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From Science & Research

From Science & Research Advances in PLA chemistry by Alexander Hoffmann Sonja Herres-Pawlis Ludwig-Maximilians University Munich, Germany New robust catalysts for lactide polymerization: Zinc complexes of neutral nitrogen donors Ring-opening polymerisation (ROP) of lactide represents a growing field of research because the resulting polymers are biodegradable and based on renewable raw materials which ensures growing attention within the context of Green Chemistry. Up to now, neutral nitrogen donor ligands have been overlooked in their potential to stabilise catalytically active systems. This contribution highlights recent developments in this area as well as the applicability in the lactide polymerisation with special regard to the reaction conditions. Targeting a use in industrial scale, the tolerance towards moisture, air, lactide impurities and high temperatures is an important issue to be considered during catalyst design. For the well-controlled synthesis of polylactide with regard to composition, molecular weight and microstructure, the coordination-insertion process is now commonly regarded as the most efficient method [1-4]. This mechanism (Figure 1) involves the coordination of the monomer to the metal centre, followed by a nucleophilic attack of the alkoxide to the acyl carbon atom and the insertion of lactide into the metalalkoxide species with retention of configuration [5]. A new metal-alkoxide species is formed which is capable of further insertion reactions. Under industrial conditions, mostly homoleptic catalysts are used like tin(II)ethylhexanoate, zinc(II)lactate and aluminium isopropoxide in combination with alcohols as initiators [6]. These catalyst systems can be conveniently synthesised and utilised in the polymerisation of cyclic esters but complicated equilibria phenomena and multiple nuclearities of the active species result in limited polymerisation control. Detrimental side reactions like transesterifications and epimerisations may occur which lead to a broadening of the molar mass distribution. Consequently, the development of new catalysts for the ring-opening polymerisation of lactide has seen tremendous growth over the past decade [1-5,7]. As amelioration, these catalysts shall enable a better control, activity and selectivity during the polymerisation by optimal adaption of the coordinating ligands. A vast multitude of well-defined Lewis acid catalysts following a coordinationinsertion mechanism has been developed for this reaction mainly based on tin,[8] zinc,[9-12] aluminium[13-15] and rare earth metals [16-20]. To develop the polymer from a specialty material to a large-volume commodity plastic the development of new polymerisation catalysts is required. Most large-scale processes are based on the use of stannous compounds as initiators [3,4,7]. For use in food packaging or similar applications, heavy metals are undesirable because of accumulation effects [3,4]. To date, the design of new catalysts mostly follows the paradigm that an efficient lactide ROP initiating system needs an anionic ligand, e.g. alkoxides, amides, ketiminates or an alcohol as co-initiator which forms the truly active species as alkoxide. The high polymerisation activity of all these systems is often combined with high sensitivity towards air and moisture. For industrial purposes and especially the breakthrough of PLA in the competition with petrochemical based plastics, there is an exigent need for active catalysts that tolerate air, moisture and small impurities in the monomer [3,4,7]. The disadvantageous sensitivity can be ascribed to the anionic nature of the ligand systems stabilising almost all of these complexes. 36 bioplastics MAGAZINE [03/13] Vol. 8

From Science & Research the use of the sterically less demanding guanidine-pyridine ligands, a multitude of zinc complexes could be isolated and trends for the ROP activity were derived [29,30]. In case of the quinoline-guanidine complexes, mono(chelate) chlorido complexes exhibit smaller activity than the mono(chelate) acetato complexes [29,30]. Especially quinoline-guanidine bis(chelate) triflato zinc complexes exhibited very high activity and robustness towards monomer impurities at the same time. Using technical quality lactide, molecular weights of 70000 and 77000 g mol -1 (M n ) with PDs of 2 could be obtained with conversions of >90% [29,30]. Together with comparative studies with guanidine mesylato complexes,[31] it came up that within the bis(chelate) triflato zinc complexes the zinc atom possesses a high positive partial charge and the guanidine a pronounced negative charge. Figure 1. Coordination-insertion mechanism for lactide ROP The role of neutral donor ligands for the stabilisation of ROP active systems has to be highlighted because this niche has been overlooked for years [21]. With regard to industrial usefulness, only systems with real applicability in lactide bulk polymerisation are discussed here (Figure 2). The scope of used neutral N donors ranges from simple alkylated amines and substituted pyridines over guanidines to sophisticated oxabispidines and oxalamidines. Historically, the pyridinecarbene zinc complexes of Tolman and coworkers are the first complexes of this class; they polymerise lactide at 140°C within minutes with a polydispersity (PD) of 2.4 [22]. The robust 9-oxabispidine zinc acetate complex has been reported as ROP active system in the lactide melt at 150°C (PD = 2) but with low yields [23]. As rather simple neutral ligand systems, the classic N donor ligands 2,2´-bipyridine and 1,10-phenanthroline were proven to stabilise zinc complexes with surprising ROP activity under challenging conditions in melt in 2009 [24]. The polydispersities of approximately 2 account for the presence of transesterification reactions. In order to overcome the limitations of anionic and other sensitive ligand systems, the potential of a neutral but highly nucleophilic ligand system was evaluated. Guanidines convince by their good donor properties and their strong nucleophilicity [25,26]. In 2007, the first cationic complex [Zn(DMEG 2 e) 2 ][OTf] 2 comprising an aliphatic bis(guanidine) has been reported as active ROP catalyst for the lactide polymerisation in melt at 150°C [27]. In following studies with the closely related but more basic imino-imidazoline 8MeBL, it appeared that the partial charge at the zinc atom as well as on the donating Nimine atom is crucial for the lactide activity [28]. Using mono(chelate) zinc imino-imidazoline complexes high conversions of 88 % were observed. With As guanidines are strong neutral donors, their nucleophilicity was proposed to help the ring-opening reaction. In all these polymerisation experiments with commercial grade PLA, no external initiator had been added. Hence, the working hypothesis implied the coordination of the lactide to the zinc centre followed by a nucleophilic attack of the guanidine on the carbonyl C atom of the lactide molecule. Guided by this idea, extensive density functional studies for the ROP with guanidine triflato zinc complexes were accomplished [32]. In fact, this computational study is the first DFT study for the ROP with neutral ligands without additional co-initiators. The fluorescence activity of the guanidine-quinoline ligands gave further mechanistic hint because the quinoline-related emission can be traced in the zinc complexes and the resulting polylactide. Moreover, the UV absorption of the guanidine-quinoline ligands was found in the corresponding lactide as well [32]. In summary, these studies showed that the guanidine zinc triflato complexes react in a variant of the coordination-insertion mechanism with the nucleophilic attack to the lactide performed by the guanidine and the classic ring-opening step as next transition state [32]. The great impact of the guanidine is expressed in two central traits: the excellent donor capacity stabilises very robust zinc complexes and the high nucleophilicity of the guanidines enables the ring-opening of cyclic esters by the guanidine donor functionality. The great advantage of guanidine systems is their extraordinary robustness towards moisture and monomer impurities. Until now, comparable robust systems have only been reported by Davidson et al.[33] who used tris-phenolate titanium complexes. However, the zinc guanidine systems combine in a unique manner many crucial features for efficient large-scale lactide ROP. In detail, the robustness of zinc guanidine complexes in lactide ROP supersedes monomer recrystallisation or sublimation and saves cost-effective processing steps. Moreover, the polymerisation can be accomplished under melt conditions at high temperatures up to 200°C without racemisation effects [32]. This is important for further applications in reactive polymer extrusion. bioplastics MAGAZINE [03/13] Vol. 8 37

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