Letter pubs.acs.org/macroletters
Well-Defined Cyclic Triblock Terpolymers: A Missing Piece of the Morphology Puzzle George Polymeropoulos, Panayiotis Bilalis, and Nikos Hadjichristidis* King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, Thuwal 23955, Saudi Arabia S Supporting Information *
ABSTRACT: Two well-defined cyclic triblock terpolymers, missing pieces of the terpolymer morphology puzzle, consisting of poly(isoprene), polystyrene, and poly(2-vinylpyridine), were synthesized by combining the Glaser coupling reaction with anionic polymerization. An α,ω-dihydroxy linear triblock terpolymer (OH-PI1,4-b-PS-b-P2VP-OH) was first synthesized followed by transformation of the OH to alkyne groups by esterification with pentynoic acid and cyclization by Glaser coupling. The size exclusion chromatography (SEC) trace of the linear terpolymer precursor was shifted to lower elution time after cyclization, indicating the successful synthesis of the cyclic terpolymer. Additionally, the SEC trace of the cyclic terpolymer produced, after cleavage of the ester groups, shifted again practically to the position corresponding to the linear precursor. The first exploratory results on morphology showed the tremendous influence of the cyclic structure on the morphology of terpolymers.
T
Scheme 1. General Reaction Scheme for the Synthesis of Cyclic Triblock Terpolymers
he synthesis of linear di/triblock co/terpolymers, as well as of the corresponding stars, has been reported in the literature, and their self-assembly properties in selective solvent/bulk have been extensively studied.1 Furthermore, the synthesis of cyclic diblock copolymers and the comparison with the corresponding linear and star structure have also been described.2 Due to the difficulty in the synthesis of cyclic terpolymers, only one group3 has reported the synthesis of only one low MW (13 000 g/mol) well-defined cyclic triblock terpolymer of PS, PI, and PMMA by intra-amidation of an anionically synthesized α-NH2-ω-COOH linear terpolymer. However, since this method was not efficient for high MW samples the cyclic synthesized was below the segregation limit and consequently not phase separated. Therefore, there was a missing piece in the puzzle of the influence of architecture on the morphology of triblock terpolymers.4 In this communication we report a straightforward approach for the synthesis of cyclic triblock terpolymers consisting of poly(isoprene) with high 1,4-microstructure (PI1,4), polystyrene (PS), and poly(2-vinylpyridine) (P2VP). Moreover, we compare the bulk morphologies of the cyclic terpolymers with the ones of the linear precursors. The cyclic triblock terpolymers were obtained in four general steps (Scheme 1 and S1 of SI): (a) Synthesis of α-tbutyldimethylsilyloxy-ω-hydroxy triblock terpolymer, t-butyldimethylsilyloxy-PI1,4-b-PS-b-P2VP-OH, by high vacuum sequential anionic copolymerization of isoprene (Is), styrene (St), and 2-vinylpyridine (2-VP) with 3-(t-butyldimethylsilyloxy)-1-propyl-lithium as initiator,5 followed by end-capping with ethylene oxide (EO); (b) deprotection of the t© XXXX American Chemical Society
butyldimethylsilyl group with hydrochloric acid; (c) esterification of the OH groups with 4-pentynoic acid, in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and N,N-dimethylpyridin-4-amine (DMAP), in order to introduce alkyne groups on both chain ends;6 and (d) intramolecular ring closure in the presence of Cu(I)Br/N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) and pyridine at room temperature.7 Received: October 22, 2016 Accepted: October 25, 2016
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DOI: 10.1021/acsmacrolett.6b00807 ACS Macro Lett. 2016, 5, 1242−1246
Letter
ACS Macro Letters
spectroscopy and more specifically by the observation of the characteristic peak of t-butylsilyl at 0.9 ppm. Treatment of the protected triblock terpolymer with 10 M HCl, at 50 °C for 24 h, led to the disappearance of the chemical shifts at 0.9 ppm (Figure 1B) indicating the complete removal of the tbutyldimethyl-silyl groups. Additional confirmation of the quantitative deprotection arises from the fact that the ratio of the integrated area at 3.5 ppm before (Figure 1A) and after (Figure 1B) deprotection to afford α−ω hydroxyl triblock terpolymer is practically 0.5 (1/1.9), indicating the existence of two hydroxyl groups per chain. The experimental procedure followed for the cleavage of the t-butyldimethyl-silyl groups is given in detail in the SI. The