Ring Opening Polymerization of Cyclic Esters

6 RING-OPENING POLYMERIZATION OF CYCLIC ESTERS 9.1.6.1 Introduction 

The high level of interest in the ring-opening polymerization (ROP) of cyclic esters (lactones) stems from the biocompatibility and biodegradability of their polymers. Resorbable aliphatic 

CL e-caprolactone, LA = lactide, GA = glycolide, 6-VL = 5-valerolactone, 7-BL = 7-butyrolactone, /3-BL =/3-butyrolactone, /3-PL /3-propiolactonc 

Other ROP reaction that will be discussed in this section was done on cyclic ester monomers to give polyesters (reaction 7.3). Currently organic chemicals are predominantly derived from petrochemicals. Polyethylene and polypropylene production are hugely increased during last decade. These polymers in addition to polyethers, polycarbonates, and polyesters are all derived from oil. But if we are going to run out of fossil fuels, we must find alternative chemicals which could be produced from renewable 

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This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

Polyesters from the Ring-Opening Polymerization of Cyclic Esters... 

Zhong Z, Dijkstra PJ, Birg C, Westerhausen M, Feijen J (2001) A novel and versatile calcium-based initiator system for the ring-opening polymerization of cyclic esters. Macromolecules 34 3863-3868... 

Connor EE, Nyce GW, Myers M, Mock A, Hedrick JL (2002) First example of N-heterocyclic carbenes as catalysts for living polymerization organocatalytic ring-opening polymerization of cyclic esters. J Am Chem Soc 124 914-915... 

For selectiveness of initiation of graft reactions, the use of a tin octoate (SnOct2) (or tin(II) 2-ethylhexanoate) catalyst is convenient for preparing this type of copolymers, polysaccharide-gra/f-PHAs, because the ring-opening polymerization of cyclic esters such as lactides and e-caprolactone (CL) can be initiated efficiently by hydroxo-initiators (i.e. based on hydroxyl groups) in the presence of SnOct2 [12-16], as exemplified in Scheme la. 

Scheme 1 a Ring-opening polymerization of cyclic esters in the presence of SnOct2 and hydroxo-initiator, and b its application to the synthesis of polysaccharide-gra/t-aliphatic polyesters (or poly(hydroxyalkanoate)s)... 

Ring-opening polymerization of cyclic esters and amides... 

Figure 15 Ring-opening polymerization of cyclic esters to make biodegradable polymers (a) ROP of e-caprolactone (b) ROP of one enantiomer of lactide to give isotactic polymer and (c) ROP of rac-lactide to form polymers of variable tacticity...

Catalysis design for ring-opening polymerization of cyclic esters 1. Group 1 metal and thallium(I) trispyrazolylborate complexes with hemilabile ligands... 

The synthesis of polyphosphates as a model for nucleic and teichoic acids by ring-opening polymerization of cyclic esters of phosphoric acid is studied by Penczek s group and was reviewed recently (43. 101. 102). 

Asandei, A. D., Saha, G. Living ring-opening polymerization of cyclic esters with epoxide-derived titanium alkoxides. 

Goldys, A. M., Dixon, D. J. Organocatatytic ring-opening polymerization of cyclic esters mediated by Eighty active bifiinctional iminophospho-rane catalysts. Macrnmnlecules. 2014.47,1277-1284. 

Thomas, C. M. Stereocontrolled ring-opening polymerization of cyclic esters Synthesis of new polyester microstructures. 

Figure 2. In situ generation of a calcium alkoxide from bis(tetrahydrofuran)calcium, bis[bis(trimethylsilyl)amide] and an alcohol, for the ring Opening polymerization of cyclic esters...

A last example to be mentioned here is the case of peculiar abnormal NHC (143) in which the carbene centre is no longer located between the nitrogen atoms but between nitrogen and carbon atoms. Compound (143) has been established as an efficient organocatalyst for ring-opening polymerization of cyclic esters.This report represents the first use of an abnormal NHC as catalyst in an organic transformation. 

Ring-Opening Polymerization of Cyclic Esters. Polyester syntheses have been achieved by enzymatic ring-opening polymerization of cyclic esters with various ring-sizes. Figure 9 summarizes cyclic monomers so far polymerized through lipase catalysis. 

Interestingly, the first NHC complexes were reported with chromium (0) carbonyl by Ofele in 1968. Relatively few NHC early-transition metal complexes were then reported in the 1990s and this number steadily increased over the past decade. This subject is now mature moreover, the coordination chemistry of NHC has been investigated with alkali metals, alkaline earth metals, lanthanides or group 13-15 metals. Applications of these NHC complexes in catalysis now include, most notably, olefin polymerization or ring-opening polymerization of cyclic esters. Some of these complexes display high activity and selectivity and, in some instances, may compete with the best systems in the field. 

Some examples of sequence-controlled polymers prepared by ring opening polymerization of cyclic esters have been reported. For instance, Thomas, Coates, and coworkers reported a very original method for the synthesis of alternating aliphatic polyesters.These authors used a stereoselective yttrium-based organometallic catalyst to control comonomer sequences. It had been previously shown that the polymerization of a racemic mixture of p-butyrolactone in the presence of this catalyst led to the formation of... 

Ae-CATALYZED IMMORTAL RING-OPENING POLYMERIZATIONS OF CYCLIC ESTERS... 

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