As we face the issues associated with fossil resource-based polymers and their environmental impact after their useful life, polyesters become more important as “greener” alternatives due to their potential hydrolytic and enzymatic degradability in various environments. Moreover, post-modifying their structure can additionally open up access to a variety of new materials. During this work the potential to post-modifying synthetic PHB made via the organocatalysed ring-opening polymerisation of β-butyrolactone (β-BL) is shown. Modification by thiol–ene ‘click’ chemistry was succesfully conducted under UV-initiation. Surprisingly, attempting the modification under thermal conditions using dimethylphenylphosphine (DMPP) as catalyst, resulted in the attachment of the phosphine, as shown via NMR spectroscopy. Control experiments using crotonic acid, methyl crotonate and n-butyric acid indicated that the presence of a carboxylic acid group is necessary in order for the phosphine addition to occur. Further, the formation of particles shown via dynamic light scattering (DLS), zeta-potential (ZP) and transmission electron microscopy (TEM) measurements suggest an amphiphilic character of the phosphine-functionalised polymers. Finally, stability studies in the presence of salt and different pH environments revealed a high responsiveness and dependency between pH and particle size as well as surface charge.