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Anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. This polymerization is carried out through a carbanion active species. Like all addition polymerizations, it takes place in three steps: chain initiation, chain propagation, and chain termination. Living polymerizations, which lack a formal termination pathway, occur in many anionic addition polymerizations. The advantage of living anionic addition polymerizations is that they allow for the control of structure and composition.

Anionic polymerizations are used in the production of polydiene synthetic rubbers, solution styrene/butadiene rubbers (SBR), and styrenic thermoplastic elastomers.

History

The early work of Michael Szwarc and co – workers in 1956 was one of the breakthrough events in the field of polymer science. When Szwarc learned that the electron transfer between radical anion of naphthalene and styrene in an aprotic solvent such as tetrahydrofuran gave a messy product, he started investigating the reaction in more detail. He proved that the electron transfer results in the formation of a dianion which rapidly added styrene to form a “two – ended living polymer.”Being a physical chemist, Szwarc set forth in understanding the mechanism of such living polymerization in greater detail. His work elucidated the kinetics and the thermodynamics of the process in considerable detail. At the same time, he explored the structure property relationship of the various ion pairs and radical ions involved. This had great ramifications in future research in polymer synthesis, because Szwarc had found a way to make polymers with greater control over molecular weight, molecular weight distribution and the architecture of the polymer.

The use of alkali metals to initiate polymerization of 1,3-dienes led to the discovery by Stavely and co-workers at Firestone Tire and Rubber company of cis-1,4-polyisoprene. This sparked the development of commercial anionic polymerization processes that utilize alkyllithium initiatiors.

Monomer Characteristics

In order for polymerization to occur with vinyl monomers, the substituents on the double bond must be able to stabilize a negative charge. Stabilization occurs through delocalization of the negative charge. Because of the nature of the carbanion propagating center, substituents that react with bases or nucleophiles either must not be present or be protected.

Examples of vinyl monomers.

Vinyl monomers with substituents that stabilize the negative charge through charge delocalization, undergo polymerization without termination or chain transfer. These monomers include styrene, dienes, methacrylate, vinyl pyridine,aldehydes, epoxide, episulfide, cyclic siloxane, and lactones. Polar monomers, using controlled conditions and low temperatures, can undergo anionic polymerization. However, at higher temperatures they do not produce living stable, carbanionic chain ends because their polar substituents can undergo side reactions with both initiators and propagating chain centers. The effects of counterion, solvent, temperature, Lew base additives, and inorganic solvents have been investigated to increase the potential of anionic polymerizations of polar monomers. Polar monomers include acrylonitrile, cyanoacrylate, propylene oxide, vinyl ketone, acrolein, vinyl sulfone, vinyl sulfoxide, vinyl silane and isocyanate.

Examples of polar monomers.

Solvent

The solvent used in anionic addition polymerizations are determined by the reactivity of both the initiator and carbanion of the propagating chain end.The stability of the anionic propagating species is also dependent on the solvent as it is significantly reduced in polar solvents such as ethers due to the presence of the nucleophilic C-O bond of the ether. Less reactive chain ends, such as heterocyclic monomers, can use a wide range of solvents.

Initiation

The reactivity of initiators used in anionic polymerization should be similar to that of the monomer that is the propagating species. The pKa values for the conjugate acids of the carbanions formed from monomers can be used to deduce the reactivity of the monomer. The least reactive monomers have the largest pKa values for their corresponding conjugate acid and thus, require the most reactive initiator. Two main initiation pathways involve electron transfer (through alkali metals) and strong anions.

Initiation by Electron Transfer

Szwarc and coworkers studied the initiation of polymerization through the use of aromatic radical-anions such as sodium naphthenate. In this reaction, an electron is transferred from the alkali metal to naphthalene. Polar solvents are necessary for this type of initiation both for stability of the anion-radical and to solvate the cation species formed. The anion-radical can then transfer an electron to the monomer.

Initation through electron transfer.

Initiation can also involve the transfer of an electron from the alkali metal to the monomer to form an anion-radical. Initiation occurs on the surface of the metal, with the reversible transfer of an electron to the adsorbed monomer.

Initiation by Strong Anions

Nucleophilic initiators include covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines and organometallic compounds (alkyllithium compounds and Grignard reagents). The initiation process involves the addition of a neutral (B:) or negative (B:-) nucleophile to the monomer.

Initiation through strong anion.

The most commercially useful of these initiators has been the alkyllithium initiators. They are primarily used for the polymerization of styrenes and dienes.

Propagation

Propagation of an anionic addition polymerization.

