Thermoresponsive materials are currently discussed for applications, e.g., in drug delivery or sensor systems. While homopolymers exhibit a certain LCST, copolymers allow adjusting the cloud points depending on the comonomer ratio. We report on degradable poly(phosphester) (PPE) copolymers exhibiting LCSTs, which can be further tuned by reversible postmodification. A library of copolymers with different cloud points is obtained from a single “precursor polymer”. A novel furfuryl-carrying cyclic phosphate was designed, and different PPE copolymers were prepared via ring-opening polymerization with adjustable furan contents (up to 25 mol % were targeted and achieved) and molecular weights up to 40 000 g/mol (Đ < 1.25). Modification was achieved by a Diels–Alder reaction with maleimides of different hydrophilicity to further alter the solubility profile of the polymers. While the postmodification is thermally reversible, the biodegradable PPE backbone remains unaffected. This first report on the Diels–Alder postmodification of PPEs to adjust LCST behavior further underlines the versatility of this polymer class and might be used in future drug delivery and temperature-responsive devices.