Catalysts as Key Enablers for the Synthesis of Bioplastics with Sophisticated Architectures.

Bioplastics are one of the answers to environmental pollution and linear material flows. The most promising bioplastic polylactide (PLA) is already replacing conventional plastics in a number of applications. The properties of PLA, however, do not fit for all potential application areas, but they can be altered by the introduction of comonomers. The copolymerization of lactide (LA) with other lactones like ϵ-caprolactone (CL) has been established for several years. Nevertheless, controlling copolymerizations remains a challenge due to the high complexity of the system. Copolymerization of LA with other monomer classes is much less investigated, but has the chance to overcome the limitations in material properties that occur when only lactones are used. The crucial factor for all copolymerizations is the catalyst. It dominates the reaction kinetics and determines the resulting microstructure. In this review, copolymerization catalysts for LA are presented divided into catalysts for the synthesis of lactone block copolymers, lactone random copolymers, and multimechanistically synthesized copolymers. The selected catalysts are highlighted either owing to their industrially applicable polymerization conditions or their non-standard mechanism.

Master of Chaos and Order: Opposite Microstructures of PCL-co-PGA-co-PLA Accessible by a Single Catalyst.

One catalyst, two reaction set-ups, three monomers and unlimited macromolecular microstructural designs: The iron guanidine complex [FeCl2 (TMG5NMe2 asme)] (1) polymerizes lactide faster than the industrially used Sn(Oct)2 and shows high activity towards glycolide and ϵ-caprolactone. Its distinguished features enable the synthesis of both block and random-like copolymers in the melt by a simple change of the polymerization set-up. Sequential addition of monomers yields highly ordered block copolymers including the symmetrical PLA-b-PGA-b-PCL-b-PGA-b-PLA pentablock copolymers, while polymerizations of monomer mixtures feature enhanced transesterifications and pave the way to di- and terpolymers with highly dispersed repeating unit distributions. A robust catalyst active under industrially applicable conditions and producing copolymers with desired microstructures is a major step towards biocompatible polymers with tailor-made properties as alternatives for traditional plastics on the way towards a sustainable, circular material flow.

Active in Sleep: Iron Guanidine Catalyst Performs ROP on Dormant Side of ATRP.

Copolymers are the answer to property limitations of homopolymers. In order to use the full variety of monomers available, catalysts active in more than one polymerization mechanism are currently investigated. Iron guanidine catalysts have shown to be extraordinarily active in ROP of lactide and herein prove their versatility by also promoting ATRP of styrene. The presented iron complex is the first polymerizing lactide and styrene simultaneously to a defined block copolymer in a convenient one-pot synthesis. Both mechanisms work hand in hand with ROP using the dominantly present FeII species on the dormant side of the ATRP equilibrium. This orthogonal copolymerization by a benign iron catalyst opens up new pathways to biocompatible polymerization procedures broadening the scope of ATRP applications.

Undiscovered Potential: Ge Catalysts for Lactide Polymerization.

Polylactide (PLA) is a high potential bioplastic that can replace oil-based plastics in a number of applications. To date, in spite of its known toxicity, a tin catalyst is used on industrial scale which should be replaced by a benign catalyst in the long run. Germanium is known to be unharmful while having similar properties as tin. Only few germylene catalysts are known so far and none has shown the potential for industrial application. We herein present Ge complexes in combination with zinc and copper, which show amazingly high polymerization activities for lactide in bulk at 150 °C. By systematical variation of the complex structure, proven by single-crystal XRD and DFT calculations, structure-property relationships are found regarding the polymerization activity. Even in the presence of zinc and copper, germanium acts as the active site for polymerizing probably through the coordination-insertion mechanism to high molar mass polymers.

Tuning a robust system: N,O zinc guanidine catalysts for the ROP of lactide.

Non-toxic, highly active and robust complexes are the holy grail as ideal green catalysts for the polymerisation of biorenewable and biodegradable polylactide. Four new zinc guanidine complexes [ZnCl2(TMG4NMe2asme)], [ZnCl2(TMG5Clasme)], [ZnCl2(TMG5Measme)] and [ZnCl2(TMG5NMe2asme)] with different electron-donating and electron-withdrawing groups on the ligand's aromatic backbone have been synthesised. Ligands are derived from low-cost commercially available compounds and have been converted by a three- or four-step synthesis process into the desired ligand in good yields. The compounds have been fully characterised and tested in the ROP of rac-LA under industrially relevant conditions. The complexes are based on the recently published structure [ZnCl2(TMGasme)] which has shown high activity in the polymerisation of lactide at 150 °C. Different substituents in the para-position of the guanidine moiety significantly increase the polymerisation rate whereas positioning substituents in the meta-position causes no change in the reaction rate. With molecular weights over 71 000 g mol-1 being achievable, the best system produces polymers for multiple industrial applications and its polymerisation rate approaches that of Sn(Oct)2. The robust systems are able to polymerise non-purified lactide. The initiation of the polymerisation is suggested to occur due to impurities in the monomer.

New Kids in Lactide Polymerization: Highly Active and Robust Iron Guanidine Complexes as Superior Catalysts.

Polylactide is a biodegradable versatile material based on annually renewable resources and thus CO2 -neutral in its lifecycle. Until now, tin(II)octanoate [Sn(Oct2 )] was used as catalyst for the industrial ring-opening polymerization of lactide in spite of its cytotoxicity. On the way towards a sustainable catalyst, three iron(II) hybrid guanidine complexes were investigated concerning their molecular structure and applied to the ring-opening polymerization of lactide. The complexes could polymerize unpurified technical-grade rac-lactide as well as recrystallized l-lactide to long-chain polylactide in bulk with monomer/initiator ratios of more than 5000:1 in a controlled manner following the coordination-insertion mechanism. For the first time, a biocompatible complex has surpassed Sn(Oct)2 in its polymerization activity under industrially relevant conditions.