Temperature-induced self-assembly of block copolymers allows the formation of smart nanodimensional structures. Mostly, nondegradable lower critical solution temperature (LCST) segments are applied to prepare such dynamic aggregates. However, degradable upper critical phase separation (UCST) block copolymers that would allow the swelling or disassembly at elevated temperatures with eventual backbone hydrolysis have not been reported to date. We present the first well-defined degradable poly(phosphonate)s with adjustable UCST. The organocatalytic anionic ring-opening copolymerization of 2-alkyl-2-oxo-1,3,2-dioxaphospholanes provided functional polymers with excellent control over molecular weight and copolymer composition. The prepolymers were turned into thermoresponsive polymers by thiol-ene modification to introduce pendant carboxylic acids. By this means, non cell-toxic, degradable polymers exhibiting UCST behavior in water between 43 and 71 °C were produced. Block copolymers with PEG as a nonresponsive water-soluble block can self-assemble into well-defined polymersomes with narrow size distribution. Depending on the responsive block, these structures either swell or disassemble completely upon an increased temperature.