ABC Miktoarm Star Terpolymers and Their Spherical Capsules Bearing pH-Responsive Nanochannels

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Date
2014-10-03
Authors
Hu, Heng
Keyword
Miktoarm Polymer , Nano Capsules , Self-Assembly
Abstract
This thesis reports the preparation of several ABC and ABCx miktoarm star terpolymers using a novel “assembly-and-reaction” strategy. Some of the prepared polymers were used to undergo another level of self-assembly to form vesicles. After chemical derivations, the vesicles were converted to capsules bearing regularly packed nano-scale channels, which were pH-responsive. This thesis is about the “double assembly” of linear block copolymer precursors and “double chemical processing” to yield functional nanostructures Triblock copolymers containing a short central carboxyl-bearing block and a diblock copolymer containing a short amine-bearing block were synthesized. One triblock and one diblock copolymer precursor was subsequently associated via electrostatic interactions between their amine- and carboxyl-bearing blocks and were covalently linked via amidization to yield ABC miktoarm copolymers. In particular, this “assembly-and-reaction” strategy was used to prepare µ-(PtBA100)(PSMA66/200)(PCEMA120) copolymers, where PtBA, PSMA, and PCEMA respectively denote poly(tert-butyl acrylate), poly(2,2-dimethyl-1,3-dioxolan-4-yl)methyl methacrylate (also called poly(solketal methacrylate)), and poly(2-cinnamoyloxyethyl methacrylate). These copolymers were ideal candidates for functional nanostructures, because all three of their arms could be derivatised. PSMA was hydrophobic but could be hydrolyzed into PGMA (poly(glyceryl methacrylate)), which contained hydroxyl groups and was soluble in water, PtBA could be hydrolyzed into PAA (poly acrylic acid), and PCEMA was photocrosslinkable. ABCx miktoarm copolymers were synthesized through the same “assembly and reaction” strategy but using different precursor feed ratios. These copolymers were pseudo-miktoarm structures because two short reactive blocks rather than a traditional trivalent functional group were used to link the long blocks. Moreover, the average number x of diblock copolymer chains that were attached to each triblock copolymer chain was not 1, but 1.14 and 1.58 for the two µ-(PtBA)(PCEMA)(PEO)x copolymers that we have prepared. Since only the poly(ethylene oxide) (PEO) block was soluble in a tetrahydrofuran/water mixture containing 80 vol% water, the two µ-(PtBA)(PCEMA)(PEO)x samples formed vesicles with PtBA and PCEMA as the wall. Using appropriate lengths for the PtBA and PCEMA blocks, we also ensured that PtBA formed cylinders that permeated across the PCEMA wall. Photocrosslinking the PCEMA wall and hydrolyzing the PtBA chains in the cylindrical domains yielded permanent capsules permeated by regularly-packed poly(acrylic acid)-gated nanochannels. These capsules were loaded with various cargoes and exhibited pH-responsive reagent release in water, releasing reagents faster and more completely at higher pH's.
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