| The main focus in this thesis is the synthesis of poly(2-ethyl-2-oxazoline) with different topologies using different functional initiators.(1) Hyperbranched poly(2-ethyl-2-oxazoline) was synthesized by a combination of cationic ring-opening polymerization and reversible thiol/disulfide exchange reaction. A three-arm star macromonomer was first synthesized using 1, 3,5-tris(bromomethyl) benzene as an initiator. The star macromonomer was end capped with potassium ethyl xanthate. Similarly, using benzyl bromide as an initiator, a linear poly(2-ethyl-2-oxazoline) was synthesized and end capped with potassium ethyl xanthate. Hyperbranched polymer was then obtained by in situ cleaving and subsequent oxidation of the star macromonomer and linear PEtOx-xanthate mixture with n-butylamine as a cleaving agent and also as a base in tetrahydrofuran. The linear PEtOx was used to prevent the formation of gel. The hyperbranched polymer can be cleaved with dithiothreitol (DTT) to trithiol and monothiol macromonomers and confirmed by SEC. The hyperbranched PEtOx shows no remaining thiols using Ellman's assay. Although there is no gel formation but at the same time no free thiols were observed, which means that monothiol macromonomers were also formed continuously during cleavage and the thiol groups of the monothiol macromnomers formed disulfide during the oxidation by oxygen gas in the air. The growing hyperbranched polymer chains were terminated by these free monothiol macromnomers. These processes stop the intense increase in the molecular weight as well as stop the process of gelation. The resulting hyperbranched PEtOx was hydrolyzed to a novel hyperbranched polyethyleneimine (PEI) with reversible disulfide linkage.(2) Cyclic poly(2-ethyl-2-oxazoline) was synthesized using cationic ring opening polymerization and reversible thiol/disulfide exchange reaction. A two arm star macromonomer was first synthesized using 1,3-bis(bromomethyl)benzene as an initiator. The star macromonomer was end capped with potassium ethyl xanthate.Cyclic polymer was then obtained by in situ cleavage and subsequent oxidation of the macromonomer using cleaving agent n-butylamine which also works as a base in the same reaction media.1H NMR, SEC, MALDI-TOF MS and UV-visible spectroscopy were used to characterize the macromonomer and cyclic polymer. Ellman's assay was used to determine the thiol content in the cyclic and thiol terminated polymer. A clean cyclic polymer was obtained with a concentration of 5 mg / mL in THF. The synthetic route was compared with a similar cyclic PEtOx synthesized using the alkyne-azide click chemistry.(3) A novel trifunctional initiator with one alkyne and two trifluoromethanesulfonate moieties was synthesized from a protected alcohol 5-hydroxyl-2-phenyl-1, 3-dioxane. The alkyne functionalized intermediate with two protected alcohol groups was synthesized by reacting with propargyl bromide. The alcohol groups were cleaved using a mixture of tetrahydrofuran and hydrochloric acid aqueous solution. In the last step the initiator was synthesized using triflic anhydride in carbon tetrachloride. The initiator was characterized by 1H NMR and used for the polymerization of 2-ethyl-2-oxazoline which gives polymers with narrow distribution.For comparison a similar initiator with two tosylates was prepared and used for the polymerization of the monomer 2-ethyl-2-oxazoline. The resulting product has a wide molecular weight distribution and most of the initiator remains unreacted after 24 h which may be due to the steric hindrance between the two tosylate groups. To further explore the steric hindrance phenomenon, a linear tosylate initiator was synthesized,but still some of the initiator remains unreacted, illustrating that both steric hindrance and electrophilic balance affect the efficiency of the cationic ring-opening polymerization. All of the polymers were characterized in detail by using 1H NMR,MALDI-TOF-MS, and SEC to confirm the purity and distribution of the polymers. |
PDF Full Text Request