chapter  5
76 Pages

Condensation (Step-Growth) Polymerization

Condensation polymerizations, also known as step-growth or simply step polymerizations, are merely classical organic reactions that are used to produce linear macromolecules starting from bifunctional monomers (that is, monomers containing two functional groups per molecule), or to produce polymer networks from mixtures of bifunctional and multifunctional monomers having three or more functional groups per molecule. The polymerization of bifunctional monomers may be described as a stepwise or progressive conversion of monomers with two reactive end groups to higher molecular-weight homologues, which themselves retain two reactive end groups. It may take place either by a polycondensation reaction, whereby a low-molecular-weight by-product is formed along with the polymer, as is exemplified by polyesterification :

n HOOC (CH2)xCOOH n HO(CH2)yOH HO-[-OC(CH2)xCOO(CH2)yO-]n-H (2n 1) H2O

or by a polyaddition reaction in which the total reactants are incorporated in the polymer chain (and no by-products are formed), as is typified by polyurethane formation :

n 1OCN (CH2)xNCO + n HO(CH2)yOH OCN-[-(CH2)xNHCOO(CH2)yOOCNH-]n-(CH2)xNCO

These equations represent the overall reactions in the respective step-growth polymerizations. The growth of polymer molecules, however, occur by a stepwise intermolecular reaction. Thus two monomer molecules react to form a dimer; a dimer reacts with a monomer to form a trimer or with a dimer to form a tetramer, and so on. In fact, any two species in the reaction mixture can react with each other. Step polymerization can therefore be expressed by the general reaction :

n-mer + m-mer n m-mer

where n and m can have any value from 1 to very large number. Thus, a polyesterification reaction mixture of diacid and diol at any instance will consist of various-sized diol, diacid, and hydroxyacid molecules. Any two of these molecules containing OH and COOH groups can react and the

chemical reaction in each step is the same, which may be written as

-------COOH + HO------- -------COO------- + H2O 5.1

Similarly, polyamidation and polyurethane-forming reactions can be written as

-------COOH + H2N------- -------CONH------- + H2O 5.2

-------NCO + HO------- -------NH-COO------- 5.3

Implicit in these equations is the assumption that the functional group on the end of a monomer has the same reactivity as a similar group on a n-mer of any size and that the reactivities of both functional groups of bifunctional species (e.g., COOH groups of a diacid and OH groups of a diol) in the reaction mixture are the same. These simplifying assumptions are known as the concept of equal reactivity of functional groups. Experimental evidence and theoretical justifications have been provided in support of this concept (Flory, 1953; Odian, 1991).