Ring-opening polymerisation monomer activation

The ring opening polymerisation of THF has as active centres tertiary oxonium cations and is based on the nucleophilic attack of the oxygen atom of the THF monomer at the a-carbon atom of the oxonium cation. The a-carbon atom is activated by the presence of the neighbouring positive charge which decreases the electron density at this carbon atom. Thus, the nucleophilic attack of the oxygen atom of the monomer takes place at the activated a-carbon atom (reaction 7.1) [3, 7] ... 

Addition of L-lactide monomer to preformed active PCL chains gave 98% conversion of lactide monomer and formation of block copolymer. That is, the PCL end groups initiated the ring-opening polymerisation of L-lactide. It was also demonstrated that there was no transesterification reaction during the polymerisation the end groups of the propagating lactide chain did not attack the preformed PCL chain. 

Incorporation of enzyme-sensitive units in the polymer side chain can be done either after the main polymer has been synthesised or the enzyme-sensitive functionality can be present in the monomer before polymerisation (Fig. 6.10). Attachment to the polymer is typically done via standard SPPS protocols. Both peptides and DNA fragments have been attached to polymers obtained through ring opening polymerisation of norbornene using either N-hydroxysuccinimide (NHS) activation or A, A -tetramethyl-... 

Monomer activation mechanism of ring-opening polymerisation catalysed by nucleophiles. 

The use of nucleophilic catalysts is promising. Indeed, ring-opening polymerisation of LA initiated by alcohols and catalysed by nucleophiles, such as tertiary amines (e.g., 4-dimethylaminopyridine, 4-pyrrolidinopyridine) (Nederberg et al, 2001) and N-imidazolium carbenes (Connor et al, 2002) is well controlled. A monomer activation mechanism (Fig. 4.11), similar to the one reported for biocatalysis, has been proposed. 

The mode of lactone ring opening depends on the kind of catalyst. It is characteristic that -lactone polymerisation with a catalyst containing a metal alkoxide active bond (Mt-X X = OR) involves C(0)-0 bond scission in the coordinating monomer (via the metal orthocarbonate species) with regeneration of the metal alkoxide active bond [scheme (7)] [87]. On the other hand, the application of a catalyst with a metal carboxylate active bond [Mt-X X = 0C(0)R] for -lactone polymerisation results in Cp — O bond scission in the coordinating monomer with regeneration of the metal carboxylate active bond [scheme (8)] [88-90],... 

The most strained cycloolefins, which are substituted cyclopropenes, e.g. 3,3-dimethylcyclopropene or 3-methyl-3-ethylcyclopropene, appeared to be polymerised readily to respective substituted poly(l,2-cyclopropene)s in the presence of Pd-based catalysts containing very bulky non-labile ligands. Such catalysts are characterised by reduced activity in order to prevent ring opening of the cyclopropene monomer [23],... 

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