Molecular Weight Distribution of Living Chains in Polystyrene

Letter pubs.acs.org/macroletters

Molecular Weight Distribution of Living Chains in Polystyrene Prepared by Atom Transfer Radical Polymerization Joongsuk Oh,† Jiae Kuk,† Taeheon Lee,‡ Jihwa Ye,‡ Hyun-jong Paik,*,‡ Hyo Won Lee,*,§ and Taihyun Chang*,† †

Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Korea ‡ Department of Polymer Science and Engineering, Pusan National University, Busan, 46241, Korea § Department of Chemistry, Chungbuk National University, Cheongju, 28644, Korea S Supporting Information *

ABSTRACT: Living and dead chains of a polystyrene synthesized by atom transfer radical polymerization were separated and characterized by high performance liquid chromatography (HPLC), size exclusion chromatography (SEC), NMR, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The bromine end group in the living chain was quantitatively converted to a hydroxyl end group via first azidation and subsequent copper-catalyzed azide−alkyne cycloaddition (CuAAC) click reaction with propargyl alcohol. The living chains bearing a polar end group are fully resolved from the unmodified dead chains by HPLC separation using a bare silica stationary phase. Molecular weight distributions (MWD) of the living and dead chain are characterized by SEC and MALDI-MS. The MWD of the living chains is close to a Poisson distribution. Interestingly, the elution peak of the living chains in the HPLC separation split into two. The peak split is attributed to the diastereomeric structure of the chain end by NMR and MALDI-MS analyses.

A

chains from the dead chains since the end group of the ATRP living chains is labile during the MALDI process.24,25 High performance liquid chromatography (HPLC) is known to be efficient to separate polymers according to CEF. The polymers with a polar CEF interact more strongly with a polar stationary phase such as bare silica than the polymers without a polar CEF. We reported earlier that polystyrene (PS) with MW over 100000 can be fully resolved by one terminal hydroxy group difference.26 Copper-catalyzed azide−alkyne cycloaddition (CuAAC) click reaction is a popular reaction since the reaction proceeds fast and almost quantitatively.27 The halogen end group of an ATRP grown living chain can be easily converted to an azide group through a reaction with sodium azide and these methods have been commonly applied to modify the CEF of polymers prepared by ATRP. CuAAC with azide chain end polymer and propargyl alcohol is the simplest method to make hydroxyl chain end in the ATRP polymer. Gao et al. reported a successful HPLC separation of dihydroxy PS synthesized via this method.28 In this paper, we report the MWD analysis of the living chains of a PS prepared by ATRP by quantitative chain end modification, HPLC separation, and MALDI-MS analysis.

tom transfer radical polymerization (ATRP) is a useful method to obtain polymers with a narrow molecular weight distribution (MWD)1,2 and various chain end functionalities (CEF).3−7 While polymer chains are growing in ATRP, dead chain formation is unavoidable, albeit suppressed, due to the termination reaction between the free radicals as well as side reactions such as loss of HBr from the chain ends8 that are responsible for imperfect CEF and a broader MWD than a living anionic polymerization.9−11 The dead chain fraction is affected by various factors including the rates of polymerization, the monomer conversion, the target degree of polymerization, and so on.12−14 The imperfect living nature of ATRP limits the purity in the synthesis of block copolymers15,16 and topological polymers.17,18 There are a few theoretical/simulation studies on the MWD of ATRP polymers,19−22 but the precise MWD of polymers prepared by ATRP is yet to be addressed experimentally since the ATRP polymers has an overlapped MWD of both living and dead chains. MWD of polymers is commonly measured by size exclusion chromatography (SEC). However, SEC separates polymers according to the chain size in solution and it is impossible to differentiate the living chains from the dead chains, let alone the limited resolution of SEC.23 Matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS) is a powerful MWD analysis technique for the polymers with low MW and narrow MWD, but it is not successful to differentiate the living © XXXX American Chemical Society

Received: June 19, 2017 Accepted: June 22, 2017

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DOI: 10.1021/acsmacrolett.7b00447 ACS Macro Lett. 2017, 6, 758−761

