Nothing is added to or removed from any one of the combining monomers. Thus, the empirical formula of an addition polymer (if a homopolymer) is theoretically a multiple of the empirical formula of the monomer; actually, the terminal groups may differ slightly.
A simple example of addition polymerization is aldol addition or aldol condensation. (Do not confuse aldol condensation with condensation polymerization, which is here called anhydro polymerization to avoid confusion.)
Molecules of aldehydes like formaldehyde and acetaldehyde (with alpha hydrogen), in the presence of H+ or OH, can link together to form dimers, trimers and long polymers with high molecular weights. By addition polymerization, formaldehyde forms solid
Ketones, alkenes and alkynes can also undergo addition polymerizations. It is evident that many complex and varied substances can be formed by simple addition reactions between relatively simple substances and under prebiotic conditions (spontaneously). Most addition polymers are easily depolymerized by heat.
2. Anhydro polymerization (Condensation polymerization):
In anhydro polymerization each monomer is combined with the next by removing H+ from one and “OH from the other, forming water and causing the two residues to combine (polymerize).
Because water is formed the process is sometimes called “anhydrosynthesis”. Unlike addition polymers, the empirical formula of the anhydro polymer is not a simple multiple of that of the monomer since x number of molecules of water have been withdrawn in the process of polymerization.
Anhydro polymerization generally involves at least two kinds of enzymes: one or more to activate the units and one to withdraw the H+ and –OH and cause the residues to combine.
The process also requires energy. This is derived from elsewhere in the cell via the ADP-ATP and similar energy-transfer systems and is used in forming the bonds between the units of the polymer.