Metal‐Size Influence in Iso‐Selective Lactide Polymerization

Abstract

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a P i value of 0.81-0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (P s = 0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La > Y > Lu.Polylactide (PLA), a degradable polymer obtained from renewable resources, is one of the leading commercial alternatives to petrochemical plastics. [1] PLA is produced by the metal-catalyzed ring-opening polymerization (ROP) of lactide (LA). [2] The central challenge in this field of catalysis is to combine high rates with excellent stereocontrol, ideally without the need for expensive chiral auxiliaries or ligands. [3] Iso-selectivity using rac-LA is especially important and useful because the product, stereoblock/complex PLA, has superior properties. [4] For example, it is a crystalline polymer with a higher melting temperature (T m ) and better mechanical properties than isotactic poly-L-lactide (PLLA). In some cases, T m is elevated by as much as 50 8C, greatly improving thermal stability and enabling PLA to compete as an engineering polymer. [5] However, rac-LA iso-selective catalysts remain scarce, with the majority being chiral aluminum salen complexes or derivatives. [3b, 4a, 6] Although these compounds show impressive degrees of stereocontrol, they are often extremely slow and require unacceptably high catalyst loadings, typically taking hours or days to reach completion even at 1 mol % catalyst loading. There are only a handful of other, non-aluminum based, iso-selective catalysts, [7] the structures of three of the most selective of these are shown in Figure 1. [8] In 2008, Arnold and co-workers reported a homochiral yttrium complex that showed good iso-selectivity (C, P i = 0.75, 298 K). [8a,b] Subsequently, we reported yttrium phosphasalen complexes (Figure 1, structures A and B), which combined very high rates with promising isoselectivity (P i = 0.74, 298 K). [8c] Herein, we report the performance of lutetium and lanthanum phosphasalen complexes. For such Group 3 lanthanide complexes, the coordination geometries are predominantly influenced by steric factors. In this regard, it is relevant that lutetium has a slightly smaller covalent radius than yttrium (1.87 versus 1.90 ), whilst lanthanum is significantly larger (2.07 ). [9] The use of different lanthanide centers enables an investigation of the influence of atomic size and coordination environment on polymer tacticity. Both metals have precedence in lactide ROP catalysis, [10] although lutetium is rarely investigated. [10b,e,f,h] Okuda a...

One‐pot regioselective and stereoselective terpolymerization of rac‐lactide, CO2 and rac‐propylene oxide with TPPMCl (M = Cr, Co, Al)/PPNCl binary catalyst

“…GPC analysis of pure copolymers using polystyrene standard in tetrahydrofuran; the experimental M n was calculated considering the Mark−Houwink corrections for M n ( M n (obs) = 0.58[ M n (GPC)]); units Daltons. Comparisons between the experimental and theoretical M n values are limited.…”

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

“…In this context, biologically benign metal-based catalysts are of great interest for the production of this bioassimilable material, with calcium [ 22 , 23 , 24 , 25 ], magnesium [ 26 , 27 , 28 , 29 ] and zinc [ 30 , 31 , 32 , 33 , 34 , 35 ] (see Chart 2 ), as the most representative centres, although other metals from group 13 (aluminium [ 36 ] and indium [ 37 ]), group 4 [ 38 ], as well as rare earth [ 39 ] metals, have been also successfully described, all of them supported by a wide variety of ancillary ligands. These examples constitute a much greener alternative to the toxic industrially employed tin(II) 2-ethylhexanoate, which, despite its robustness, offers poor control of polymer parameters [ 40 ].…”

Ring opening polymerisation of lactide with uranium(iv) and cerium(iv) phosphinoaryloxide complexes

“…6 Although often overlooked due to misconceptions of scarcity and excessive oxophilicity, the f-block cations possess a unique capability to tune the ROP performance due to the available range of size and Lewis acidity. [7][8][9][10] This variability is difficult for other metals; the propensity of In III to adopt a coordination number of 5 limited the scope in our system. 11 While rare, both Ce III and Ce IV are known to initiate ROP reactions, and judicious choice of ligands can dramatically alter activity.…”