Chapter 12
Polymerization and Copolymerization of Lactides and Lactones Using Some Lanthanide Initiators Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 29, 2016 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch012
Nicolas Spassky and Vesna Simic Laboratoire de Chimie des Polymères, U M R 7610, Université Pierre et Marie Curie, 4, place Jussieu, 75252 Paris Cedex 05, France
A series of lanthanide alkoxides were used as initiators for the polymerization of lactides and several lactones (ε-caprolactone, δ-valerolactone, β-butyrolactone). Most of these initiators allow a controlled polymerization in mild conditions, i.e., reaction in solution (dichloromethane, toluene) at room temperature. Yttrium tris-(alkoxyethoxides) appear to be among the most satisfactory initiators as concerning reactivity and selectivity for the polymerization of lactide, ε-caprolactone and also trimethylene carbonate. Block copolymers were prepared from ε-caprolactone and (D) or (L)-lactides. The blending of these block copolymers leads to the formation of stereocomplexes.
Polyesters derived from lactides and lactones are interesting materials for biomedical, pharmacological and packaging applications (7-5). They are usually obtained from the corresponding monomers by ring opening polymerization using different types of initiators as decribed in a recent review (6). In order to obtain polymers with well defined characteristics and to be able to synthesize block copolymers, initiators leading to a living type process arc desirable. Aluminum alkoxides were found to exhibit a controlled polymerization of some lactones and lactides (7-9) and the livingness of the polymerization systems was discussed in terms of the selectivity of the involved elementary reactions (10). Lanthanide alkoxides exhibit a much higher reactivity than other metal alkoxides for the polymerization of lactones and lactides and a living type behavior is observed for some of them (11-18). In this paper we want to discuss the results taken from the literature on the use of lanthanide initiators in the polymerization of cyclic esters and to present our recent investigations in this field.
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© 2000 American Chemical Society Scholz and Gross; Polymers from Renewable Resources ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
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Lanthanide alkoxide initiators Although other derivatives of lanthanides, such as halogenides (19) cyclopentadienyl (16,20) and bimetallic complexes (27), are studied in the polymerization and copolymerization of cyclic esters, lanthanide alkoxides appear to give up to now the more reproducible results. The structure cf the latter initiators, their functionality has been described in the literature. In our studies we have mainly used two families of lanthanide alkoxides, i.e. the oxoalkoxide clusters of Ln5^-0)OiPr)i3 type and the yttrium trisalkoxyethoxides of Y(OCH2CH20R)3 type. Some results obtained with a bimetallic aluminum-yttrium alkoxide will be also reported in this paper. Almost all of the lanthanide initiators used in this work were prepared in the laboratory of Prof. L.G. Huberl-Pfalzgraf (Université Sofia-Antipolis, Nice). The oxoalkoxide clusters ίη5(μ-0)(ΟΐΡΓ)ΐ3 where Ln is La, Sm, Y and Yb were prepared according to the procedure described by Poncelet et al for yttrium derivative (22). In these compounds five lanthanide atoms are linked to a single central oxygen atom. The isopropoxy ligands of these pentanuclear oxoaggregates are distributed in three different groups corresponding to the formula ίη5(μ5-0)(μ3-ΟίΡΓ)4(μ2-ΟίΡΓ)4(ΟιΡΓ)5. All lanthanide atoms are octahedrally surrounded by six isopropoxide groups. The overall functionality (number of active metal-alcoholate groups) cf these initiators is 2.6 (13/5). These pentanuclearoxoaggregate structure 1 3
determined by % and C NMR and X-ray diffraction remains in solution (23). Yttrium tris-alkoxyethoxides initiators [Y(OC2H40R)3]n were prepared from the reaction of yttrium with the corresponding alcohol according to the procedure described by Poncelet et al (24). The nuclearity (n) of these compounds depends on the R substituent and was found to be equal to n=2 for R=iPr, but reaches η = 10 for R=Me. The heterometallic derivative Y[Al(OiPr)4]3 was obtained from exchange reaction between yttrium cluster compounds and aluminum (25).
Homopolymerization alkoxides.
of
lactides
with
lanthanide
The polymerization of lactides using lanthanide alkoxides as initiators was recently reported in several works. In order to compare the reactivity of these compounds and kinetic results, a careful attention must be devoted to the experimental conditions used, e.g., monomer concentration [M] , monomer over initiator [M]/[I] ratio, temperature, solvent used etc. In fact, only general trends can be established since experimental conditions usually differ between groups. Concerning cluster compounds the commercially available Y5(0)(OiPr)o was used for the first time in the group of Feijen (75) for the polymerization of L-lactide. Later, a novel catalyst system derived from the reaction of tris (2,6-di-tert-butyphenoxy) yttrium and 2-propanol in situ was used and the kinetic data of both systems were compared (18). The 0
Scholz and Gross; Polymers from Renewable Resources ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
148 latter in situ system was much more reactive and conversion of 98% can be obtained in 5 min in dichloromethane solution at 22°C (with [M] = 0175 M and [M]/[I] = 166). Living type behavior with a narrow MWD (1.14) is observed. In our work recently reported (26) we have compared the polymerization cf (D,L)-lactide with différent lanthanide oxoalkoxide clusters where Ln was La, Sm, Y and Yb in dichloromethane solution at room temeprature. In the same experimental conditions [M] = 0.9M, and [M]/[I] = 68) the La initiator was found to be the most reactive and Yb initiator the less reactive. Based on the time of half reaction a comparison of reactivities gives the following range : 0
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 29, 2016 | http://pubs.acs.org Publication Date: January 15, 2001 | doi: 10.1021/bk-2000-0764.ch012
0
Ln 11/2 (min)
La
