A Facile Synthetic Route to Functional Poly(phenylacetylene)s with Tunable Structures and Properties
Pentafluorophenyl (PFP) ester-functionalized poly(phenylacetylene)s (PPAs, P1, P2, and P3) were designed and synthesized in desirable yields and molecular weight by using organorhodium complexes as catalysts. Furthermore, these PFP-containing PPAs were used as precursors to prepare a series of mono- and dual-functionalized PPAs by the substitution of the activated ester moieties with functional amines. The structures of the PFP-containing PPAs and the derived functional PPAs were characterized by using multiple spectroscopic techniques including GPC, FTIR, 1H NMR, 13C NMR, and 19F NMR. The experimental details and the characterization data demonstrate that activated ester synthetic route to functional PPAs is facile (just stirring the precursor polymer with proper amine(s) at room temperature for hours), efficient (complete transition from ester to amide has been confirmed), and quantitative (the relative content of a specific functionality can be precisely preset by controlling the feed ratio of the functional amines). By reacting three PFP-containing PPAs with chiral amines or with chiral and nonchiral alkyl amines in a step-by-step way, a series of seven different PPAs with asymmetric carbon in the side chains were obtained. CD measurements indicated that the incorporation of chiral amine into polymer side chains induced helicity formation of P1 backbone. P1-C*Ph(L) and P1-C*Ph(D) backbones adopt predominantly right-handed and left-handed helical conformation, respectively. While the flexible spacer between the chiral center and the rigid PPA backbone blocked the induction of main-chain helicity by chiral pendants, thus no CD signals were recorded for P2-C*Ph(L) and P3-C*Ph(L). Substitution of PFP ester with amine-functionalized PEGs transited the hydrophobic PPAs to hydrophilic. All of the PEG-containing PPAs can be dissolved in water and form clear solutions. Meanwhile, all of the aqueous solutions exhibit LCST behavior and the hydrophilic PEG chains and hydrophobic alkyl spacers have positive and negative impact on the cloud point, respectively. Contact angles measurements showed that the length and content of the PEG chains contribute greatly to the hydrophilic property, and the length of the alkyl spacers and the content of the alkyl amine component played a contrary role. By controlling the ratio of the PEGylated and alkyl amines, the amphiphilic property of the PPAs can be well tailored.
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