Precise Synthesis Of Molecular Bottlebrush Polymers Based On NBD-Mal Click Reaction And Its Application As Drug Carrier

Bottlebrush polymers（BBPs）,with side chains highly grafted to the polymer backbone,has been attracted more and more attention in the application of nanomedicine due to its precise structures,tunabe shapes and unimoleuclar nanosize（10-100 nm）.However,several limitations hindered the development of BBPs synthesis,including the use of metal catalysts and low grafting density.Therefore,it is of great research value to develop a fast,efficient and metal-free system to synthesize BBPs.In this thesis,we focus on the construction of an efficient,convenient and clean methodology for the synthesis of BBPs,and a preliminary study on the drug loading and release of BBPs synthesized by this method.The details are given as follows.1.Through the D-A reaction of Norbornadiene-Maleimide（NBD-Mal）in the presence of tetrazine,a efficient and clean universal method for synthesis of BBPs was developed.In this approach,the polymer backbones PHEA-NBD with NBD pendant group and the polymer side chains with Mal end group were obtained by reversible addition-fragmentation chain transfer（RAFT）polymerization and post-modification of esterification reaction,respectively.The polymer side chains including hydrophobic acrylate polymer poly（tert-butyl acrylate（Pt BA））,hydrophilic polyethylene glycol（PEG）,acrylamide polymer poly（N,N-dimethylacrylamide（PDMA））and acrylate polymer poly（dimethylaminoethyl acrylate（PDMAEA））.Then under the catalysis of small molecule tetrazine,the NBD group of the backbone undergoes a retro-D-A reaction to form a highly active diene cyclopentadiene group,and coupled with the highly reactive dienophile Mal group at the end of the side chains to obtain the BBPs with different length of the backbones and side chains,including PHEA₁₀₅-g-PEG₄₆,PHEA₂₁₀-g-PEG₄₆,PHEA₄₁₉-g-PEG₄₆,PHEA₄₁₉-g-Pt BA₃₂,PHEA₄₁₉-g-PDMA₂₈ and PHEA₄₁₉-g-PDMAEA₂₁.The grafting density can reach 99%after 5 hours of reaction,when the feed ratio of the polymer backbones to the side chains is 1/1.5 and the reaction temperature is 20.The results of AFM characterization showed that the BBPs showed worm-like morphology.2.A multiblock BBPs was synthesized by combining P2Ox-HRP enzymatic cascade catalysis polymerization and NBD-Mal click reaction.First,the backbone of(A₅₀B₅₀A₅₀B₅₀)₂A₁₀₀ multiblock copolymer was obtained by P2Ox-HRP enzymatic cascade catalysis polymerization and post-modification of esterification reaction,where the A block was a polymer PHEA-NBD with NBD pendant group and the B block was poly（methoxypoly（ethylene glycol）acrylate）（POEGA）,and then the polymer backbone and the side chains are coupled through the NBD-Mal click reaction,which A block were respectively Pt BA,PDMA and PDMAEA,to obtain a multiblock BBPs.Owing to certain steric hindrance of the POEGA block,the reaction is extended to 9 hours,and finally 99%of the polymer side chains grafting density can be achieved.At the same time,AFM characterization results shown that the multiblock BBPs was present a special rosary-like morphology.3.The application of BBPs as drug carriers was preliminarily explored（including the cytotoxicity and the loading and in vitro release behavior）.In order to increase the drug loading capacity of the BBPs,firstly,a multiblock BBPs with the side chain of the amphiphilic block copolymer Mal-Pt BA₂₀-b-PEG₂₃ was synthesized,and then combined with the PHEA₄₁₉-g-PEG₄₆ was used for comparative study.The anti-tumor model drug CPT was loaded onto the BBPs through hydrophobic interaction.The shape and size were characterized using Atomic force microscopy（AFM）and dynamic light scattering（DLS）techniques.Finally,the in vitro release behavior of the two kinds of BBPs was studied separately.The results shown that the BBPs can maintain a good unimolecular morphology.In addition,for the amphiphilic multiblock BBPs,it not only has higher DLC and DLE,but also in vitro release than PHEA₄₁₉-g-PEG₄₆.This may provide a new types and direction for designing nanomaterials in nanomedicine.