Construction Of Grafted Polymers With Cyclic Topological Structures On Main Or Side Chains

Grafted polymers,in conjunction with conventional linear polymers,have many side chains on their main backbones.They are easily designed to polymeric architectutes with two or more different chemical compositions and structures to construct precise microphase that exhibits unique and remarkable properties in both solution and solid states.Grafted polymers have broad potential applications such as super-soft elastomers,drug-delivery materials,hybrid inorganic/organic materials,nanomaterials,lubricants and biosensors,which have gained significant interest from scientists and researchers.According to the structure of polymer mainchain,grafted polymers can be roughly divided into linear-,star-,cyclic-,dendritic-,hyperbranched-grafted polymers and so on.With continuous development of new methods and technologies,novel grafted polymers have exhibited more and more different properties and exploration for new strategies to construct novel grafting polymers has never stopped Recently,combining cyclic and grafted architectures,cyclic grafted polymers composed of one cyclic polymer backbone and many grafted side chains,have potential applications in biomaterials.Synthesis and properties of grafted polymers with cyclic polymers on main chains or side chains certainly has a potential significance and research merit.In this thesis,we designed a series grafted polymers with cyclic polymers on main chains or side chains via combing different cyclization strategies,efficient chemicals reactions and controlled radical polymerization techiniques.Firstly,the controllability of grafting density from cyclic grafted polymers was realized by controlling the feed ratio of Whol-Ziegler reaction.Secondly,new cyclic-grafted polymers with high grafting density and cyclic double-grafted polymers were successfully constructed by using active ester chemistry.Thirdly,based on electrostatic self-assembly and covalent fixation(ESA-CF),a novel cyclic polymer containing both one azide group and one cleavable point was successfully constructed,which provided a basis for preparation of grafted polymers with cyclic topological structures on side chains.The detailed researches were summarized as the following:(1)Combining Cu-catalyzed azide-alkyne cycloaddition(CuAAC)ring-closure method with the "grafting from" technique,a novel synthetic strategy for preparing cyclic-brush polymers with controlled grafting density was successfully demonstrated.The cyclic polystyrene(c-PS)precursor was synthesized using the general methods of activators generated by electron transfer for atom transfer radical polymerization(AGET-ATRP)and CuAAC ring-closure method.The benzylic hydrogen atoms of c-PS were substituted with bromine based on Wohl-Ziegler bromination to generate a tertiary bromide ATRP macroinitiator(c-PSBr),and the bromine content can be adjusted to 18.4%,25.3%,31.9%and 50.3%by using different reaction conditions.This ATRP macroinitiator can also be converted into a macro-RAFT agent with azole(c-PSN)or ethyoxyl(c-PSO)as the Z groups based on the styrenic polymers as the R group.Using the as-prepared ATRP macroinitiator or macro-RAFT agents,a series of cyclic-brush polymers were successfully prepared.The grafting density can be facilely tuned by using variable Br content of c-PSBr or Z group content of the macro-RAFT agents.The work illustrated a first example of versatile synthesis of cyclic-brush polymers with predictable molecular weight,grafting density and different grafting polymer chains.(2)Combining light-induced Diels-Alder ring-closure approach and activated ester chemistry as the post-modification method,novel cyclic-grafted polymers with high grafting density were successfully constructed via the grafting onto approach.The cyclic activated ester containing polymer precursor[poly(pentafluorophenyl 4-vinylbenzoate),c-PPF4VB]was synthesized by combining the reversible addition-fragmentation chain transfer(RAFT)polymerization and light-induced Diels-Alder click reaction.Using activated ester chemistry,this cyclic polymer(c-PPF4VB)was post-modified by PEG-NH2 to obtain cyclic-grafted polymer(c-PPF4VB-g-PEG)with hydrophilic grafting polymer chains and hydrophobic cyclic polymer backbone.Meanwhile,the c-PPF4VB was post-functionalized using propargyl amine to obtain clickable cyclic polymer(c-P1 and c-P2).The cyclic polymer(c-P1 and c-P2)was further coupled with azide-containing polymeric chain(l-PS-N3)to produce the cyclic-grafted polymer(c-P1-g-PS and c-P2-g-PS).Notably,these two cyclic-brush polymers exhibited high grafting density(almost quantitively)and narrow molecular weight distribution(Mw/Mn<1.10).This work illustrated one concept for constructing cyclic-brush polymers by combining ring-closure strategy and activated ester chemistry.(3)Combining light-induced Diels-Alder ring-closure approach and activated ester chemistry,novel cyclic double-grafted polymers with polymeric double-chains were successfully constructed via the grafting onto approach.Hydroxyl-containing polymeric double-chain(l-PS-PhOH)was prepared by reacting azide-containing polymeric chain(l-PS-N3)with 3,5-bis(propynyloxy)phenyl methanol,and further modified by azide group to prepare azide-containing polymeric double-chain(l-PS-PhN3).The cyclic main chain(c-P2)was coupled with azide-containing polymeric chain(l-PS-N3)using CuAAC reaction to construct novel cyclic double-grafted polymers(c-P2-g-Ph-PS).(4)Based on ESA-CF cyclization,we designed and synthesized a novel cyclic polymer(c-PTHF-N3)containing one azide group and one cleavable point,and the corresponding linear polymer(l-PTHF-N3)was also prepared.The cyclic polymer(c-PTHF-OTBDMS)and the corresponding linear counterpart(l-PTHF-OTBDMS)were prepared by ring-opening polymerization of tetrahydrofuran,and ion-exchanged with sodium 9-anthracene carboxylate and sodium 9-anthracene carboxylate dimer respectively,followed by heating for covalent fixation.After deprotection reaction,the hydroxyl group in the obtained polymers was further esterified to generate an azide group on polymeric chains.The gained cyclic polymers(c-PTHF-N3 and l-PTHF-N3)will be applied in the preparation of grafted polymers with cyclic structure on side chains(PCL-g-(c-PTHF))and corresponding linear-grafted polymers(PCL-g-(l-PTHF)to realize ring-to-line conversion from side chains of grafted polymers.