Study of Ligand Substituent Effects on the Rate and Stereoselectivity Of
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Study of ligand substituent effects on the rate SPECIAL FEATURE and stereoselectivity of lactide polymerization using aluminum salen-type initiators Pimpa Hormnirun, Edward L. Marshall, Vernon C. Gibson*, Robert I. Pugh, and Andrew J. P. White Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AY, United Kingdom Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved June 26, 2006 (received for review April 5, 2006) -A series of aluminum salen-type complexes [where salen is N,N bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that dif- fer in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geome- tries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron- withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho sub- stituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselec- tivity generally increases with increased flexibility of the backbone CHEMISTRY linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor. catalysis ͉ polyesters n recent years, Al(salen) complexes [where salen is N,NЈ- Ibis(salicylaldimine)-1,2-ethylenediamine] have been widely in- vestigated for their ability to initiate the stereocontrolled poly- merization of lactide (LA) (1–17) to give a material, polylactide (PLA), which has a range of biomedical, pharmaceutical, and agricultural applications (18–21). A convenient synthetic route to PLA is the ring-opening polymerization (ROP) of LA, the cyclic diester of lactic acid, which is derivable from renewable resources such as corn starch, sugars, and dairy produce. Metal alkoxides, for example, those of Al, Zn, Mg, Y, Ln, Sn(II), Sn(IV), Fe(II), and Fe(III), are typically used to catalyze the ROP of LA and related cyclic esters. Several reviews describing the mechanism of ROP and the types of initiators and catalysts have been published recently (22–27). Fig. 1. Aluminum initiators and pro-initiators used in the ROP of rac-LA. Following initial findings that simple (salen)Al complexes such as I–III (Fig. 1) could initiate a relatively controlled polymerization of rac-LA to moderately isotactic PLA (1, 2), shown to polymerize rac-LA with high isoselectivity and excel- interest developed in the potential of such initiators to mediate lent control in both solvent-based and solvent-free polymeriza- the stereoselective polymerization of rac-LA by exploiting chiral tions (14, 15). ligand backbones. Thus, the enantiomerically pure chiral alumi- An interesting and potentially useful aspect of (salen)Al num complex, (R)-IV, was shown to selectively consume D-LA systems is their apparent ability to mediate isoselective poly- from a racemic mix (kD͞kL ϭ 20) to give optically active isotactic merizations of rac-LA by using an achiral ligand system such as poly(D-LA) (9). The related derivative, (R)-V, was exploited to obtain syndiotactic PLA from meso-LA with an enantiotopic Conflict of interest statement: No conflicts declared. selectivity of 96% (10, 11). By using the optically inactive racemate, (rac)-VI (12, 13), a tapered stereoblock copolymer was This article is a PNAS direct submission. obtained from rac-LA. A polymer-exchange mechanism was Abbreviations: LA, lactide; PLA, polylactide; ROP, ring-opening polymerization; salen, N,NЈ-bis(salicylaldimine)-1,2-ethylenediamine; TBP, trigonal bipyramidal. proposed where each enantiomer of (rac)-VI preferentially Data deposition: Crystallographic data for the structures reported in this work have been polymerizes one enantiomer of rac-LA, but the growing chains deposited in the Cambridge Structural Database (CSD), Cambridge Crystallographic Data undergo exchange at aluminum centers of opposite chirality to Centre, Cambridge CB2 1EZ, United Kingdom [CSD reference nos. 292440–292447 (struc- produce blocks composed of the other enantiomer of LA with tures 6, 7, 11, 13, 17, 19, 20, and 24, respectively)]. averaged block length of 11 monomer units (12). More recently, *To whom correspondence should be addressed. E-mail: [email protected]. the cyclohexane derivatives, (R)-VII and (rac)-VIII, have been © 2006 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602765103 PNAS ͉ October 17, 2006 ͉ vol. 103 ͉ no. 42 ͉ 15343–15348 Fig. 2. Synthetic pathway for the preparation of complexes 1–24. those contained in IX and X (6). The isoselectivity is enhanced for initiators bearing more flexible C3 linkers, and introducing large ortho-phenoxy substituents also was found to increase the isotacticity. Further tacticity enhancements have been reported for the dimethyl-substituted derivative XI (7). The mechanism of stereoselective polymerization using these achiral ligand-based complexes is postulated