Final Report Summary - CATYLENE (New Catalysts for Degradable Polyethylene)
This project was focused on two main parts based on the nature of the catalyst used with special focus on the polymerisation of monomers derived from renewable resources such as pentadecalactone (PDL), ambrettolide and globalide. In this context and as part of our project's objective detailed in the annex 1 (Marie Curie grant agreement), our first interest was focused on the catalytic ring-opening polymerisation (cROP) of the fully biobased PDL monomer, a 16-membered macrolactone that produces a polyester with polyethylene-like properties. Thus, during the first year, two strategies have been investigated for the polymerisation of macrolactones: metal free catalysis using organic molecules and single-site metal catalysts based on zinc and aluminum supported by multi-dentate ligands. Both routes toward those polyesters preparation are investigated as alternative strategies to the enzymatic catalysis, which is by far, the most reported route for macrolactones polymerisation. However, this biocatalytic route has its drawbacks such as enzyme costs, limited control over molecular weight and polymer microstructure and restricted reaction temperature (no melt polymerisation) which makes the research of alternative methods particularly interesting in term of economical and practical issues.
The first interest was given to the use of organic catalysts such as guanidines, amidines and N-heterocyclic carbenes (NHCs) for the ROP of PDL macrolactone, caprolactone (CL) and the PDL/CL copolymerisation. Among all the organic molecules screened in the first part of this project, TBD in combination with an initiator alcohol (BnOH, triol), proved to be the only active system for the ROP of PDL as well as for the opolymerisation of PDL and CL. The kinetic studies revealed first-order behaviour of the TBD/BnOH system in the first stage of the reaction or at high catalyst concentration. Unfortunately, using the organic catalyst TBD for PDL polymerisation resulted in significant transesterification as a side reaction and consequently polymers with only moderate molecular weight were formed. Particularly, at higher conversions, transesterification starts to play an increasingly important role leading to broader polydispersities and a decrease in molecular weight. Transesterification also became more prominent at higher polymerisation temperatures but more importantly also at low temperatures in the presence of conventional protic solvents. On the other hand, using the binary system TBD/BnOH as catalyst / initiator and a mixtures of PDL and CL, poly(PDL-co-CL) random copolymers with tunable comonomer composition and melting temperatures can be obtained. The random copolymers poly(PDL-co-CL) remain highly crystalline over the whole composition range and show a linear relationship between the random copolymers melting temperatures (Tm) and comonomer content. Although rapid competitive transesterification prevents the formation of block copolymers, the preparation of polypentadecalactone (PPDL) homo- or copolymers with specific functional end groups and variable architectures (i.e. branched) is potentially possible by a meticulous selection of the initiator group. The detailed results concerning this part have been published recently in Macromolecules (2012, 45, pp. 3356 - 3366).
After the recent breakthrough reported by our group in the cROP of macrolactones using aluminum salen complexes, which were found to be remarkably efficient catalysts for the cROP of PDL and medium-sized lactones (CL, L-lactide, etc.) producing high molecular weight PPDL, and in order to expand our interest to the valorisation of those promising renewable macrolactones, the second part of this project has been dedicated to the study of the ROP of macrolactones using metallic catalysts. This part is conducted in collabouration with the petrochemical company Sabic Europe (Geleen, the Netherlands) and was mainly focused on the preparation of new zinc and aluminum metal complexes supported by bidentate (guanidinate and amidinate) and tridentate (amine functionalised phenoxy-imine) ligands and their performances in the cROP of macrolactones derived from renewable resources with particular emphasis on the cROP of PDL monomers.
In the first instance, all the zinc and aluminum complexes prepared during this study have been found as efficient catalysts for the cROP of PDL and CL both in toluene solution and bulk molten monomer. Particularly, from the polymerisation reactions carried out in bulk at 100°C, two catalysts (ZnGuan-NTMS and FIZn-Et, Chart 1) have been found as truly efficient both in homo- and copolymerisation reactions of those large lactones (PDL, ambrettolide, globalide). Interestingly, the formation of random poly(PDL-co-CL) and block poly(PDL-b-CL) copolymers has been observed using the zinc catalysts supported either by guanidinate and phenoxy-imine ligands.
Varying the experimental conditions, highly crystalline random and block copolymers have been prepared using the binary system catalyst / BnOH. The blocky structure of poly(PDL-b-CL) prepared using ZnGuan-NTMS has been randomised by the addition of a catalytic amount of TBD, highlighting the high degree of polymerisation control for the metallic catalysts reported in this study. For the random copolymer poly(PDL-co-CL), the linear relationship between the random copolymers melting temperatures (Tm) and comonomer content was revealed by the thermal analysis. Interestingly, high molecular weight polymers could only be obtained using the zinc and aluminum catalysts supported by amine functionalised phenoxy-imine ligands both in PDL polymerisation and in PDL/CL copolymerisation.
