Preparation Of Multi-responsive Polymers From Functional Monomers Of Ethylene Glycol Functional Oligomers And Their Characterization

Stimuli-responsive fluorescent polymers(SRFP)can change their fluorescence properties upon some external stimulus.It is widely used in biological fields such as cell imaging,biomarkers,drug delivery system and analysis.Traditional preparation of SRFP is to incorporate conjugated organic chromophores into stimuli-responsive polymers without fluorescent emission by physical or chemical ways.The conjugated organic chromophores used for this purpose bring about often the ACQ(aggregation-caused-quenching)effect,besides difficulties at different extent in their syntheses.In contrast,polymers with non-traditional chromophores(carbonyl,tertiary amino,ester,and amide)and characteristic of cluster-triggered emission(CTE)exist in large quantities in nature,and they generally have hydrophilicity,non-toxicity,good biocompatibility and processability.Therefore,the synthesis of these SRFP polymers with CTE emission characteristic is of important academic significance and application value.Poly(ethylene glycol),PEG,has been known as a thermo-responsive polymer with good biocompatibility and non-toxicity.PEG polymers with different functional groups have received great attention in recent years,because these studies are interesting in view of their fluorescence mechanism and their application as a different type of SRFP material.Nevertheless,the studies on the correlation between their CTE properties and phase transition behavior are rarely available.In this work,P(OEGA-MAA),a linear copolymer responsive to temperature,pH and salt,was prepared by free radical copolymerization of oligo(ethylene glycol)methyl ether acrylate(OEGA)with methylacrylic acid(MAA)in ethanol.The structure of P(OEGA-MAA)was characterized by ~1H NMR and its molecular weight determined by GPC.The evolution of light transmittance of the water solution of P(OEGA-MAA)with temperature was measured under different conditions and its fluorescence performance was characterized.The results show that P(OEGA-MAA)is not only responsive to temperature,p H and salt,but also emits visible blue fluorescence under UV.Effects of the polymer composition,pH and salt concentration on the lower critical solution temperature(LCST)and fluorescence properties are studied.And the relationship between fluorescence properties and LCST is also descibed.With MAA content increased in the polymer,the LCST in its aqueous solution(2.0 mg/m L,pH=1)decreased,and the corresponding fluorescence intensity increased first with MAA content increased up to 67mol%(OEGA/MAA=3.3/6.7,sample P7),where it reached a maximum,and followed by a decrease with further increase in MAA.Effects of pH and NaCl concentration were also studied with P7 as an example.With increased pH in the aqueous solution(2.0 mg/mL)of P7,the LCST increased and the fluorescence intensity decreased;with the increase in NaCl concentration,the LCST decreased,while no obverious change was detected for the maximal fluorescence emission.Cross-linked copolymers based also on PEG oligomers,P(OEGDA-MAA),were then prepared through the same way but with oligo(ethylene glycol)diacrylate(OEGDA)with methylacrylic acid(MAA),which were also responsive to temperature,pH and salt and with fluorescent emission under given conditions.The structure of P(OEGDA-MAA)was characterized by NMR.LCST and fluorescent emission of the cross-linked P(OEGDA-MAA)in aqueous solution were characterized under different conditions.The results show that P(OEGDA-MAA)is not only responsive to temperature,pH and salt with also fluoresct emission under UV.With MAA content increased in the polymer,the LCST in its aqueous solution(pH=1)decreased from 58oC in DP3(OEGDA/MAA=7.0/3.0)to 29oC(DP7,OEGDA/MAA=2.9/7.1),while the emission intensity in constant decrease.This was quite different from the linear P(OEGA-MAA)where the emission increased first and followed by a decrease with increased MAA in the copolymer.LCST of P(OEGDA-MAA)in aqueous solution was in constant increase with pH.The LCST of DP7 was therfore increased from 29oC at pH=1 to 45oC at pH=7,and disappeared thereafter.The emission demonstrated a maximum at pH=9.An important effect of the presence of NaCl in the cross-linked polymer was also detected.Taking DP7 as an example,its LCST decreased from 30oC to 26oC in accordance with NaCl concentration inscreased from null to 500 mmol/L,where as the fluorescence emission remained at 312 nm with an significant increase in emission intensity.The properties and emission behaviors were compared for the two series of PEG-based SRFP,and it was concluded that the fluorescent emission was owing to the cluster-formation of the oxygen atoms in PEG chains,in accordance with CTE mechanism.The difference between the emission behaviors of the linear and the cross-linked polymer was based from the cross-linking of the polymer chains in P(OEGDA-MAA),which imposed restriction to the mibility of their chain segments,and led to different cluster formation under stimuli of external conditions,such as polymer concentration,temperature,pH and the presence of the salt NaCl.This study provides therefore a novel type of materials for potential applications in biomedical fields,and it is also of great significance for understanding the fluorescence mechanism of PEG-based stimuli-responsive polymers.