Study On Tailoring Chain Topologies Of Ethylene Copolymers And Their Functionalization

Olefinic polymers are the largest plastic product in the world and their developmental degree measures the economic level of a country. The use of cheap and readily available olefin monomers as raw materials to tailor novel polymer products with excellent performance and high additive value enhances product profits substantially. The field also provides unique academic avenues for research. Enhanced material properties can be achieved via a precise engineering of the chain functionality, sequence, and topology of polyolefins. The major theme of this work is tailoring of chain topology and functionality. Ethylene copolymers with hyperbranched, cross-linked, tree, and star structures have been fabricated and their application in the fields of catalyst loading and nanomaterial preparation were carried out. The research content is divided into the following two parts:(Ⅰ) Pd-diimine complexes were used to catalyze the chain walking copolymerization of ethylene and acrylic monomers, producing hyperbranched polyethylene (HBPE) copolymers and polyethylene (PE) gels in one step. These polymers can be applied as new carriers to load organic and inorganic catalysts for the development of polymer-supported catalysts with high activity/selectivity and good recyclability. We developed three unique supported catalysts:(1) HBPE-supported L-prolines, which were capable of catalyzing the organic aldol reaction between p-nitrobenzaldehyde and cyclohexanone with a high product selectivity of anti/syn=98:2and ee>99%;(2) An HBPEs copolymer containing disulfide groups which in-situ encapsulate Pd(II) ions from Pd-diimine complexes through a coordination complexation between the disulfide and Pd atom, resulting in an HBPE-supported Pd catalyst. The catalyst can efficiently catalyze the Heck coupling reaction between iodobenzene and butyl acrylate in a biphasic solvent system comprised of n-heptane and dimethyl formamide (DMF). A rapid and convenient recovery and reuse (>10cycles) of the catalysts was carried out by phase separation of the solvents when cooled to room temperature after the reaction; (3) A HBPE based gel containing Pd(0) nanoparticles which were generated by the in-situ reduction of Pd-diimine’s Pd(II) ions. The supported Pd nanoparticles efficiently catalyzed the Heck and Suzuki coupling reactions of aryl bromides in both DMF and water and could be recycled numerous times (>10).(Ⅱ) Section Ⅱ focused on a two step polymerization method consisting of a Pd-catalyzed "living" coordination polymerization of ethylene followed by an atom-transfer radical polymerization (ATRP) of functional monomers. Using this method we generated5novel functional PEs with tree or star topologies, they are as follows:(1) A tree block copolymer of HBPE-b-PS or HBPE-b-PMMA each containing a HBPE unit and a polystyrene (PS) or polymethyl methacrylate (PMMA) segment respectively;(2) An amphiphilic block copolymer of PE-b-POEGMA consisting of a linear or hyperbranched PE unit and a poly[oligo(ethylene glycol) methyl ether methacrylate](POEGMA) segment;(3) A multi-arm star polyethylene polymer containing a cross-linked polydivinylbenzene (PDVB) core and multiple linear PE arms;(4) A miktoarm star copolymer containing a cross-linked PDVB core and multiple linear PE and PS arms;(5) An amphiphilic star copolymer of HBPE-co-(PDMAEMA)n consisting of a HBPE core and multiple poly(2-dimethylaminoethyl methacrylate)(PDMAEMA) arms. By controlling various factors including the type of Pd-diimine catalyst, the polymerization conditions (ethylene pressure, time, and temperature), the commoner concentrations and feeding strategies in the ethylene polymerization, well-defined PE macroinitiators (MIs) with tailored molecular weights (MWs)=6-50kg/mol, MW distributions<1.5, chain topologies (linear or hyperbranched) and functionalities (content and distribution) have been prepared. Excellent tailoring of the5above mentioned functional PE products was achieved through ATRP by controlling the feeding amounts/concentrations of PE MIs and functional monomers, copper catalytic systems (amount and type of ligands and Cu catalysts used), and reaction conditions (temperature, solvent, and time).This study found that the synthesized polymers contained many unique properties such as:The liposoluble and chemically stable HBPE and its gel can offer the supported catalyst a hydrophobic microenvironment and enrich the oil-soluble reactants of the aqueous system. This makes the supported catalysts highly active, selective, recyclable and reusable;(PE)n-PDVB and (PE)n-PDVB-(PS)m have good solubility in their good solvent and very low dilute solution viscosities; Amphiphilic PE-b-POEGMA and HBPE-co-(PDMAEMA)n possess stimuli-responsive properties; PE-b-POEGMAs can be assembled into thermal-sensitive micelles in water, while HBPE-co-(PDMAEMA)n show a CO2-responsive micellization capability in aqueous solution. The unique properties of these novel ethylene polymer materials establish a good foundation for their application in various fields including catalyst supports, drug carriers and nanoreactors amongst others.