Cationic coordination polymerization lactam

Some heterocycles have both nucleophilic and electrophilic atoms in their molecule. Thus they can be opened and polymerized by the anionic, cationic or coordination mechanisms. Examples are lactams, lactones, and cyclic siloxanes. Investigations of the mechanism of lactam propagation are complicated by the occurence of side reactions. In principle, the mechanism described in Chap. 3 by the schemes (55)—(57) and (71) is accepted. Anionic polymerization of cyclic esters consists, in most cases (see Chap. 4, Sect. 2.2) of repeated reversible attacks on the carbonyl carbon by the anion 0]-. From e-caprolactone, polyester chains grow according to [315]... 

The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids, salts of primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split off water at elevated temperatures. The initiators react by coordinating with and fonrung rapid pre-equilibrium lactam cations. These cations are the reactive species in the polymerizations. Initiations of this type are also possible with weakly acidic compound, but such compounds ate not able to transfer protons to the lactam. They are capable, however, of forming hydrogen bonds with the lactams. The high reactivity of the lactam cations may be attributed to the decreased electron density at the carbonyl carbon atoms. This makes them more subject to nucleophilic attacks. Protonations of the amides occur at the oxygens, but small fractions of N-protonated amides are also presumed to exist in tautomeric equilibrium. To simplify the illustrations, all lactams will be shown in this section as... 

These acidic initiators will coordinate with a lactam molecule in a rapid preequilibrium to give a lactam cation which is the reactive species in the polymerization. This type of initiation may also take place with weakly acidic compounds which cannot actually transfer a proton to the lactam, but which eire able to form a hydrogen bond with it. 

In the conventional less strained lactams, protonation of the oxygen atom or its coordination to Lewis acids raises the elec-trophilicity of hoth the carhonyl and the carbon atom linked to nitrogen (C, ), and increases the nudeophilidty of the carbon linked to the carbonyl (Q,) accordingly, the nudeophilidty of nitrogen is decreased. The O-protonated amide group is characterized hy a lower energy compared to the N-protonated one consequendy, O-protonation is preferred as the initiation step in the cationic polymerization (see Section 4.14.5). 

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