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ACS Macro Letters A PS (Mw = 3300, Đ = 1.04) was synthesized by ATRP (conversion 28%) with pentamethyldiethylenetriamine (PMDETA) ligands and ethyl α-bromoisobutyrate (EBib) initiator. The bromine CEF in the living chains in the PS was modified to a polar CEF by the two-step substitution reactions shown in Scheme 1. Details of synthesis and characterization procedures are described in the Supporting Information. Scheme 1. Preparation of PS-OH by ATRP and CEF Modification

Figure 1. Characterization of PS synthesized by ATRP during the chain-end modification reactions: (a) SEC and (b) 1H NMR.

silica column (Nucleosil, 250 × 4.6 mm, 50 Å pore) with THF/ n-hexane mixed solvent was chosen to separate them.25 As displayed in Figure 2a, the dead chains (F1) eluted out at tE ∼ 9 min under the eluent composition of THF/n-hexane (42/58, v/v), while the living chains are retained in the column due to the strong interaction between the polar CEF and the polar stationary phase. Upon increasing the THF composition, the living chains elute out. Interestingly, the elution peak of the living chains split into two, F2 and F3. The three fractions (F1, F2, and F3) are collected over the range shown with vertical bars for further analyses. 1H NMR spectra of each fraction are shown in Figure 2b. F1 does not show any peaks of Hb nor Hc, indicating that it is the dead chain fraction. In the case of F2 and F3, the peak areas of Hb are exactly half of Ha and the peak areas of Hc are the same as Ha. It means that both F2 and F3 are living chains with quantitative CEF. It is interesting to note that the NMR peak shapes of Hb and Hc in F2 and F3 differ one another and it will be discussed later. In Figure 3a, SEC chromatograms of the three fractions are displayed together with the as-prepared PS-OH (before the HPLC separation). The intensities of the living (F2 and F3) and the dead chains (F1) are adjusted so that the sum of the two chromatograms reproduces the as-prepared PS-OHs. F2 and F3 show exactly the same peak shape, indicating that they have an identical MWD. The perfect match of the as-prepared PS-OHs and the reconstructed chromatogram supports the quantitative nature of this analysis. The amount (integrated

During the substitution reaction of the living chains, MWD of PS-Br, PS-N3, and PS-OH are not changed according to the SEC measurements, as shown in Figure 1a. Their 1H NMR spectra in Figure 1b prove that the CEF of the living chains are converted quantitatively within the precision of the NMR measurement. The normalized NMR peak area of the Hb proton shows the same value of 0.86 during the two substitution reactions when normalized at two Ha protons at the initiator moiety, while the chemical shift of the Hb proton changes (4.5 → 3.9 → 5.1 ppm) according to the CEF change. The peak of two Hc protons appears in the final product (PSOH) at 4.7 ppm, and the relative peak intensity (1.73) is also in accord with the Hb intensity. It means that ∼15% of the PS is the dead chain that does not have a bromine chain end. After the substitution reactions, all living chains carry a triazole ring and a hydroxyl end group, while dead chains should remain unchanged in the sample. HPLC can separate the dead chains from the living chains carrying the polar CEF that interacts strongly with a polar stationary phase. A bare 759

DOI: 10.1021/acsmacrolett.7b00447 ACS Macro Lett. 2017, 6, 758−761

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ACS Macro Letters

Figure 3. (a) SEC chromatograms of the dead chain (red, F1) and living chain (blue, F2 and F3) with the as-prepared PS-OH (black). Summation of reconstructed chromatograms (green dashed) of dead chain and living chain is well matched with as-prepared PS-OH. Inset: SEC curve comparison of the dead chains (red) and the living chains (blue) with equally adjusted peak heights. (b) MWD of living chain by MALDI-MS (black bar) and theoretical Poisson distribution (red line).

