Unexpected Temperature Behavior of Polyethylene Glycol Spacers in Copolymer Dendrimers in Chloroform

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The dendrimer macromolecules were investigated in a dilute chloroform solution by 1H NMR methods (spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is “frozen” at high temperatures (above 260 K), but it unexpectedly becomes “unfrozen” at temperatures below 250 K (i.e., melting when cooling). The transition between these two states occurs within a small temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1 and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by dendrimer conformations, in which the PEG group concentration in the layer increases with growing G. We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practical applications such as nanocontainers or nanoreactors.


Structure and NMR spectra of carbosilane dendrimers with terminal phenylbenzoate group connected by oligo(ethylene) glycol (PEG) spacer.
Fig. S1 shows the 1 H NMR spectra of the dendrimers at 298 K and 218 K. We identified spectral peaks with different molecular groups of dendrimers using materials of Refs. 10,22 . The numbering of peaks of the NMR spectrum in Fig. S1 corresponds to one of the groups in Fig. S2. Spectra of the studied dendrimers can be divided into three main parts. The peaks of the dendrimer core are mainly located in strong fields (0-2 ppm). Accordingly the identification in Ref. 10 peak α corresponds to the inner Si-CH 3 groups.
Peak β represents the signal from the external Si-CH 3 groups, which are the connectors between the dendrimer core and the PEG spacer. The inner CH 2 groups contribute peaks to 1 and 3. Peaks 2, 4, and 6 appear in NMR spectrum of mesogenic groups with butyl (BUT) groups (see, for instance, Fig. S7 in "Supplementary Information" of Ref. 22 ). It is natural to identify the line with CH 3 groups of BUT groups because this line is narrow and possesses the smallest chemical shift in this groups (~1 ppm). Chemical shifts are used for the recognition of CH 2 groups of BUT tail. we identify the last peak 5 in this region as belonging to the first CH 2 group of PEG spacer (without neighboring oxygen atom).
Peaks, γ, γ', γ", 7 (corresponding to the PEG spacer), and peak ε, (corresponding to the group CH 2 -O of the tail of terminal segments) are located in the range of 3 to 5 ppm.
The 1 H NMR spectra for the carbosilane dendrimer with the same mesogenic groups (with and without BUT tail), connected by aliphatic spacers, allow one to identify lines ε and γ".
In the case of absence of BUT tails only one peak is observed in the spectrum (~4.3 ppm, see Fig. S6 in "Supplementary Information" of Ref. 22 ). Thus, this peak (γ") corresponds to the CH 2 -O-C(O) groups. In our case this peak has higher chemical shift (4.45 ppm) due to the influence of PEG fragment in the studied dendrimer. Second peak in the region appears for the dendrimer structure with BUT tail at 4.05 ppm (see Fig. S7 in "Supplementary Information" of Ref. 22 ). Due to this fact, we can accurately identify the peak ε at 4.05 ppm in our case. Other groups of the PEG spacer (7, γ, and γ') we assigned with the structural formula (Fig. S2) in accordance with the increase in chemical shifts.
In the diapason of 6-9 ppm there are a few peaks, corresponding to proton signals of aromatic mesogenic groups. For more details " Supplementary Information Fig. S1).

Temperature evolution of PEG peaks in G5 TG1
Fig. S3 reflects the final changes of PEG moiety in G5TG1 dendrimer from room temperature to 218 K. At room temperature the spectrum of the groups practically is not resolved in contrast to the spectrum at low temperature.
The temperature evolution of the shape of the peaks is shown in Fig. S4. It can be seen that the changes in the spectrum occurs in a fairly wide range of temperatures (250 K