Polylactones

PCL is prepared by the ring-opening polymerization of the cyclic monomer e-caprolactone, and it can be copolymerized with numerous other monomers because of its high affinity with them.  

PCL retains mechanical properties for 5-6 months and then gradually loses its physical properties until completely metabolized over a period of 2 years, and for this reason it is suitable for bone tissue engineering. Indeed, PCL degrades at a much slower rate than PLA and is therefore most suitable for the development of long-term implantable systems. By copolymerization of caprolactone with dilactide, it is also possible to tune the device degradation rates.  

Despite these advantages, in orthopedic applications the use of pure PCL is sporadic because considerable drawbacks arise from its low chemical versatility and ability to withstanding mechanical loads. Examples are pure PCL pins implanted in rabbit humeri, which revealed minimal inflammation and abundant periosteal callus production.Nevertheless, the feeble ossification rates indicated that these devices showed insufficient mechanical strength for load-bearing applications.  

Other attempts were made to synthesize solvent-free template arrays constituted of PCL nanowires, which would be able to encapsulate active ingredients and enhance mesenchymal stem cell performances. Zein et al. created alternating ply designs of PCL using the fused deposition model (FDM) method. Their results demonstrated that FDM could be used to produce scaffolds having adequate porosity for bone tissue engineering, while having mechanical properties just equal to soft trabecular bones. 

Several studies followed another philosophy to solve mechanical problems in the use of PCL copolymer instead of changing the fabrication technique. Rat bone marrow cells were seeded and cultured on a porous polymeric system composed of cornstarch blended with PCL. Results revealed that 

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Thermodynamics of fusion of poly-p-propiolactone and poly-e-caprolactone. Comparative analysis of the melting of aliphatic polylactone and polyester chains, Eur. Polym. J.. 8, 449-463, 1972. 

Anionic block copolymerizations of MM A with lactones proceeded smoothly to give copolymers with Mw/Mn = 1.11-1.23 when the monomers were added in this order. However, when the order of addition was reversed, no copolymerization took place [3c], i.e., no addition of MMA to the polylactone active end group occurred (Scheme 12). 

Dubois P, Jerome R, Teyssie P (1989) Macromolecular engineering of polylactones and polylactides. I. End-functionalization of poly-e-caprolactone. Polym Bull 22 475 82... 

Kricheldorf HR, Strieker A, LangankeD (2001) Polylactones, 50. The reactivity of cyclic and noncyclic dibutyltin bisaUcoxides as initiators in the polymerization of lactones. Macromol Chem Phys 202 2525-2534... 

Kricheldorf HR, Jonte JM, Dunsing R (1986) Polylactones. 7. The mechanism of cationic polymerization of (3-propionolactone and E-caprolactone. Makromol Chem 187 771-785... 

Kricheldorf HR, Sumbel MV (1988) Polylactones, 15. Reactions of 5-valerolactone and E-caprolactone with acidic metal bromides. Makromol Chem 185 317-331... 

Kricheldorf HR, Fechner B (2002) Polylactones. 59. Biodegradable networks via ring-expansion polymerization of lactones and lactides. Biomacromolecules 3 691-695... 

Kricheldorf FIR, Lee SR (1996) Polylactones. 40. Nanopretzels by macrocyclic polymerization of lactones via a spirocyclic tin initiator derived from pentaerythritol. Macromolecules 29 8669-8695... 

Kricheldorf HR, Lee SR (1995) Polylactones. 35. Macrocyclic and stereoselective polymerization of P-D,L-butyrolactone with cyclic dibutyltin initiators. Macromolecules... 

Kricheldorf HR, Eechner B (2001) Polylactones. 51. Resorbable networks by combined ring-expansion polymerization and ring-opening polycondensation of -caprolactone or DL-lactide. Macromolecules 34 3517-3521... 

Macrocyclic polycarbonyls are cyclic ligands containing carbonyl functionalities. Several categories come under this heading the macrocyclic oligoketones (7), the polylactones (8) and the polylactams (9). 

Polylactones (for an example, see Fig. 14.4) are synthetic analogues of naturally occurring ionophores such as enniatin (species that transport ions across biological membranes). Molecular mechanics calculations have been used to predict the stability and selectivity with respect to Li+, Na+, and K+ of a series of new polylactones12661. Metal-ligand interactions were again modeled using a combination of van der Waals and electrostatic terms. 

Star-shaped polylactones with functional end groups via ring-expansion polymerization of f-caprolactone or y-o,i-butyrolactonc with a spiroinitia-tor was synthesized as shown in Fig. 44 [146]. The spirocyclic polylactones formed were reacted with various carboxylic acid chlorides and yielded fourarmed stars with the elimination of Bu2SnCl2. Star arms with chloroacetate, 4-bromobenzoate, 4-nitrobenzoate, cinnamate, stearate, or methacrylate end groups were obtained by varying the acid chlorides. 

Polylactones and polylactides of high molecular weight are exclusively produced by the ROP of the corresponding cyclic monomers. A polyester is formed when cyclic esters are reacted with a catalyst or initiator. Scheme 1 presents the reaction pathway for the ROP of a cyclic ester. 

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Polyoxa macrocycles, known more commonly as the crown ethers, comprise an extensive area of research, with a repertoire of types and variations. Some of these macrocycles have been utilized predominantly for purposes other than metal ion complexation, and these will not be discussed in depth in this review. Included in this latter category are the polycarbonyls, polylactones,polylactams and carcerands. " ... 

The results of Figure 7 demonstrate that CO2 evolution from polyester films can also be measured and offers a convenient way of monitoring the photodegradation of polymers such as polylactones which are of increasing interest because of their biodegradability. However, unlike polyalkenes. 

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