Figure 2. (a) Solvent gradient HPLC separation and fractionation of PS-OH; (b) 1H NMR spectra of the three fractions.

area) of the dead chains (F1) is ∼15%, which is consistent with the NMR analysis shown in Figure 1b. In the inset, the SEC chromatograms of the dead chains (F1) and the living chains (F2 or F3) are compared after the peak heights are adjusted for an easy visual comparison. The dead chains (Mw = 3600, Đ = 1.16) have a much broader MWD than the living chains (Mw = 3300, Đ = 1.03), with both higher and lower MW portions that reflect both coupling and disproportionation reactions occurred in the termination mechanism. The chain structure and MWD of the fractions were investigated by MALDI-MS. As shown in Figure S1, F2 and F3 show the identical mass spectra and the m/z values are well matched with the molecular structure of the PS-OH chain (theoretical m/z of 30mer C249H255O3N3Na+: 3360.0, observed: 3360.0). On the other hand, F1 is identified as dead chains by MALDI-MS in Figure S2. In Figure 3b, MWD of the living chains show Mw = 3383 and Mn = 3278 (Đ = 1.03). The MWD matches quite well with a Poisson distribution drawn in red line. Poisson distribution is the MWD expected in an ideal anionic polymerization.29,30 It was predicted earlier that the MWD of an ATRP grown polymer approaches a Poisson distribution as the number of activation/deactivation cycles increases if no termination reaction occurs.20,31 Since the MWD of the living chains is equivalent to the MWD in the absence of termination reaction, the Poisson distribution is feasible for the living chains. However, it needs further investigations since MWD of ATRP grown polymers should be affected by a number of

experimental parameters unlike anionic polymerization. Furthermore, MALDI-MS analysis is not a standard technique to measure MWD of polymers due to mass discrimination in the MALDI process, although it is known to be not serious for a polymer with narrow MWD.32 Both F2 and F3 are identified as the living chains from their identical MALDI mass spectra (Figure S1). However, they split into two peaks in the HPLC separation. Furthermore, the NMR peaks of Hb and Hc protons have different shapes although they show the same chemical shift (Figure 2b). The two differently shaped peaks add up to reproduce the peak shape of the as-prepared PS-OH well (Figure S3). These interesting observations can be attributed to the different stereochemistries of the CEF. In the HPLC separation, the major contribution to the separation of living chains from the dead chains is the interaction between the polar chain end and the stationary phase.26 Therefore, the two diasteromeric configurations of the chain end (Figure 4) can split the elution peak of the two polymer species. The initial intention of the chain end modification was the introduction of a hydroxyl group as a polar CEF, but the triazole ring itself is interactive enough to show the diastereomeric peak splitting at this separation condition (see Figure S4). 760

DOI: 10.1021/acsmacrolett.7b00447 ACS Macro Lett. 2017, 6, 758−761

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ACKNOWLEDGMENTS T.C. acknowledges support from NRF-Korea (2015R1A2A2A0100 4974). H.j.P. acknowledges support from the Industrial Strategic Technology Development Program (10063082, 10070127) funded by the Ministry of Trade, Industry and Energy (MOTIE) of Korea.

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Figure 4. Stereoisomeric structures of PS-OH at the chain end.

The different configurations of the triazole ring must have influenced the NMR peak shape of Hb and Hc, too. Hb is the proton adjacent to the triazole ring, and the different peak shapes of Hb in the two stereoisomers can be accepted easily. Stable conformation structures of a model compound, 1-(1,3diphenylbutyl)-1H-1,2,3-triazol-4-yl)methanol, obtained by a DFT calculation, show different shielding environments for Hc protons in the two stereoisomers that could explain the different NMR peak shapes of the Hc protons in the two split peaks. (Figure S5). In summary, we separated the living chains of an ATRP grown PS from the dead chains. After azidation and CuAAC click reaction with propargyl alcohol, bromine chain end in the living chains was converted to a polar chain end. Living and dead chains were separated by HPLC utilizing the interaction of the polar chain ends in the living chains with the polar HPLC stationary phase. The living chains were further resolved according to the stereochemistry of the chain end. The MWD of the ATRP grown living chains was as narrow as the one prepared by anionic polymerization and in good agreement with the Poisson distribution although a further study with higher MW polymers is required to address the details of the MWD of ATRP grown polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00447. Experimental. MALDI mass spectra of living chains and dead chains (1, 2). Expanded view of NMR spectra of living chains (3). HPLC separation of PS-triazole (4). 3D structure of model molecules for PS-OH (5) (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Taihyun Chang: 0000-0003-2623-1803 Notes

The authors declare no competing financial interest. 761

DOI: 10.1021/acsmacrolett.7b00447 ACS Macro Lett. 2017, 6, 758−761