Synthesis Of Functionalized Polymers Via Living ROMP And Their Applications In Preparing Nanoparticles

Ring-opening metathesis polymerization (ROMP), a widespread tool tosynthesize well-defined and highly functionalized polymers, has expanded the realmof polymer synthesis. In particular, Grubbs catalysts were found with well-defined,stable and highly active characteristics and thereby providing more opportunities forROMP. Norbornene-based monomers are commonly employed in ROMP because oftheir high ring strain and various functional groups. In general, the functional groupsfollow the order according to the tolerance of Grubbs catalysts: acids> aldehydes>ketones> esters or amides. The2ndgeneration Grubbs catalyst (G2) containingN-heterocyclic carbene ligand (NHC) exhibits higher activity and functional grouptolerance and is more likely to be used in the preparation of polymers with differentfunctions.In our previous studies, homopolymers and block copolymers were synthesizedin the use of such polymerization method and the surface grafting modification ofhalloysite nanotubes (HNTs) was investigated. It was found that functional polymerscontaining sugar groups, ester groups, or amide groups can be obtained effectivelyand the surface-initiated ring-opening metathesis polymerization (SI-ROMP) onHNTs was successfully performed through a “graft from” approach. However, therewere difficulties in the polymerization of monomers containing nitrile groups, wherethe monomer conversion was only4.6%. Considering this challenge, Lewis-acids,which will more easily coordinate with nitrile groups, can be introduced to reactionsystems prior to ROMP, and then as a result, the living ROMP ofnitrile-functionalized monomers can be successfully accomplished. In the thesis, theapplications of free and semifree polymers to assist the preparation of nanoparticleswere explored.Firstly, the nitrile-functionalized norbornene (M1) was successfully synthesizedby Diels-Alder adduct and the purity was95%from the results of FT-IR and1H NMR.Then Fe2+and Fe3+were coordinated with nitrile groups of monomers to gain the coordination compounds M1-Fe2+and M1-Fe3+, respectively. From the UV-Visresults, it can be concluded that the coordination between M1and iron ions followedthe stoichiometry and the coefficient was about1. Besides, the living ROMP of threemonomers initialized by G2was studied. The monomer conversion is significantlyimproved with the assistance of Fe2+or Fe3+and Fe3+has a better performance thanFe2+.Secondly, the sugar-functionalized norbornene (M2) and amide-functionalizednorbornene (M3) were synthesized. The homopolymerization of M1, M3andcopolymerization of M2and M1, M2and M3were performed respectively under therole of G2, obtaining four kinds of free polymers, namely, P(M1), P(M3), P(M2M1)and P(M2M3). The results of GPC characterization show that the PDI values ofhomopolymers P(M1) and P(M3) are less than gradient diblock copolymers P(M2M1)and P(M2M3). Subsequently, Fe3+was chelated to the polymer chains and thenreduced by NaBH4to produce stable and monodispersed Fe0nanoparticles. TEMimages show that block copolymers can exhibit better control of nanoparticlemorphology and size than homopolymers. Fe0nanoparticles were oxygenated by(CH3)3NO to obtain the γ-Fe2O3and subsequently the resulting γ-Fe2O3nanoparticleswere utilized as photocatalysts for hydrogen production with a well catalyticefficiency.Finally, the preparation of semifree polymers was carried out. After anchoringinitiator G2to the surface of HNTs, the monomer M1-Fe3+and M2were addedsequentially to prepare the copolymer brushes on HNTs (HNTs-PM1M2). The resultsof FT-IR and TGA show that polymer brushes have been successfully grafted onto thesurface of the HNTs. Moreover, Fe0nanoparticles can be obtained through thereduction of Fe3+contained in polymer brushes.