Synthesis of Thermoplastic Xylan-Lactide Copolymer with Amidine-Mediated Organocatalyst in Ionic Liquid

Xueqin Zhang,Aiping Zhang,Junli Ren,Chuanfu Liu,Huihui Wang

doi:10.1038/s41598-017-00464-6

Abstract

Ring-opening graft polymerization (ROGP) of l-Lactide (l-LA) is a practical method of altering the physical and chemical properties of lignocellulose. Previous studies have mainly investigated cellulose and tin-based catalysts, particularly of tin(II) 2-ethylhexanoate (Sn(oct)2), at high temperatures and reported low graft efficiencies. In the present study, ROGP of l-LA was successfully achieved on xylan-type hemicelluloses in ionic liquid (IL) 1-allyl-3-methylimidazolium chloride ([Amim]Cl) using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an effective organic catalyst. Mild reaction condition (50 °C) was used to limit transesterification, and thus enhance the graft efficiency. The hydroxyl groups on xylan acted as initiators in the polymerization, and DBU, enhanced the nucleophilicity of the initiator and the propagating chain. Xylan-graft-poly(l-Lactide) (xylan-g-PLA) copolymer with a degree of substitution (DS) of 0.58 and a degree of polymerization (DP) of 5.51 was obtained. In addition, the structures of the xylan-g-PLA copolymers were characterized by GPC, FT-IR and NMR, confirming the success of the ROGP reaction. Thermal analysis revealed that the copolymers exhibited a single glass-transition temperature (Tg), which decreased with increasing molar substitution (MS). Thus, modification resulted in the graft copolymers with thermoplastic behavior and tunable Tg.

Highlights

Recent advances in polymerization techniques have enabled more economical production of PLA and have broadened its applications[28, 32, 33]
Attack of the nucleophilic nitrogen of DBU on the carbon of the acetyl group in l-LA generated zwitterion 1, analogous to the mechanisms proposed for the activation of acyl halides and dialkyl carbonates with DBU49
At 50% weight loss, the decomposition temperatures of xylan and the grafted copolymers 24, 25, and 21 were 275 °C, 279 °C, 295 °C, and 300 °C, respectively. These results indicated that the thermal stability of xylan was enhanced by the increased degree of substitution (DS) of the xylan-g-PLA copolymers, in contrast to the decreased thermal properties of cellulose-g-PLA copolymers with increased DS14, 35

Summary

Mediated Organocatalyst in Ionic

Ring-opening graft polymerization (ROGP) of l-Lactide (l-LA) is a practical method of altering the physical and chemical properties of lignocellulose. Previous studies have mainly investigated cellulose and tin-based catalysts, of tin(II) 2-ethylhexanoate (Sn(oct)2), at high temperatures and reported low graft efficiencies. ROGP of l-LA was successfully achieved on xylan-type hemicelluloses in ionic liquid (IL) 1-allyl-3-methylimidazolium chloride ([Amim]Cl) using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an effective organic catalyst. Chemical modifications have been applied to confer desirable properties and functionalities to hemicelluloses, resulting in the improved performance of composite materials[9,10,11,12], among which graft polymerization provides a significant route to covalently modify the surface of hemicelluloses with polymers[4, 13,14,15]. We extend the study of ROGP of l-LA onto xylan-type hemicelluloses with the organic catalyst DBU to improve the graft efficiency under mild conditions. The physicochemical properties of the xylan derivatives were characterized by gel permeation chromatography (GPC), FT-IR, 1H-NMR, 13C-NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC, thermogravimetric analysis (TGA/DTG), differential scanning calorimetry (DSC) and X-ray diffraction (XRD)

Results and Discussion

Conclusion

Additional Information