Coordinative Ring-Opening Polymerization of Lactones

Coordinative initiation differs from ionic polymerization in that the propagating species consists of a covalent bond species. This generally reduces the reactivity and the polymerization rate. Decreased reactivity also leads to fewer amounts of side reactions and the often-living ROP of lactones may take place under these conditions. Chedron, in the early 1960s, showed that some Lewis acids, such as triethylaluminum and water or ethanolate of diethylaluminum, were effective initiators for lactone polymerizations. Tin(IV) alkoxides and phenox-ides, [92,93] aluminum alkoxides, mainly aluminum / so-propoxide, and soluble 

Block copolymers of e-CL with D,L-lactide, styrene, or butadiene have been synthesized using these initiators. Efficient and versatile initiators based on a,(3,y,6-derivatives of tetraphenylporphinato-aluminum for the polymerization of e-CL, (3-lactones, 6-lactones, and lactides have been reported [99,100]. 

The living ROP of e-CL is usually initiated by aluminum isopropoxide, [Al(0 Pr)3] in toluene at 0-25 °C. Under these conditions this initiator exists as an aggregate of trimers and tetramers. However, freshly distilled Al(0 Pr)3 consists mainly of trimers, and is a more reactive initiator for ROP. The initiation rate is high compared to the rate of propagation so that a narrow molecular weight distribution is obtained in the polymer. There is no termination reaction and 3 chains grow per Al atom. Block polymers have been prepared by sequential polymerization of e-CL (monomer A) and DXO (monomer B) using Al(0 Pr)3 as an initiator in THF at 0 °C to yield AB or BA di-block copolymers [95]. 

In (blactones, scission of either the acyl-oxygen bond or the alkyl-oxygen bond may take place leading to the formation of alkoxide- or carboxylate-grow-ing chains [ 115]. Methylene chloride end groups were observed in the ZnCl2-in- 

Recent studies of the tetrabutyltin-initiated polymerization of e-CL have indicated that Bu2SnO dissolved in Bu4Sn is the main initiator [123]. Almost 100% conversions are observed in the living macrocyclic polymerization of e-CL in bulk at 80 °C with 2,2-dibutyl-2-stanna-l,3-dioxepane as initiator [124]. 

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