Synthesis of poly(2-oxazolines) with tunable architectures and their potential biomedical applications
Description
Polymers of 2-oxazolines have shown immense potential in synthetic and biomedical applications. Exhibiting a cationic ring-opening polymerization reaction that follows a 'living' mechanism, the polymerization of 2-oxazolines allows for the synthesis of well-defined poly(N-acylethylenimine) (PNAI) polymers with low polydispersities. This mechanism allows for end group control while the N-acyl side chain of 2-oxazolines allows for introduction of desired functionalities. The ease with which the backbone and end groups of poly(oxazolines) can be modified makes this family of polymers attractive as a tool in the synthesis of diverse polymer architectures with potential biomedical applications. Polymers of 2-oxazolines have been shown to exhibit water solubility and biocompatibility depending on the N-acyl side chain rendering them a class of polymers comparable to poly(ethylene glycol) (PEG). In contrast to PEG, where functionalization can only be achieved through the end groups, modification through the N-acyl side chain in oxazoline polymers enables the introduction of diverse functionalities along the polymer backbone. Through the use of thiol-ene coupling between a thiol present in a small molecule or amino acid and the alkene of 2-isopropenyl-2-oxazoline, three new 2-oxazoline monomers with different functionalities were synthesized followed by polymerization to yield 2-substituted oxazoline polymers with aryl, ester, amine, and carboxylic acid functionalities, including a 2-oxazoline polymer with a protected cysteine attached at each repeat unit. MALDI-TOF mass spec was then used to elucidate a chain transfer mechanism resulting from the thioether bond of the thiol-ene coupling reaction. The living cationic ring opening of polymers of 2-oxazolines also offer versatility and control of the polymer end groups contingent on the nature of initiating and terminating agents. Utilizing this end group control, linear PNAI was synthesized containing both alkyne and azide end groups. The linear polymers were then cyclized via a combination of slow addition and the fast, high yielding Cu(I)-catalyzed 2 + 3 cycloaddition click (CuAAC) reaction to yield cyclic, water soluble polymers with directly comparable linear analogs. The N-acyl side chains of PNAIs can undergo hydrolysis to yield well defined polyethyleneimine (PEI), a polymer know to show potential as a non-viral gene delivery vector. Though, traditionally, branched PEI has been shown to have a high degree of transfection and DNA complexation, it has also been shown to exhibit high cytotoxicity. To probe the effect of polymer architecture of PEI as a potential gene delivery vehicle, cyclic PNAI and its linear analogs were hydrolyzed with aqueous acid to yield directly comparable cyclic and linear PEI of varying molecular weights. In vitro studies with two different cells lines revealed that cyclic PEI display higher transfection efficiencies than their linear counterparts and cyclic PEI above 70 repeating units exhibit comparable transfection efficiencies to a 25K sample of branched PEI. Cytotoxicity experiments with HFF-1 cells showed promising results, with higher molecular weight cyclic PEI (70 to 80 DP) showing comparable transfection at different N:P ratios as a 25K branched PEI standard. These high molecular weight cyclic PEI samples also exhibited lower cytotoxicity than the branched PEI Standard. Studies with 3 additional cell types showed similar results, suggesting cyclic PEI at higher molecular weights has potential to be an improved gene delivery vehicle for genetic therapies