After successful polymerization of Poc-Sar with 4MTPPC, the activation (via oxidation) of the terminal MTP esters was performed. As was reported previously, the different reagents can be used for oxidation. Johnson and Jacob treated N-carbobenzoxy amino acid MTP esters of lysine, alanine, leucine, etc., with 30 % H2O2 in glacial acetic acid for 12 h to obtain the corresponding MTO2P esters . Still, these conditions were found to be more harsh Popovic and colleagues introduced oxone as the oxidizing agent for 4MTP activation . In a slightly different but related approach, Wu et al. reported a very interesting PPM of thiocarbamate derivatives of poly(2-oxazoline)s, which were also oxidized using m-CPBA to make them amendable for nucleophilic substitution . Accordingly, we chose to utilize the same strategy to prevent polymer degradation and confirmed that the sulfone is generated under mild conditions with 3.0 eq. m-CPBA (Figure ). GPC traces of the oxidized polymer remained monomodal, narrow and Mn remained essentially unchanged (Figure ), which indicates that polymer degradation or cross-linking does not occur . It should be noted, when we increased the oxidation time up to 24 h, GPC analysis suggests polymer degradation as evidenced by a decrease of the molar mass, as well as the presence of the high molecular weight shoulder point on the interaction of macro chains, ends with each other (Supporting information Figure ). After successful activation to yield oxidized polysarcosine (PSaroxid), we performed nucleophilic substitution with different low and higher molecular weight N-and S-nucleophiles. In the first experiments, PSaroxid was incubated with 3.0 eq. of N-nucleophile (benzylamine, neopentylamine and dansylcadaverine) overnight, showing a satisfying degree of functionalization (Figure ). 1 H NMR spectra of PSaroxid treated with benzylamine in MeCN