Propagation in anionic addition polymerization results in the complete consumption of monomer. It is very fast and occurs at low temperatures. This is due to the anion not being very stable, the speed of the reaction as well as that heat is released during the reaction. The stability can be greatly enhanced by reducing the temperatures to near 0˚C. The propagation rates are generally fairly high compared to the decay reaction, so the overall polymerization rates is generally not affected.

Termination

Anionic addition polymerizations have no formal termination pathways because proton transfer from solvent or other positive species does not occur. However, termination can occur through unintentional quenching due to trace impurities. This includes trace amounts of oxygen, carbon dioxide or water. Intentional termination can occur through the addition of water or alcohol. Another method of termination, chain transfer, can occur when an agent can act as a Bronsted acid. In this case, the pKa value of the agent is similar to the conjugate acid of the propagating carbanionic chain end. Spontaneous termination occurs because the concentration of carbanion centers decay over time and eventually results in hydride elimination. Polar monomers are more reactive because they are stabilized by their polar substituents. These polar substituents can react with nucleophiles which results in termination as well as side reactions that compete with both initation and propagation.

Kinetics

The kinetics of anionic addition polymerization depend on whether or not a termination pathway occurs.

Kinetics of Living Anionic Addition Polymerization

In general, the reaction mechanism for living anionic addition polymerization are as follows:

$\textstyle \ {\begin{aligned}&{\mbox{I}}^{-}+{\mbox{M}}{\overset {k_{init}}{\longrightarrow }}{\mbox{M}}^{-}\\&{\mbox{M}}^{-}+{\mbox{M}}{\overset {k_{prop}}{\longrightarrow }}{\mbox{M}}^{-}\end{aligned}}$

where I = initiator, k_init = the initiation reaction rate constant, M = monomer, M^-= propagating species, and k_prop = the propagation reaction rate constant.

As most polymerizations of this type do not have a termination pathway, the rate of polymerization is the rate of propagation:

$\textstyle \ {\mbox{rate(prop)}}=k_{p}[{\mbox{M}}^{-}][{\mbox{M}}]$

where k_p is the rate of constant of propagation, [M^-] is the total concentration of propagating centers, and [M] is the concentration of monomer. Since there is no termination pathway in living polymerizations, the concentration of propagating centers is equal to the concentration of initiator ([I]). Thus,

$\textstyle \ {\mbox{rate(prop)}}=k_{p}[{\mbox{I}}][{\mbox{M}}]$

The degree of polymerization, X_n is also affected by no termination pathway. It is the ratio of concentration of reacted monomer ([M]_o) to initiator([I]_o) times the percent conversion p. In this case, the chain length (ν) is equal to X_n.

$\nu ={\frac {[{\mbox{M}}]_{o}}{[{\mbox{I}}]_{o}}}\rho$

When conversion, p = 1 (100% conversion), chain length is simply the ratio of reacted monomer to initiator.

$\nu ={\frac {[{\mbox{M}}]_{o}}{[{\mbox{I}}]_{o}}}$

Kinetics: Termination due to Impurities

When termination occurs due to impurities, the impurities must be taken into account in determining the reaction rate. The reaction mechanisms would begin the same as that of a living anionic addition (initiation and propagation). However, there would now be a termination step to account for the effect of the impurities on the reaction.

$\textstyle \ {\mbox{M}}^{-}+HX{\overset {k_{term}}{\longrightarrow }}{\mbox{M-H}}+{\mbox{X}}^{-}$

where M^-= propagating species, HX = impurity and k_term = the termination reaction rate constant.

Using the steady-state approximation, the rate of propagation becomes

$\textstyle \ {\mbox{rate(prop)}}={\frac {k_{init}k_{prop}[{\mbox{I}}][{\mbox{M}}]^{2}}{k_{term}[{\mbox{H-X}}]}}$

Since

$\textstyle \ \nu ={\frac {rate(prop)}{rate(term)}}={\frac {k_{prop}[{\mbox{M}}]}{k_{term}[{\mbox{H-X}}]}}$

Thus chain length and rate of propagation are negatively impacted by the presence of impurities in the reaction.

Uses of Living Anionic Polymerization

Living polymerization was first introduced by Szwarc and co workers in 1956. Their initial work was based on the polymerization of styrene and dienes. One of the remarkable features of living anionic polymerization is that the mechanism involves no formal termination step. In the absence of impurities, the carbanion would still be active and capable of adding another monomer. The chains will remain active indefinitely unless there is inadvertent or deliberate termination or chain transfer. This gave rise to two important consequences:

1. The number average molecular weight, Mn, of the polymer resulting from such a system could be calculated by the amount of consumed monomer and the initiator used for the polymerization, as the degree of polymerization would be the ratio of the moles of the monomer consumed to the moles of the initiator added.
   $M_{n}=M_{o}{\frac {[{\mbox{M}}]_{o}}{[{\mbox{I}}]}}$, where M_o = formula weight of the repeating unit, [M]_o = initial concentration of the monomer, and [I] = concentration of the initiator.
2. All the chains are initiated at roughly the same time. The final result is that the polymer synthesis can be done in a much more controlled manner in terms of the molecular weight and molecular weight distribution (Poisson distribution).

