Preparation Of Colored Functional Polylactic Acid By Metal Complex-catalyzed Ring-opening Polymerization Of Lactide

Bio-based materials are receiving more and more attention as a result of the global resource shortage caused by the massive exploitation of non-renewable resources such as oil.Bio-based materials are derived from natural substances with regenerative properties such as plants,reducing dependence on petroleum.Polylactic acid（PLA）,as a bio-based environmental material,can achieve complete natural recycling,high biocompatibility and easier processing.However,functionalized PLA materials often have problems such as low color fastness and impaired mechanical properties.In order to improve the disadvantages,this paper focuses on the UV resistant PLA polymerization dyeing process.Firstly,the aluminum metal complexes with anti-UV function were prepared as catalysts,and the azo disperse dyes with hydroxyl groups were synthesized as initiators.Then,by ligand-insertion ring-opening polymerization（ROP）mechanism of lactide（LA）,the polymerization was achieved and the functional catalyst and dye were attached to the long chain as a capping group to give PLA UV-absorbing and color properties.The details of the study are as follows:1.Preparation of functional catalysts:Choosing 2,2’-dihydroxybenzophenone,2-hydroxybenzophenone,Phenyl salicylate,2-（2’-Hydroxy-5’-methylphenyl）benzotriazole and 2-（2’-hydroxy-3’,5’-dipentylphenyl）benzotriazole as ligands to be coordinated with trimethylaluminum respectively,synthesizing five aluminum-based complexes（U₁-U₅）.The complexes were characterized by ¹H NMR,¹³C NMR and FTIR to determine the structures,and the UV-Vis Spectra was used to demonstrate the UV absorption property.2.Preparation of dye initiators:Six azo disperse dyes（R₁-R₆）were synthesized by using p-nitroaniline and aniline as diazo components and N-methyl-N-hydroxyethylaniline,N-ethyl-N-hydroxyethylaniline and N,N-dihydroxyethylaniline as coupling components.The dyes were detected by ¹H NMR and UV-Vis Spectra.3.Preparation of functional dyed PLA polymerization:（1）Investigate the effect of catalyst and initiator dosage on the ROP reaction.Among the five metal catalysts,2,2’-dihydroxybenzophenone aluminum complex（U₁）and 2-（2’-hydroxy-3’,5’-dipentylphenyl）benzotriazole aluminum complexes（U₅）were selected representatively.The effects of the dosage of the two catalysts on the molecular weight,UV-visible absorption capacity and thermal stability of PLA products were investigated respectively.Through the six dye initiators,R₁ was chose as a representative,and the effects of the dosage of the initiator on the molecular weight,UV-visible absorption capacity and thermal stability of PLA products were explored.The optimal reaction molar ratio for U₁ was found to be LA/U₁/R₁=100/1.0/0.5,and for U₅ was LA/U₅/R₁=100/1.3/0.5.（2）Researching the feasibility of functional catalysts and dye initiators on the ROP.Five aluminum complexes with anti-UV function and six dye initiators were applied to the polymerization of LA,and dyed PLA with anti-UV function were successfully synthesized.The polymers were characterized by ¹H NMR,FTIR,GPC,UV-Vis Spectra,and TG-DTA.The final results showed that,the functional catalyst and dye initiator attached to the long chain of PLA as capping groups.Among five metal catalysts,the effect of the U₅ was optimal and had good molecular weight control,the PLA Mn was 24,863 g/mol and PDI=1.05 when R₁ was used as the initiator.Through the six dye initiators,the effect of the R₅ was optimal,and the PLA Mn was 62,906 g/mol and PDI=1.08 when U₁ was used as the catalyst.UV-Vis absorption spectra demonstrate that the functionalized dyed PLA films are able to cover the UV-B and UV-A ranges with transmission rates below 20%.The TG-DTA results showed that the introduction of functionalized end groups had some effect on the thermal stability of PLA,which was able to increase the T_m of PLA up to 173.1℃,but decreased the T_d.