next step toward the cyclic structure was the synthesis of the dialkyne triblock terpolymer. For this reason, the hydroxyl chain-end groups of the telechelic triblock terpolymers were esterified with pentynoic acid in the presence of DCC and DMAP at room temperature for 48 h.6 The successful esterification was confirmed by 1H NMR and FT-IR spectroscopy. Comparing the 1H NMR spectra of the dihydroxy with the corresponding dialkyne sample (Figure 1B and 1C) new chemical shifts appeared at δ = 2.3 and 2.5 ppm, corresponding to the terminal proton of the alkyne functional groups and to the methylene protons adjacent to the triple bond, respectively. FT-IR spectroscopy was also employed to verify the complete transformation of the hydroxyl into alkyne groups. The FT-IR spectrum reveals the disappearance of the broad peak at 3400 cm−1, which is attributed to the stretching of the hydroxyl groups (Figure S3A), with the simultaneous appearance of a new peak at 3300 cm−1 corresponding to the alkyne stretching (Figure S3B). The cyclic triblock terpolymers (hereafter noted as c-PI1,4-bPS-b-P2VP) were synthesized by the intramolecular Glaser coupling reaction of the linear telechelic alkyne-functional precursors.7 The oxidative alkyne dimerization was promoted by oxygen, using pyridine as solvent in the presence of
Experimental details are given in the Supporting Information (SI). Two linear triblock terpolymers with different molecular weights but the same mass fractions were synthesized and used as the precursors for the cyclic structures. The molecular characteristics of all precursors and final products are given in Table 1. Table 1. Molecular Characteristics of Linear (l-PI1,4-b-PS-bP2VP) and Cyclic Triblock Terpolymers (c-PI1,4-b-PS-bP2VP) sample
(M̅ n)SECa (g/mol)
l-PI-b-PS-b-P2VP-1 c-PI-b-PS-b-P2VP-1 l-PI-b-PS-b-P2VP-2 c-PI-b-PS-b-P2VP-2
34.000 23.000 60.000 50.500
Đ
f(PI)b (1H NMR) % (w/w)
f(PS)b (1H NMR) % (w/w)
1.05 1.06 1.06 1.06
0.31 0.31 0.31 0.32
0.30 0.31 0.32 0.32
Size exclusion chromatography in THF at 35 °C using polystyrene standards. bMass fraction was calculated via 1H NMR spectroscopy in CDCl3 at 25 °C.
a
In the case of the linear precursors, all SEC traces correspond to monomodal narrow molecular weight distributions (Figure S1) indicating a high degree of homogeneity. The successful synthesis was also confirmed by 1H NMR spectroscopy. The 1 H NMR spectrum of the l-PI1,4-b-PS-b-P2VP-1 sample (Figure 1A) reveals the characteristic chemical shifts of all blocks, as well as of the t-butyl silyl group at 0.9 ppm and the EO unit at 3.5 ppm. The double peak at 4.7 ppm is attributed to the PI with 3,4 microstructure. The 1H NMR spectrum of the l-PI1,4-b-PS-b-P2VP-2 sample is given in the SI (Figure S2A). The quantitative deprotection of the t-butyldimethyl-silyl groups was accomplished using hydrochloric acid (HCl).5 The progress of the reaction was monitored by 1H NMR
Figure 1. 1H NMR spectra of the: (A) linear precursor t-butyl-dimethylsiloxy-propyl-functionalized triblock terpolymer (l-PI1,4-b-PS-b-P2VP-1) with ω-hydroxyl chain end; (B) the corresponding deprotected α−ω hydroxyl triblock terpolymer; and (C) the dialkyne triblock terpolymer. 1243
DOI: 10.1021/acsmacrolett.6b00807 ACS Macro Lett. 2016, 5, 1242−1246
Letter
ACS Macro Letters
precursor was higher (Figure 2A). The differences in the apparent molecular weights between the cyclic triblock terpolymers and their linear analogues, calculated by SEC analysis, are given in Table 1. In order to further confirm the synthesis of the cyclic terpolymers, additional SEC traces were taken from a mixture of cyclic/linear as well as from individual linear and cyclic samples (Figure S5). Figure 2A demonstrates the main peak that corresponds to the cyclic triblock terpolymer along with two minor peaks. The first one, with double molecular weight with respect to the main peak, is attributed to the polycondensation byproducts (less than 2%) and the second one to the unreacted dialkyne triblock terpolymer (less than 4%). Similar SEC traces are given in the SI for the linear and cyclic samples PI-b-PS-b-P2VP-2 (Figure S6). The very low percentage of both peaks, compared to the one of the cyclic polymer, indicates that the adopted synthetic procedure led to uniform cyclic terpolymers, in high yield, with significant reduction of the undesirable intermolecular reactions. Consequently, the Glaser coupling reaction in combination with anionic polymerization leads to almost pure cyclic polymers (
