Poly(alkyl ethylene phosphonate)s with different alkyl side chains exhibit significant differences in their degradation behavior. Three novel 2-alkyl-2-oxo-1,3,2-dioxaphospholanes, cyclic monomers for the ring-opening polymerization (ROP) toward poly(alkyl alkylene phosphonate)s, were synthesized by robust two- or three-step protocols in reasonable yields and high purity. The polymerization was promoted by the organocatalysts 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and proceeded with high control over molecular weight and narrow molecular weight distributions (A 1.2) up to full conversion. These polymers with methyl, ethyl, and isopropyl side chains are perfectly soluble in water (up to 25 mg mL-1) without a temperature-dependent phase separation. They showed no toxicity against HeLa cells after 24 h of incubation at any tested concentration. Polymers with butyl side chains exhibit decreased solubility and concentration-dependent cloud point temperatures and show toxicity against HeLa cells at concentrations above 25 mL-1. The polymers showed no acetylcholinesterase inhibition. All polymers exhibited significantly different degradation times under both neutral as well as basic conditions (variation of the alkyl side chain allowed stabilities from 8 h up to 6 days).