clay than the organic monomer, so it preferentially adsorbs to the surface of the clay. The entire mixture of clay, organic monomer, initiator, catalyst, and water solvent is heated to initiate polymerization. Polymers grow off of the initiators which are in turn bound to the clay. Due to recombination and disproportionation reactions, growing polymer chains bind to one another, forming a strong, cross-linked network polymer, with clay particles acting as branching points for multiple polymer chain segments. Free radical polymerization used in this context allows the synthesis of polymers from a wide variety of chemical substrates (the chemistries of suitable clays are quite varied), and the termination reactions unique to chain growth polymerization are taken advantage of for the actualization of a material with flexibility, mechanical strength, and biocompatibility.
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