Poly(methacrylic acid) (PMAA) is a

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

made from

methacrylic acid Methacrylic acid, abbreviated MAA, is an organic compound. This colorless, viscous liquid is a carboxylic acid with an acrid unpleasant odor. It is soluble in warm water and miscible with most organic solvents. Methacrylic acid is produced indu ...

(preferred

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

name, 2-methylprop-2-enoic acid), which is a carboxylic acid. It is often available as its sodium salt, poly(methacrylic acid) sodium salt. The monomer is a viscous liquid with a pungent odour. The first polymeric form of methacrylic acid was described in 1880 by Engelhorn and Fittig. The use of high purity

monomers In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

is required for proper polymerization conditions and therefore it is necessary to remove any inhibitors by extraction (phenolic inhibitors) or via distillation. To prevent inhibition by dissolved oxygen, monomers should be carefully degassed prior to the start of the polymerization.

Polymerization

PMAA has a pKa of ~4.8, meaning that at neutral pH the MAA groups in the network are almost entirely deprotonated making it an

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

polymer. PMAA can act as a

polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are ...

and has the ability to absorb and retain water. These properties are strongly affected by the pH and therefore many

hydrogels A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...

are composed of PMAA copolymers. These hydrogel capsules can act as carrier vessels for confined drugs and act as microreactor reservoirs. For certain applications the sodium salt form of PMAA is used, in order to minimize side effects occurring from the anionic charge of the polymer or in applications where solubility in different solvents is required. The conventional synthesis method of PMAA is

free radical polymerization In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks ( repeat units). Free radicals can be formed by a number of different mechani ...

. In aqueous solution, substantial differences have been described in the polymerization rate of non-ionized and fully ionized MAA (pH effect). For the non-ionized scenario, a kinetic model has been well described. Recent progress has been made for (partially) ionized MAA by introducing a new rate law for

propagation Propagation can refer to: * Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials * Propaganda, non-objective information used to further an agenda * Reproduction, and other fo ...

where electrostatic and non-electrostatic effects are explicitly considered. In addition, the rate constant of propagation (kp) during free radical polymerization of methacrylic acid is dependent on the monomer concentration. Using pulsed layer polymerization size-exclusion chromatography techniques, it was determined that there is a minor decrease in kp for partially ionized MAA as monomer concentration increases while kp increases for fully ionized MAA as monomer concentration increases. The latter is in accordance with transition state theory for propagation. Controlled polymerization techniques, such as

RAFT A raft is any flat structure for support or transportation over water. It is usually of basic design, characterized by the absence of a hull. Rafts are usually kept afloat by using any combination of buoyant materials such as wood, sealed barrels ...

and NMP can be used for the direct polymerization of MAA. In contrast, polymerization of acidic monomers, such as MAA, has traditionally posed a challenge with, for example,

anionic polymerization In polymer chemistry, anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but tradi ...

, group transfer polymerization (GTP, see

living polymerization In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer ...

) and ATRP. The latter is not currently well understood but reasons hypothesized include ligand protonation at low pH, competitive coordination of carboxylate moieties to the copper and displacement of halide anions from the Cu(II) deactivator complex. Protecting group chemistry is commonly used for the polymerization of acidic monomers (using alkyl esters), followed by deprotection and purification, but other methods have also been explored. PMAA cyclization proved to be the main cause of termination, and this was reduced by changing the leaving group and the nucleophile, lowering the pH to reduce concentration and carboxylate anions, and accelerating the rate of polymerization. This work overcame one of the main limitations in ATRP and showed that water can be used as solvent for the polymerization of polar monomers using ATRP.

References

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Polymerization

References

See also