Thermal And Enzymatic Dual-stimuli Responsive Linear-dendritic Block Copolymers Based On Poly(N-vinylcaprolactam)

Linear-dendritic block copolymers?LDBCs?combine properties of dendritic macromolecules and linear polymers.Amphiphilic LDBCs can self-assemble to form aggregates with special core–shell structures,which have broad application prospects in biomedicine,nanomaterials and catalysis.Stimuli-responsive polymers could change their structure or state through small changes in the environment,thereby affecting their physicochemical properties.Because of overexpression of enzymes and increase of temperature in some diseased tissues,thermal and enzymatic dual-stimuli responsive LDBCs are attractive for potential application in drug delivery system,disease diagnosis and other fields.However,thermal and enzymatic dual-stimuli LDBCs have been only limited exploration.In this thesis,two poly?N-vinylcaprolactam??PNVCL?linear chains with different molecular weights?M_n=14 kDa,21 kDa?were firstly synthesized by reversible addition-fragmentation chain transfer?RAFT?polymerization.They were used as the polymeric supporter,respectively,and then linear-dendritic block copolymers?LDBCs?PNVCL_m-b-?Phe-Lys?_n?m=14k,21k,n=1-3?with linear PNVCL and dendritic phenylalanyl-lysine?Phe-Lys?dipeptides were prepared via stepwise peptide chemistry.The structures of the LDBCs were confirmed by 1H NMR spectra and GPC.Their self-assembly behavior,enzyme and temperature responsive properties were investigated via transmission electron microscope?TEM?,fluorescence spectroscopy,and UV-Vis spectroscopy.The anticancer drug doxorubicin?DOX?was used as a model drug to study the drug loading and release properties,cytotoxicity and cell uptake properties of these LDBCs.The results show that both PNVCL_14k-b-?Phe-Lys?_1-3 and PNVCL_21k-b-?Phe-Lys?_1-3 can self-assemble into spherical nanomicelles in aqueous solution.The low critical micelle concentration?CMC?and particle size of the micelles decrease with increasing the generation of LDBCs.When the generation is same,the CMC and particle size of the micelles increase with increasing molecular weight of PNVCL.The maximum drug loading of PNVCL_14k-b-?Phe-Lys?₃ and PNVCL_21k-b-?Phe-Lys?₃ reach 34.9%and26.7%.The micelles are able to release the encapsulated anticancer drug doxorubicin?DOX?upon trypsin or cathepsin B treatment.When the generation is same,PNVCL_21k-b-?Phe-Lys?_n released DOX faster than PNVCL_21k-b-?Phe-Lys?_n upon the addition of trypsin or cathepsin B.In addition,DOX was released faster and more from the micelles upon the addition of cathepsin B than upon the addition of trypsin.The results of in vitro cytotoxicity and cell uptake experiments show that the LDBCs have good biocompatibility and the drug-loaded micelles can be enriched in tumor cells.Moreover,PNVCL_m-b-?Phe-Lys?_n has thermal responsive behavior,the low critical solution temperatures?LCSTs?of the LDBCs were depended on their concentration,generation and molecular weight of PNVCL.The LCST values of the LDBCs decrease with increasing their generation and concentration,and decreasing the molecular weight of PNVCL.Furthermore,the LCSTs of PNVCL_m-b-?Phe-Lys?_n increased with the time of enzymatic hydrolysis,which indicated that the copolymers PNVCL_m-b-?Phe-Lys?_n displayed thermal and enzymatic dual-stimuli responsiveness.Finally,the effects of different concentrations of aprotic solvents such as dimethyl sulfoxide?DMSO?,N,N-dimethylformamide?DMF?,tetrahydrofuran?THF?,1,4-dioxane and protic solvents?low molecular weight alcohol?on the LCST of poly?N-vinylcaprolactam??PNVCL?aqueous solution were studied by turbidimetry.The results showed that the cononsolvency effect occurred upon addition of any of the tested solvents to PNVCL-water solutions.The LCSTs of PNVCL increased monotonically with increasing concentration of added DMF,THF,1,4-dioxane and CH₃OH to the aqueous solution of PNVCL.Moreover,among these solvents tested,the addition of DMF and 1,4-dioxane had the greatest effect on the LCSTs of PNVCL,followed by THF,while methanol has the least effect on the LCSTs.The addition of small amounts of ethanol,1-propanol,and 2-propanol to an aqueous PNVCL solution initially decreases the transition temperature,and a further addition increases it.Unlike these alcohols,the LCST of PNVCL first increases and then decreases with increasing concentration of added DMSO.