Lactide Polymerisation

 

In the new century, two severe problems have to be solved: the society’s dependence on petrochemical resources and the growing amounts of non-compostable and sometimes poisonous waste spreaded in the environment. Due to the growing ecological awareness, the sustainable development leads to an increasing demand for biodegradable plastics based on renewable raw materials. In an effort to solve these problems, numerous studies have been undertaken on the synthesis of biodegradable polymers. Over the last 30 years, great advances in the synthesis, manufacture, and processing of these materials have enabled a broad range of practical applications from short-term packaging to highly sophisticated biomedical devices.

Among the variety of biodegradable plastics, aliphatic polyesters have conquered the bioplastics market because hydrolytic and/or enzymatic chain cleavage yields w-hydroxyacids, which in most cases are completely metabolised.Within the intensive research in the field of aliphatic polyesters, polylactide or poly(lactic acid) (PLA) has gained the greatest attention due to its favourable chemical and mechanical properties. PLA shows mechanical similarities to poly(ethylene terephthalate) (PET) but also to poly(propylene) (PP). It can be processed in most polymer processing equipment which is a critical factor for industrial use. But even more important: It appears to be the polymer with the broadest range of applications because of its ability to be stressed or thermally crystallised, filled and copolymerised.Until now, PLA has found many fields of application, for example as biomedical implants and sutures (owing to its biocompatibility), or for packaging, films and fibres.

PLA can be produced from inexpensive, annually renewable raw materials. Corn or sugar beets are the most significant sources of lactic acid, but agricultural waste and non-food quality carbohydrates may be used as well. After bacterial fermentation of the carbohydrate feed, lactic acid is readily available and converted to lactide. PLA can be produced by ring-opening polymerisation (ROP) of lactide (LA), the cyclic diester of lactic acid, or by direct polycondensation of lactid acid. The polylactide can be either recycled or composted after use and therefore is CO2 emission neutral. Hence, PLA is less environmentally costly than common recyclable polymers which can be only recycled by a limited number of times before losing the desired material properties. Among the numerous polyesters studied to date, PLAs have proven to be the most attractive and useful class of biodegradable polyesters.

We study pyridine and guanidine stabilised zinc complexes which are active without the presence of anionic co-ligands. Chloride, acetate and triflate served here as charge-compensating unit. Screening of a multitude of these zinc guanidine complexes allowed to evaluate a structure-reactivity relationship. The variety within this ligand class encompasses aliphatic and aromatic framework as well as polydentate systems and combinations with other donors. In fact, quinoline-guanidine zinc triflate complexes show the best performance with high molecular weights and controlled polymerisation behaviour. A combined spectroscopic, kinetic and theoretical study revealed that the coordination-insertion mechanism is valid for these complexes. Kinetic studies demonstrated that the reaction obeys first order kinetics and proceeds with living character. UV/Vis and fluorescence measurements indicated that the ligand binds to the polymer and remains as chain end. This together with the absence of racemisation reactions during the polymerisation of pure l-lactide leads to the proposition that the polymerisation proceeds via a coordination-insertion mecha­nism. The guanidine functions of the coordinated ligand act as nucleophile and accomplish the ring-opening. The role of the only weakly coordinating triflate as non-nucleophile has to be highlighted as it does not coordinationally compete with guanidine or lactide. Hence, these initiators allow the ROP without additional co-catalysts like alcohols or alkoxides. These studies are currently extended towards other cyclic esters and they demonstrate that the classical paradigm of anionic ligands for ROP can be overcome. For the first time we have proven that a class of complexes containing neutral ligands is able to promote lactide polymeri­sation in an efficient way.

In summary, the comprehensive concept of robust zinc guanidine systems has been proven to yield efficient and versatile ROP active catalysts. 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 Nimine donor functionality. In general, the importance of neutral ligands for the ring-opening polymerisation of lactide cannot be underestimated as further examples of ROP active complexes using simple diamine ligands have demonstrated. With regard to the major breakthrough of bioplastics for the substitution of petrochemical plastics in the commodity market, every robust catalyst system represents a huge step towards greater sustainability of our society. In the mid-term, this will improve the economic situation for biodegradable polymers from renewable resources and establish polylactide in many fields of polymer applications.