to operate by means of a chain end control process (6), whereas an enantiomorphic-site control mechanism is proposed for the chiral salen-based complexes (10). However, recent studies have highlighted some complexi- ties in ascribing the mechanism to chain-end control or enanti- omorphic-site control (16). The chirality of the ligand bonded to the metal, the chirality of the end group of the growing polymer chain, and the solvent all seem to play a complex and rather unpredictable role in influencing the stereopreference in a racemic monomer mixture. With regard to the productivities of (salen)Al systems, in the rather limited number of studies to date it has been found that the rate of polymerization is enhanced by appending electron- withdrawing substituents to the phenoxide donor (4). Enhanced activities also have been observed by using a salicylketimine derivative containing neither bulky nor electron-withdrawing substituents on the phenoxide donor (5). Although Al(salen) complexes are attractive as initiators because of their ease of preparation and the stability afforded by the tetradentate ligand, it is clear that significant gaps remain in Fig. 3. Complexes 1–24 synthesized as part of this study from Groups A–H. our knowledge of the fundamental factors influencing activity and selectivity. We therefore initiated a systematic investigation knowledge, only complexes 1, 2, 4, and 10 have been reported into the LA polymerization behavior of a family of Al(salen) previously (29–31). initiator systems, in which the phenoxy substituents and the The 1,2-ethylenediamino-based complexes 1–7 were all ob- length and nature of the diimino-linking units are varied, with a tained as yellow crystalline materials after recrystallization view to obtaining an improved understanding of the influence of either directly from the reaction medium for the less-soluble salen-type ligands on the rate and stereoselectivity of rac-LA examples (e.g., 3) or from MeCN (full experimental details for polymerization. complexes 1–24 are provided in Supporting Text, Figs. 10–42, and Tables 2–7, which are published as supporting information on Results the PNAS web site). The presence of the methyl substituents on Pro-Initiator Synthesis. The salicylaldimines used in this study were the backbone of 5–7 renders these complexes significantly more prepared through condensation of two equivalents of commercially soluble than their nonsubstituted counterparts, and hence re- available salicylaldehydes (␣-hydroxybenzaldehydes) with an ␣,- crystallized yields of these were generally lower than for 1–4 diamine according to standard literature procedures (28). The (e.g., 37%, 6; 45%, 7). The difference in solubility is particularly salicylaldimines then were reacted with one equivalent of trimethy- apparent for the dichlorophenoxy derivatives 3 and 6, the former laluminum at 110°C for 18 h to afford the (salen)Al(CH3) com- being insoluble in hot toluene, whereas complex 6 is soluble in plexes in moderate to high yields according to Fig. 2. The complexes room-temperature toluene solution. This observation suggests synthesized (1–24) (Fig. 3) span C2 and C3 backbones of varying that the low solubility of other metal salen complexes, a problem degrees of rigidity (Groups A–F) along with two additional families commonly encountered with such species, may be rectified by containing arylene linkers (Groups G and H). Attempts to synthe- introducing alkyl groups onto the diimino linker. size the R1 ϭ R2 ϭ H and R1 ϭ R2 ϭ Cl members of Group F led The 1H NMR spectra of complexes 1–4 all feature complex to intractable mixtures of low-solubility products. To the best of our second-order multiplets in the region ␦ 2.0–4.0 for the protons 15344 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602765103 Hormnirun et al. SPECIAL FEATURE Fig. 5. Molecular structure of 17. Selected bond lengths (Å) and angles (°) are Fig. 4. Molecular structure of 6. Selected bond lengths (Å) and angles (°) are as follows: Al–O(1) 1.835(2); Al–N(7) 1.995(2); Al–N(15) 2.096(2); Al–O(21) as follows: Al–C 1.949(7); Al–O(1) 1.825(4); Al–N(7) 1.997(5); Al–N(10) 2.042(5); 1.786(2); Al–C(38) 1.958(3); C(7)–N(7) 1.288(4); C(15)–N(15) 1.289(4): O(21)–Al– Al–O(16) 1.808(4); C(7)–N(7) 1.290(9); N(10)–C(10) 1.293(9): C–Al–O(1) O(1) 89.09(8); O(21)–Al–C(38) 122.74(14); O(1)–Al–C(38) 99.77(11); O(21)–Al– 103.2(3); C–Al–N(7) 110.7(3); C–Al–N(10) 94.9(3); C–Al–O(16) 120.8(3); O(1)– N(7) 121.60(10); O(1)–Al–N(7) 88.72(9); C(38)–Al–N(7) 115.10(13); O(21)– CHEMISTRY Al–N(7) 90.3(2); O(1)–Al–N(10) 161.0(2); O(1)–Al–O(16) 89.4(2); N(7)–Al–N(10) Al–N(15) 87.32(9); O(1)–Al–N(15) 168.79(10); C(38)–Al–N(15) 91.09(11); N(7)– 77.8(2); N(7)–Al–O(16) 127.2(2); N(10)–Al–O(16) 86.2(2).