Synthesis And Temperature Sensitivity Of Lysine-based Polymers For Food Smart Packaging

The by-products of grain processing have high utilization value and are the main source of important energy,nutrition and available substances.Among them,corn is China’s largest food crop,the annual output of more than 200 million tons,is China’s main food and industrial sources,deep processing produced by the by-product L-lysine annual output of more than 1.5 million tons.However,current technical deficiency in our country in the utilization leads to the lack of L-lysine functional high value-added products,and it is of great urgency to carry out researches and development of L-lysine high value utilization technology.Stimuli-responsive polymers is one kind of functional polymer,which changes its physical and chemical property by specific environmental stimuli（such as temperature,pH）.Among them,thermo-sensitive polymer can make a reversible response through changing external temperature.Lower Critical Solution Temperature（LCST）is the most important feature of thermoresponsive polymer.At temperatures below LCST,these polymers are soluble in H₂O.Upon increasing temperatures above the LCST,such polymers will experience a conformational transition from loose solvated coils to compact dehydrated globules,collapse and become insoluble in H₂O.Based on this feature,the temperature-sensitive polymer can be used as an important smart material,and it is not harmful to human body in food packaging.In addition,temperature-sensitive polymers have broad prospects in the fields of food microcapsule wall materials,agricultural product storage,biological separation,tissue engineering,and immunoassay.Firstly,renewable L-lysine was cyclized to its correspondingα-amino-ε-caprolactam and then underwent the reaction with acryloyl chloride to harvest a 2-acrylamido-caprolactam（ACL）monomer.And the 2-（2-methoxyethoxy）ethyl acrylate（OEG₂A） and 2-（2-（2-methoxyethoxy）ethoxy）ethyl acrylate（OEG₃A）were synthesized.All the monomers were characterized by ¹H NMR and ¹³C NMR.Subsequently,the P（ACL-co-OEG₂A）and P（ACL-co-OEG₃A）with narrow polydispersities were prepared via RAFT polymerization.The structure and composition of the polymer were characterized by ¹H NMR and GPC.The results show that the incorporation level of OEG₂A and OEG₃A could be well-controlled（from 20%-80%）,the copolymers possessed moderate to high molecular weight（3500 g/mol-9500 g/mol）and narrow polydispersity（1.1-1.3）.In order to study and compare the temperature-sensitive properties of the copolymer,we synthesized the homopolymers P（ACL）,P（OEG2A）and P（OEG3A）by RAFT polymerization,and the structure and composition of the polymer were characterized by ¹H NMR and GPC.The results show that the polymer possessed a higher molecular weight of 16600 g/mol-28600g/mol and narrow molecular weight distribution of 1.1-1.2.Secondly,the resultant copolymers of P（ACL-co-OEG2A）and P（ACL-co-OEG3A）displayed a thermo-sensitive feature,and the LCST of the copolymer exhibited a wide temperature distribution range（from 33℃ to 79℃）.Moreover,the incorporation level of hydrophobic caprolactam ring,the pH value and the ion concentration display a large influence on the LCST.Increasing the content of hydrophobic caprolactam ring in the polymer,the decreased LCST was observed.Due to the restricted hydrogen-bond interaction between the polymer and the water molecule,the LCST decreased under the high ion concentration condition.Comparing to the ion concentration and the fraction of polymer,the LCST increased with reducing the hydrophobicity of the side chain caprolactam ring at elevated pH value condition.These results indicate that these regenerative L-lysine-based temperature-sensitive polymers have a great potential to be used as new materials in the fields of food intelligent materials,food microcapsule wall materials.