The following experimental criteria have been proposed as a tool for identifying a system as living polymerization system.

• Polymerization until the monomer is completely consumed and until further monomer is added.
• M_n is a linear function of conversion
• Constant number of active centers or propagating species.
• Poisson distribution of molecular weight
• Chain end functionalization can be carried out quantitatively.

However, in practice, even in the absence of terminating agents, the concentration of the living anions will reduce with time due to a decay mechanism termed as spontaneous termination.

Synthesis of complex architectures

Polymerization reactions excluding a termination and transfer step are particularly useful for the synthesis of functionalized polymers with well defined architectures. . The consumption of the monomer results in stable, anionic polymer chain ends, allowing reactions with a variety of electrophilic functional groups post polymerization.

${\begin{aligned}{\mbox{PLi}}+{\mbox{X-Y}}{\longrightarrow }{\mbox{P-X}}+{\mbox{LiY}}\end{aligned}}$

However the efficacy of these reactions depends on a number of variables such as chain-end structure, solvent, temperature, and concentration. Alternatively, by controlling the functionality of the initiator, one can prepare polymers having different geometries such as symmetric and asymmetric stars, comb shaped, etc.

Block copolymers

Synthesis of block copolymers by sequential monomer addition is one of the most important applications of living polymerization as it offers the best control over structure. The nucleophilicity of the resulting carbanion, will govern the order of monomer addition as the monomer forming the lower nucleophilic propagating species may inhibit the addition of the more nucleophilic monomer onto the chain. An extension of the above concept is the formation of triblock copolymers where each step of such a sequence aims to prepare a block segment with predictable, known molecular weight and narrow molecular weight distribution without chain termination or transfer.

Triblock copolymer - reaction scheme.

Star Shaped Polymers

General Star Shaped Polymer

A star shaped polymer is a polymeric structure in which several chains emanate from a single junction point known as the core. The control offered by anionic polymerization makes it a very popular pathway to synthesize molecules with such complex geometry. It allows quantitative studies of the degree of branching of the polymer on the overall properties of the substance.

One of the the first pathways explored was the use of a multifunctional initiator , but it was limited by the insolubility of such compounds and there was no control over the reactivity of each branch. A second, more efficient way was proposed - the addition of a multifunctional electrophilic terminator at the end of the polymerization of a linear polymer. This is analogous to the convergent synthesis of dendrimers, and is efficient as long as the stoichiometry between the terminating agent and the starting monomers are maintained.

The third route is through the addition of small amounts of cross – linking agents to the polymeric precursor (for example, addition of divinyl benzene to polystyryl lithium). There are three prime reactions that take place:

• The crossover of the polystyryl chain to divinylbenzene
• The block copolymerization of divinylbenzene
• The reaction of the pendant vinyl groups of divinyl benzene with linear polystyryl branches

Addition of divinyl benzene to polystyryl lithium by cross-linking agents to polymeric precursor

The uniformity in the structure is a function of the rate of the crossover reaction compared to the other two reactions. The number of branches of the star molecules cannot be precisely predicted, as it is a complex function of the reaction variables. For example, the amount of divinyl benzene added in the above pathway, compared to the number of active chains, is a key factor governing the overall degree of branching of the polymer.

The method is widely used for the synthesis of star shaped polystyrene with divinyl benzene with low molecular weight distributions. Star shaped polymethyl methacrylate was similarly synthesized using ethylene glycol dimetthacrylate as a crosslinker. The molecular weights obtained are comparatively high (~40kDa), which is thought to be necessary to avoid the gelation due to inter core reactions. The protection of the core groups by the branches gave such polymers the name “porcupine polymers”.

End-Group Functionalization

Living anionic polymerization can also be used to incorporate functional groups. These end-groups are usually added post-polymerization. End-groups that have been used in the functionalization of α-haloalkanes include hydroxide, -NH₂, -OH, -SH, -CHO,-COCH₃, -COOH, and epoxies. oach for functionalizing end-groups is to begin polymerization with a functional anionic initiator.

Addition of hydroxide group through an epoxide.

An alternative approach for functionalizing end-groups is to begin polymerization with a functional anionic initiator. In this case, the functional groups are protected since the ends of the anionic polymer chain is a strong base.oach for functionalizing end-groups is to begin polymerization with a functional anionic initiator. This method leads to polymers with controlled molecular weights and narrow molecular weight distributions.

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