Activated monomer mechanism AMM

Anionic polymerization of 2-hydroxymethyloxetane is imsuccessfiil (34). The failure of such a reaction is most likely due to the fact that oxetanes are not known to ring-open imder basic conditions. The successful cationic ring-opening polymerization of 3-ethyl-3-(hydroxymethyl)oxetane gave hydroxy-functional hyperbranched polyethers (45,46) (Fig. 7). The cationic polymerization can proceed according to two different mechanisms, activated chain end (ACE) or activated monomer mechanism (AMM) (45) (Fig. 8). 

Fig. 8. Cationic ring-opening polymerization can proceed in accordance to two different mechanisms, activated chain end (ACE) or activated monomer mechanism (AMM).

Polymerization proceeding by activation of the monomer is referred to as the activated monomer mechanism (AMM) -both nudeophilic and electrophilic - whereas polymerization by activation of the rnitiator/potymer chain ends is referred to as the active chain end mechanism (ACEM). Note that, in the context of tme organocatalysis, only basic catalysts are able to... 

The two mechanisms already discussed above for other organocatalysts were eventually proposed to account for the controlled/living character of NHC-catalyzed ROPs of cydic esters, in general. In the AMM, the carbonyl group of the cyclic ester substrate is first subjected to a nucleophilic attack by the carbene catalyst (Path A, Scheme 37). The activate initiator/ chain end mechanism (ACEM) involves interaction of the O-H group of the alcohol initiator with the NHC lone pair (Path B, Schemes ) and the nucleophilic attack of the monomer by the oxygen of this alcohol. 

Similarly to ROP of cyclic esters, two distinct mechanisms of initiation and chain growth were proposed (Scheme 42). NHCs might be nucleophilic and silicophilic enough to activate the electrophilic part (E = H or SiMe3) of NuE chain regulators. Thus, both trimethylsilylated and hydroxylated chain regulators can be activated by NHCs, followed by monomer insertion (ACEM, path B). However, a nucleophilic attack by the NHC catalyst on the monomer is also plausible (AMM, path A). Protonation (or silylation) of the zwitterionic intermediate species by an alcohol (or a silyl ether), followed by addition of the resulting... 

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