Radical polymerization

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.[1]

Free radical polymerization is a type of chain growth polymerization, along with anionic, cationic and coordination polymerization.

IUPAC definition

A chain polymerization in which the kinetic-chain carriers are radicals.

Note: Usually, the growing chain end bears an unpaired electron.[2]

Initiation

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.[1] 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

Initiator Efficiency

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.

Figure 8: Primary recombination of BPO; brackets indicate that the reaction is happening within the solvent cage.
Figure 9: Recombination of phenyl radicals from the initiation of BPO outside the solvent cage.
Figure 10: Reaction of polymer chain, R, with other species in reaction.

Propagation

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).[8] 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.[9]

Figure 11: Phenyl initiator from benzoyl peroxide (BPO) attacks a styrene molecule to start the polymer chain.
Figure 12: An orbital drawing of the initiator attack on ethylene molecule, producing the start of the polyethylene chain.

Once a chain has been initiated, the chain propagates (Figure 13) until there are no more monomers (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.[10][11] The mechanism of chain propagation is as follows:

Figure 13: Propagation of polystyrene with a phenyl radical initiator.

Termination

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.[3]

Chain transfer

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.[3][12]

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): \frac{1}{x_n}=\left(\frac{1}{x_n}\right)_o+\frac{k_{tr}[solvent]}{k_p[monomer]}. 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.[3]

Methods

There are four industrial methods of radical polymerization:[3]

Other methods of radical polymerization include the following:

Reversible deactivation radical polymerization

Also known as living radical polymerization, controlled radical polymerization, reversible deactivation radical polymerization (RDRP) relies on completely pure reactions, preventing termination caused by impurities. Because these polymerizations stop only when there is no more monomer, polymerization can continue upon the addition of more monomer. Block copolymers can be made this way. RDRP allows for control of molecular weight and dispersity. However, this is very difficult to achieve and instead a pseudo-living polymerization occurs with only partial control of molecular weight and dispersity.[14] ATRP and RAFT are the main types of complete radical polymerization.

Kinetics

In typical chain growth polymerization, the reaction rate for initiation, propagation and termination can be described as follows.

v_i={\operatorname{d}[M\cdot]/\operatorname{d}t}=2k_df[I]
v_p=k_p[M][M\cdot]
v_t={-\operatorname{d}[M\cdot]/\operatorname{d}t}=2k_t[M\cdot]^2

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•] 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.

[M\cdot]=\left(\frac{k_d[I]f}{k_t}\right)^{1/2}

In this case, the rate of chain propagation can be further described using a function of the initiator and monomer concentration

rate={k_p}\left(\frac{fk_d}{k_t}\right)^{1/2}[I]^{1/2}[M]

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.

\ v = \frac{R_p}{R_d}=\frac{k_p[M][M\cdot]}{2fk_d[I]}=\frac{k_p[M]}{2(fk_dk_t[I])^{1/2}}

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:

 P_n = v

Termination by combination leads to one polymer molecule per two kinetic chains:

 P_n = 2v

Any mixture of these both mechanisms can be described by using the value δ, and the contribution of disproportionation to the overall termination process:

 P_n = \frac{2}{1+\delta} v

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:

 \frac{1}{P_n} = \frac{2k_{t,d}+k_{t,c}}{{k_p}^2[M]^2}R_p + C_M +C_S \frac{[S]}{[M]}+C_I \frac{[I]}{[M]}+C_P \frac{[P]}{[M]}+C_T \frac{[T]}{[M]}

It is usual to define chain transfer constants C for the different molecules

 C_M=\frac{k^M_{tr}}{k_p} ,  C_S=\frac{k^S_{tr}}{k_p} ,  C_I=\frac{k^I_{tr}}{k_p} ,  C_P=\frac{k^P_{tr}}{k_p} ,  C_T=\frac{k^T_{tr}}{k_p}

Thermodynamics

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

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.[16] 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.[17]

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.[5] 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).[5]

Figure 25: (Top) formation of isotactic polymer; (bottom) formation of syndiotactic polymer.

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).[5]

Figure 26: Penultimate unit interactions cause monomer to add in a way that minimizes steric hindrance between substituent groups. (P represents polymer chain.)

Reactivity

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

 k_{12} = Q_1Q_2 exp(-e_1e_2)

The monomer reactivity ratios for the copolymerization of monomers 1 and 2 can be given by

 r_{1} = Q_1/Q_2 exp(-e_1(e_1-e_2))

Applications

Free radical polymerization has found applications including the manufacture of polystyrene, thermoplastic block copolymer elastomers,[18] cardiovascular stents,[19] chemical surfactants[20] and lubricants. Block copolymers are used for a wide variety of applications including adhesives, footwear and toys.

Free radical polymerization has uses in research as well, such as in the functionalization of carbon nanotubes.[21] CNTs intrinsic electronic properties lead them to form large aggregates in solution, precluding useful applications. Adding small chemical groups to the walls of CNT can eliminate this propensity and tune the response to the surrounding environment. The use of polymers instead of smaller molecules can modify CNT properties (and conversely, nanotubes can modify polymer mechanical and electronic properties).[18] For example, researchers coated carbon nanotubes with polystyrene by first polymerizing polystyrene via chain radical polymerization and subsequently mixing it at 130 °C with carbon nanotubes to generate radicals and graft them onto the walls of carbon nanotubes (Figure 27).[22] Chain growth polymerization ("grafting to") synthesizes a polymer with predetermined properties. Purification of the polymer can be used to obtain a more uniform length distribution before grafting. Conversely, “grafting from”, with radical polymerization techniques such as atom transfer radical polymerization (ATRP) or nitroxide-mediated polymerization (NMP), allows rapid growth of high molecular weight polymers.

Figure 27: Grafting of a polystyrene free radical onto a single-walled carbon nanotube.

Radical polymerization also aids synthesis of nanocomposite hydrogels.[23] These gels are made of water-swellable nano-scale clay (especially those classed as smectites) enveloped by a network polymer. They are often biocompatible and have mechanical properties (such as flexibility and strength) that promise applications such as synthetic tissue. Synthesis involves free radical polymerization. The general synthesis procedure is depicted in Figure 28. Clay is dispersed in water, where it forms very small, porous plates. Next the initiator and a catalyst are added, followed by adding the organic monomer, generally an acrylamide or acrylamide derivative. The initiator is chosen to have stronger interaction with the clay than the organic monomer, so it preferentially adsorbs to the clay surface. The mixture and water solvent is heated to initiate polymerization. Polymers grow off the initiators that 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.[24] Free radical polymerization used in this context allows the synthesis of polymers from a wide variety of substrates (the chemistries of suitable clays vary). Termination reactions unique to chain growth polymerization produce a material with flexibility, mechanical strength and biocompatibility.

Figure 28: General synthesis procedure for a nanocomposite hydrogel.

Electronics

The radical polymer glass PTMA is about 10 times more electrically conductive than common semiconducting polymers. PTMA is in a class of electrically active polymers that could find use in transparent solar cells, antistatic and antiglare coatings for mobile phone displays, antistatic coverings for aircraft to protect against lightning strikes, flexible flash drives, and thermoelectric devices, which convert electricity into heat and the reverse. Widespread practical applications require increasing conductivity another 100 to 1,000 times.

The polymer was created using deprotection, which involves replacing a specific hydrogen atom in the pendant group with an oxygen atom. The resulting oxygen atom in PTMA has one unpaired electron in its outer shell, making it amenable to transporting charge. The deprotection step can lead to four distinct chemical functionalities, two of which are promising for increasing conductivity.[25]

See also

External links

References

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