Abstract
The synthesis and characterization of a series of 8-quinolinolato gallium complexes is presented, and the complexes are analogous to a series of aluminum complexes previously reported. The complexes have been shown to be active initiators for the ring-opening polymerization of rac-lactide. High degrees of polymerization control are demonstrated, as exemplified by the linear evolution of molecular weight as the polymerization progresses, narrow polydispersity indices, and molecular weights corresponding to those predicted on the basis of initiator concentration. Some of the initiators show iso-selective polymerization of rac-lactide, with Pi = 0.70. The polymerization rates have been monitored, and the pseudo first-order rate constants are compared to those of analogous aluminum compounds. The 8-quinolinolato gallium initiators show rates approximately 3 times higher than those of the series of aluminum compounds, while maintaining equivalently high iso-selectivity (Pi = 0.70) and polymerization control.
Highlights
As the demand and usage of polymers increases, so do issues associated with the sustainability of their source materials, their end of life fates, and costs
Polylactic acid (PLA) is manufactured by the ring-opening polymerization (ROP) of lactide (LA) (Figure 1), a process that is initiated by Lewis acidic metal alkoxide complexes.1b,2 Quite a range of metals have precedent for initiating ROP, and for further details on the scope of this catalysis, the reader is referred to several comprehensive reviews.1b,2,3 There has been a long-standing academic interest in aluminum-based initiators, due to the excellent control conferred by this metal and the potential for some Al complexes to enable stereocontrolled ROP of rac-lactide.[4]
We have previously reported a series of bis(8-quinolinolato) aluminum ethyl complexes, which, in the presence of iso-propyl alcohol, were efficient, iso-selective initiators (Pi = 0.76), for the ROP of rac-LA.[11]
Summary
As the demand and usage of polymers increases, so do issues associated with the sustainability of their source materials, their end of life fates, and costs. From the materials property perspective, the production of stereoblock or stereocomplex PLA, via the iso-selective ROP of rac-LA, remains a high priority target Such polymers exhibit significantly improved thermal-mechanical properties as compared to isotactic PLA; for example, the Tm for isotactic PLA is 170 °C, whereas that for stereoblock/stereocomplex PLA has been reported from 170 to 230 °C.5. The first report of a dialkyl gallium alkoxide complex, C (Figure 2), demonstrated the potential of this group 13 congener for fast, controlled polymerization catalysis.[9] A recent comparative study of aluminum and gallium complexes, with the structures represented by compound D (Figure 2), showed that the Ga catalysts were faster and well controlled as the Al counterparts (Ga >99% conversion, 100 equiv of LA, 353 K, toluene 1 h, versus Al 45% conversion under the same conditions).[10] Despite the difference in rate, the change in metal center did not affect the polymerization stereoselectivity, with moderate iso-selectivity being observed for both the Al and the Ga initiators (Pi = 0.70). In this Article, the preparation and application of a series of gallium complexes, with some of the same 8-quinolinato ligands, is presented
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