# What are the molecular requirements for condensation polymerization?

I know that in order to have addition polymerization, the monomers must have a C=C double bond. However, I don't know the molecular requirements for condensation polymerization. Must the monomers contain a functional group at each and?

Any help is appreciated. Thanks.

• You are misapplying the categories. Addition polymerisations often involve CC double bonds, but that is by no means a requirement. For condensations, it is hard to imagine a mechanism without two functional groups, but that is not the point! – Karl Apr 19 '17 at 15:57

Monomers must contain a functional group at each side (at least bifunctional) and these functions have to react with each other (like amine and carboxylic acid to yield amide group or alcohol and carboxylic acid to yield ester group,...) .

1. We can start with two types of monomers that hold identical functional groups at each side.

One example of that is the reaction of 1,6-diaminohexane with hexandioic acid to yield Nylon-6,6.

1. We can also start from one type of monomers with two different functional groups at each side.

One example of that is the reaction of 11-aminoundecanoic acid to yield Nylon-11.

The condition that the reaction should give off a small molecule is no longer valid. The counter example is the formation of polyurethane by reaction of diisocyanates and diols without giving off small molecules. We prefer name this type of polymerization "a step growth polymerization" in opposite to "chain growth polymerization" (known before as polyaddition).

Condensation polymerization can be shortened by having two monomers which are bifunctional that can react giving off small molecules like $\ce{H2O}$ or methanol.

This can be exemplified by amino acids which have a carboxyl group ($\ce{-COOH}$) and an amine group ($\ce{-NH2}$). When making the polymer the carboxylic group reacts with the amine group of the neighboring moleucle giving a dimer which is bonded via an amide bond ($\ce{-CONH-}$). This dimer has an amine group on one side and a carboxylic group on the other making it susceptible for further reaction thus making a trimer and a tetramer... and so on.

Carbon-carbon double bonds are not necessary for condensation polymerisation. The reaction usually involves the elimination of water. When two different functional groups are present on a molecule a polymer results, for example with $\omega$-hydroxyhexanoic acid repeated condensation yields a polyester, $$\ce{HO(CH2)COOH \rightarrow HO-(CH2COO)_n-H}$$

With two different bifunctional molecules are used such as an amine and an acid, such as adipic acid and hexamethylenediamine, nylon can be made;

$$\ce{ HOOC(CH2)_4COOH + H2N(CH2)_6NH2 \rightarrow HOOC(CH2)_4CONH(CH2)_6NH2 + H2O}$$ and this is followed by further steps where the initial (acid -amine) product reacts with more adipic acid.

Polyesters, polyamides, polyurethanes and silicones are made this way. If more that two functional groups are present insoluble, infusible polymers such as Bakelite are formed.

The kinetics do not follow that of chain reactions, there is no initiation or termination and the general mechanism is $\ce{M_j + M_k \rightarrow M_{j+k}}$. The kinetics can be analysed by assuming that only the end groups are important, thus the rate constant is assumed to be independent of molecular size then the rate is found to be bimolecular, (at least for acid catalysed reactions) i.e. $\ce{d[A]/dt=-$k$[A][B]}$ for reacting species A and B.