## 3 Results and Discussion
### Synthesis of Alkyne-Functionalized Arborescent PBG Cores  
Alkyne-functionalized PBG substrates were obtained by reacting the carboxylic acid groups on the substrates with propagylamine via carbodiimide activation. The synthesis of arborescent G1 PBG, either randomly or chain end-functionalized with carboxylic acids, was discussed in detail in an earlier publication [16]. A schematic representation of the preparation of the two types of alkyne-functionalized G1 PBG substrates is provided in Figure 2 as an example. Both syntheses start from a G0 PBG substrate randomly functionalized with carboxylic acid groups, obtained by coupling linear partially-deprotected PBG with linear PBG side chains, followed by partial deprotection [16]. In Figure 2A, a G1 PBG substrate with randomly-distributed carboxylic acid groups is generated by grafting non-functional (hexyl-terminated) linear PBG on the G0 substrate and partial deprotection with HBr. In Figure 2B, PBG side chains derived from an initiator with two _tert_-butyl ester-protected carboxylic acid functionalities are used to obtain the G1 PBG substrate. This allows selective deprotection of the _tert_-butyl ester protecting groups at the chain ends with trifluoracetic acid (TFA). A one-fold excess of propagylamine was added in the reaction to ensure complete conversion of the carboxylic acid groups to alkyne amide functionalities. The carboxylic acid functionalities on the substrates were easily accessible to the small molecule propagylamine, so that complete conversion was expected. For convenience, a preparative SEC instrument was used to isolate the alkyne-functionalized PBG substrates from excess propagylamine prior to \({}^{1}\)H NMR analysis, albeit this could also have been achieved by successive precipitations of the sample. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Polymer** & \multicolumn{2}{c}{**SEC 1** ** & \multicolumn{2}{c}{**1H NMR**} \\ \hline  & \(M_{\mathrm{n}}\) (g/mol) & \(M_{\mathrm{w}}/M_{\mathrm{n}}\) & \(X_{\mathrm{n}}\)2  & \(M_{\mathrm{n}}\) (g/mol) 3 [FOOTNOTE:3]Footnote  
\({}^{1}\)H NMR spectra illustrating the synthesis of a randomly alkyne-functionalized G1 PBG substrate are provided in Figure 3. The spectrum for G1 PBG before removal of a portion of the benzyl ester protecting groups is displayed at the top, whereas the spectrum for partially-deprotected G1 PBG is shown in the middle. The integrals for the benzyl ester protons at 5.0 and 7.2 ppm decreased in intensity, relatively to the methine proton at 4.4-3.7 ppm (integral value set to 1.00), from 5.08 and 2.01 before deprotection to 4.09 and 1.63 after partial deprotection, respectively. This corresponds to a deprotection level of 19%. The spectrum for randomly alkyne-functionalized G1 PBG, provided at the bottom of the figure, has an alkyne proton resonance appearing at 3.0 ppm and a signal for the protons adjacent to the alkyne group near 3.8 ppm. The alkyne substitution level was determined by integration of the signal at 3.0 ppm and the peptidic methine proton peak from 4.4 to 3.7 ppm. An overlapping peak at 3.0 ppm is from \(n\)-hexylamine, the initiator serving in the ring-opening polymerization of benzyl glutamate to synthesize the PBG substrates. This signal was present before the propagylamine reaction and was, therefore, subtracted from the integration value. The overlapping peaks at 3.8 ppm from the propagylamine protons were likewise subtracted from the peak integral for the methine proton at 4.4-3.7 ppm. The carboxylic acid and alkyne functionality levels determined by that method were both 19 mol %, indicating 100% conversion. The \({}^{1}\)H NMR spectra referring to the synthesis of G1 PBG functionalized with alkyne groups at the chain ends are provided in Figure 4. The top spectrum is for the G1 polymer with _tert_-butyl group-protected chain ends, to be selectively deprotected by dissolution in neat TFA (providing carboxylic acid functionalities) while leaving the benzyl ester substituents on the chains unaffected (middle spectrum on Figure 4).