On the other hand, reactions conducted with different amount of BnOH as initiator / chain transfer agent have been conducted with the zinc-guanidinate complex (ZnGuan-NTMS) and the results confirm the effective immortal character of this catalyst since the addition of an excess of the protic BnOH increase the reaction rate with complete conversion up to 600 equivalent of PDL to catalyst molar ratio. This property could be used to minimise the amount of the catalyst and contribute to the limitation of the contamination of the final polymer by the metallic residue. In conclusion, the major trends concerning this part were established and high interest has been expressed by our industrial partner and a patent application will be filled in order to claim some of our catalysts and copolymer materials.
The first interest was given to the use of organic catalysts such as guanidines, amidines and N-heterocyclic carbenes (NHCs) for the ROP of PDL macrolactone, caprolactone (CL) and the PDL/CL copolymerisation. Among all the organic molecules screened in the first part of this project, TBD in combination with an initiator alcohol (BnOH, triol), proved to be the only active system for the ROP of PDL as well as for the opolymerisation of PDL and CL. The kinetic studies revealed first-order behaviour of the TBD/BnOH system in the first stage of the reaction or at high catalyst concentration. Unfortunately, using the organic catalyst TBD for PDL polymerisation resulted in significant transesterification as a side reaction and consequently polymers with only moderate molecular weight were formed. Particularly, at higher conversions, transesterification starts to play an increasingly important role leading to broader polydispersities and a decrease in molecular weight. Transesterification also became more prominent at higher polymerisation temperatures but more importantly also at low temperatures in the presence of conventional protic solvents. On the other hand, using the binary system TBD/BnOH as catalyst / initiator and a mixtures of PDL and CL, poly(PDL-co-CL) random copolymers with tunable comonomer composition and melting temperatures can be obtained. The random copolymers poly(PDL-co-CL) remain highly crystalline over the whole composition range and show a linear relationship between the random copolymers melting temperatures (Tm) and comonomer content. Although rapid competitive transesterification prevents the formation of block copolymers, the preparation of polypentadecalactone (PPDL) homo- or copolymers with specific functional end groups and variable architectures (i.e. branched) is potentially possible by a meticulous selection of the initiator group. The detailed results concerning this part have been published recently in Macromolecules (2012, 45, pp. 3356 - 3366).
After the recent breakthrough reported by our group in the cROP of macrolactones using aluminum salen complexes, which were found to be remarkably efficient catalysts for the cROP of PDL and medium-sized lactones (CL, L-lactide, etc.) producing high molecular weight PPDL, and in order to expand our interest to the valorisation of those promising renewable macrolactones, the second part of this project has been dedicated to the study of the ROP of macrolactones using metallic catalysts. This part is conducted in collabouration with the petrochemical company Sabic Europe (Geleen, the Netherlands) and was mainly focused on the preparation of new zinc and aluminum metal complexes supported by bidentate (guanidinate and amidinate) and tridentate (amine functionalised phenoxy-imine) ligands and their performances in the cROP of macrolactones derived from renewable resources with particular emphasis on the cROP of PDL monomers.
In the first instance, all the zinc and aluminum complexes prepared during this study have been found as efficient catalysts for the cROP of PDL and CL both in toluene solution and bulk molten monomer. Particularly, from the polymerisation reactions carried out in bulk at 100°C, two catalysts (ZnGuan-NTMS and FIZn-Et, Chart 1) have been found as truly efficient both in homo- and copolymerisation reactions of those large lactones (PDL, ambrettolide, globalide). Interestingly, the formation of random poly(PDL-co-CL) and block poly(PDL-b-CL) copolymers has been observed using the zinc catalysts supported either by guanidinate and phenoxy-imine ligands.
Varying the experimental conditions, highly crystalline random and block copolymers have been prepared using the binary system catalyst / BnOH. The blocky structure of poly(PDL-b-CL) prepared using ZnGuan-NTMS has been randomised by the addition of a catalytic amount of TBD, highlighting the high degree of polymerisation control for the metallic catalysts reported in this study. For the random copolymer poly(PDL-co-CL), the linear relationship between the random copolymers melting temperatures (Tm) and comonomer content was revealed by the thermal analysis. Interestingly, high molecular weight polymers could only be obtained using the zinc and aluminum catalysts supported by amine functionalised phenoxy-imine ligands both in PDL polymerisation and in PDL/CL copolymerisation.
On the other hand, reactions conducted with different amount of BnOH as initiator / chain transfer agent have been conducted with the zinc-guanidinate complex (ZnGuan-NTMS) and the results confirm the effective immortal character of this catalyst since the addition of an excess of the protic BnOH increase the reaction rate with complete conversion up to 600 equivalent of PDL to catalyst molar ratio. This property could be used to minimise the amount of the catalyst and contribute to the limitation of the contamination of the final polymer by the metallic residue. In conclusion, the major trends concerning this part were established and high interest has been expressed by our industrial partner and a patent application will be filled in order to claim some of our catalysts and copolymer materials.