Polymerizations lactones

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

There is still a great deal of effort being made on the study of the mechanisms and catalysis of lactide, glycolide, and lactone polymerizations,367,379-383 and many reviews have been published on this topics.365,384 386 Since it is basically a chain-growth, rather titan a step-growth, process, it is beyond the scope of this book and will not be extensively discussed here. Nevertheless, it is worth mentioning that very close resemblances exist between the mechanisms... 

L-Lactate. See Poly(L-lactate) (PLLA) Lactone polymerization, 84 Lacty, 28... 

The general subject of lactone polymerization has been reviewed (7, 19). Polymerization of e-caprolactone can be effected by at least four different mechanisms categorized as anionic, cationic, coordination, and radical. Each method has unique attributes, providing... 

Fig. 9. Postulated mechanism of lactone polymerization catalyzed by lipase...

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

The use of these initiators to polymerize LA814 and methylglycolide815 has been reported to proceed in a well-controlled fashion. Block copolymers such as PCL-b-PLA have also been prepared. Elimination of PrOH from the reaction of (270) with preformed hydroxyl terminated polymers, followed by lactone polymerization, yields diblocks of CL with polystyrene or polybutadiene.816 The preparation of an ABA triblock has also been reported (A = CL, B = LA) since propagating chains of PLA do not initiate CL ring opening, (270) was pretreated with hydroxy terminated (PCL-b-PLA)-OH 814... 

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

Scheme 10. Proposed mechanism for initiation of lactone polymerization... 

Brode, G. Lactone polymerization and polymer properties. J. Macromol. Sci. Chem. A6 1109-1147, 1972. 

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... 

ROP of lactones and lactides using lanthanide alkoxide-based initiators is a relatively recent discovery. The first example of lactone polymerization by lanthanide alkoxide complexes was reported in a DuPont patent written by McLain and Drysdale in 1991 [89]. In general, the activity of these catalysts is much higher than that determined for aluminum alkoxides, especially in lactide polymerization [90-92]. Polymers of relatively high molecular weight and narrow MWD are formed. The negative side-reactions such as macrocycle formation, transesterification, and racemization are absent. 

Evidence on association in cationic polymerizations is less frequent. Nevertheless it does exist, and the occurrence of the effect is beyond any doubt. Belenkaya et al. described ion aggregates in cationic lactone polymerization... 

Both initiation and propagation reaction in lactone polymerization may involve an endo- or exocyclic oxygen atom ... 

On the other hand, cationic lactone polymerization may be also used to prepare cyclic oligomers of specific ring size with reasonable yield. Polymerization of bicyclic lactone, 6,8-dioxabicyclo[3.2.1]octan-7-one, generates a repeating unit containing ester and ether bonds ... 

McLain, S.J., Ford, T.M., and Drysdale, N.E. (1992) Rare earth metal coordination compounds as lactone polymerization catalysts. American Chemical Society Polymer Preprints, 33, 463. 

It was stated earlier that termination reactions are absent in lactone polymerizations. This is so at solution polymerization temperatures. However, Mayne45) has shown that during phosphine initiated bulk polymerization at 300 °C the following reaction occurs ... 

As stated above. It has been established that carboxylate Ions are the true Initiators in B lactone polymerization. In an early study. Hall ( ) stated that In Intermediate pH ranges anions or water Itself attack the CH2 group with alkyl-oxygen cleavage by SN2 reaction that proceeds most rapidly In anhydrous acetonitrile or tetrahydrofuran. From heats of combustion he determined the heat of polymerization of plvalolactone AH j. to be -20.1 kcal mol l. 

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