# Lastingly Colored Polylactide Synthesized by Dye-Initiated Polymerization
## 3 Results and Discussion
Furthermore, the GPC traces and experimental Mn estimated within both detection systems are shifted toward higher molar masses, once the ratio of [\(L\)-LA]\({}_{0}\)/[ROH]\({}_{0}\) increased. Figure 6: Polylactide (PLA)-n-DR1 conjugate samples with controlled molecular weights. Figure 7: PLA-n-DR13 conjugate samples with controlled molecular weights. Then, a plot of relative absorbance at 470 nm versus the inverse DPn was also prepared for all samples broken down by the type of DR. Relative absorbance was determined by dividing the area under the molar mass distribution measured with GPC, equipped with a PDA detector at 470 nm by the sample concentration and injection volume. The experimental data of both DR series fit famously to a linear trend (determination coefficients R2 \(>\) 99%); however, the slopes of the series are slightly different due to the nature of the chromophore (Figure 9). The plot shows that a color intensity of the polymer may be precisely adjusted by the initial ratio of monomer to initiator, as well as blending with undyed PLA. Moreover, the kind of metal does not affect the trend, so from that aspect, both types of initiators are appropriate. Figure 8: Gel-permeation chromatography (GPC) traces for selected PLA-n-DR obtained from various detection systems: refractometer (solid lines) and photo diode array (PDA) detector at 470 nm (dashed lines) for various ratios of [\(L\)-LA]\({}_{0}\)/[ROH]\({}_{0}\) for polymerization initiated with DR1 (left plot) or DR13 (right plot) series. Figure 9: Relative absorbance of the PLA-n-DR at 470 nm versus inverse DPn for DR1 (red circles) and DR13 (black squares) series; points are experimental data, whereas dashed lines are linear fits for that data. MALDI-ToF analysis of obtained products revealed that the population of linear macromolecules initiated with respective DR, and the series of macrocycles of even and odd numbers of lactic acid monomeric units were present (Table 2, Table S1, Figures S17-S24). The linear products, which formally initiated with water molecules, were discovered only in the case of DR13 series with low DP\({}_{\rm n}\) (Table 2, no. 1 and 2); however, a fraction of a number of molecules in that population was merely 2%. It proves that the ROP was exclusively initiated with DR alcohols. The presence of chains comprising odd numbers of lactic acid repeating units, as well as macrocyclic products in all samples, indicate that transesterification processes took place in the system, which broadened distributions of molar masses. The fraction of number of molecules in a population for macrocyclic products increases for a higher ratio of [\(L\)-LA]0/[ROH]0. However, those data are strongly distorted by the poor ability of long linear chains present in those samples to undergo flight in the MALDI system, comparing with the macrocyclic population which is rather of similar and quite low Mn in all samples (Table S1). Therefore, we think that the most accurate results among the set of experiments presented in Table 2 are for samples nos. 1, 2, 6, and 7, in which 0%, 4%, 0%, and 2.4% of cyclic macromolecules have been demonstrated, respectively. The macrocyclization processes caused the odd and even numbers of lactic acid monomeric units in PLA molecules to be comparable to each other in most of the cases. The samples containing very low DP\({}_{\rm n}\), had even numbers, which were c.a. 1.5-1.7 times higher than odd ones. The \({}^{1}\)H NMR spectra with proton signals assigned to the structures of exemplary oligomeric conjugates PLA-10-DR1 and PLA-10-DR13 are shown in Figures 11 and 12. The spectra showed the expected chain ends: resonances due to dye molecules (denoted as red letters D-K) and a hydroxyl group (blue letter A). The most intensive signals are multiplets at 5.04 ppm (blue B\({}_{3}\)) and 1.33 ppm (blue C\({}_{3}\)), corresponding to methine (CH) and methyl (CH\({}_{3}\)) groups, respectively. These groups are present in the oligolactide, which consists of eight repeating units.