Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Minimizing Star−Star Coupling in Cu(0)-Mediated Controlled Radical Polymerizations Bas G. P. van Ravensteijn,†,§,∥ Raghida Bou Zerdan,†,∥ Matthew E. Helgeson,§ and Craig J. Hawker*,†,‡ †
Materials Research Laboratory, ‡Department of Materials, and §Department of Chemical Engineering, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States
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S Supporting Information *
ABSTRACT: Conditions for the synthesis of star-shaped polymers via controlled radical polymerization (CRP) with minimized star−star coupling are presented. By systematically grafting a variety of polyacrylates (methyl acrylate, tert-butyl acrylate, hexyl acrylate, 2ethylhexyl acrylate) from cores carrying 4−8 initiating sites, it was found that traditional factors increasing star−star coupling, (i) the number of arms, (ii) the length of the arms, and (iii) the steric bulk of the monomer side-chain, could be controlled. This allows high monomer conversion (>95%) and low-dispersity polymers (Đ < 1.08) to be obtained for molecular weights up to ∼300 000 g mol−1. In addition to the suppression of coupling events, these improved reaction conditions maximize chain-end fidelity and permit the synthesis of well-defined (multi)block copolymer stars through in situ chain extension reactions. These advantages significantly broaden the synthetic scope and structural integrity for CRP-derived star polymers.
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INTRODUCTION Star polymers with linear chains (arms) radiating from a central core are one of the simplest realizations of a branched polymeric architecture.1,2 Their spherical, three-dimensional structure, and high degree of functionality compared to linear counterparts, endows star polymers with unique solution and solid-state characteristics, rendering them ideal candidates as rheological modifiers,3,4 model systems for soft colloidal particles,5,6 and delivery vehicles.7,8 Star-shaped polymers have been traditionally prepared using two distinct synthetic strategies.1 The arm-f irst approach involves existing end-functionalized polymers cross-linked together or tethered to a core molecule, allowing for tailoring and facile characterization of the individual polymer arms. Alternatively, the core-first approach involves growing polymer arms from an initiator-functionalized core, enabling greater control over the number of arms and structure of the core.1 Fueled by the recent advances in controlled radical polymerizations (CRP),9−13 the synthesis of functional (star) polymers has become more accessible to nonexperts. This is exemplified by the broad monomer scope, high tolerance toward functional groups, and synthetic ease when compared to traditional ionic polymerization techniques.14,15 A variety of reports exploiting atom transfer radical polymerization (ATRP),16−20 reversible addition−fragmentation chain-transfer polymerization (RAFT), 8,21,22 and nitroxide-mediated polymerization (NMP)23−25 for the synthesis of star polymer have been recently published. However, a long-standing challenge for these radical-based strategies is termination via bimolecular chain−chain coupling © XXXX American Chemical Society
(radical combination). Statistically, the tendency for these deleterious termination events scales with the number of propagating chain-ends per polymer. For multiarm star polymers, these unwanted coupling reactions increase with increasing number of arms (f) and become even more probable at high monomer conversions, or when higher molecular weights are targeted. A number of work-arounds have been developed to suppress star−star coupling, including quenching at low monomer conversions (X = 10−40%) and/ or polymerization under dilute conditions.20,21 More recent studies have shown that Cu(0)-mediated CRP combines high chain-end fidelity with controlled chain growth processes, enabling the in situ formation of (multi)block copolymers.10,26,27 Boyer, Monteiro, and Whittaker were among the first to exploit these advantages for the synthesis of welldefined star polymers.28−30 In these systems, reactions with up to 50 vol % monomer and high monomer conversions (≥90%) were achievable with acceptable levels of star−star coupling (1.15 < Đ > 1.03) when targeting homopolymer stars up to 105 g mol−1 or low-molecular-weight block copolymer stars (Mw < 104 g mol−1). In this paper we further optimize and broaden the molecular weight and functional scope for Cu(0)-based routes to welldefined (block-co)polymer stars by employing ultra-highpressure size exclusion chromatography (UHP-SEC) as an analytic tool for detecting the onset of star−star coupling Received: November 7, 2018 Revised: December 11, 2018
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DOI: 10.1021/acs.macromol.8b02375 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) Graphical representation of the Cu(0)-mediated controlled radical polymerization (CRP) synthesis of star-shaped polymers carrying 4−8 arms (f). Acrylate monomers with R = methyl, t-butyl, n-hexyl, and 2-ethylhexyl were used. (b) Size exclusion traces highlighting the onset of star−star coupling detected by ultra-high-pressure size exclusion chromatography (UHP-SEC). The identification of these higher-molecular-weight side-products was enhanced by the high separation resolution of UHP-SEC.
leading to improved polymerization conditions (Figure 1). While the chemical nature of the core (pentaerythritol-, lactose-, or resorcinarene-based) did not influence the polymerization behavior, it was observed that, by increasing the steric bulk of monomer side-chain, star−star coupling could be suppressed ensuring high chain-end fidelity and full functionalization of the cores. These features facilitate the growth of well-defined block copolymers stars and allow a variety of 8-armed di- and tetrablock star polymers with molecular weights of ∼100 000 g mol−1 to be prepared.
Scheme 1. Schematic Representation of the Synthesized Star Initiatorsa
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RESULTS AND DISCUSSION Initiator Synthesis. A series of initiators with distinctly different core chemistries (pentaerythritol derivatives, lactose, and resorcinarene), functionality (f = 4, 6, or 8), and threedimensional distribution of initiating sites were synthesized following literature procedures (Scheme 1, see the Supporting Information, section S1, for synthesis and sections S2−S6 for detailed molecular analysis).31−35 The cores with 4 (I-4), 6 (I6), and 8 (I-8, I-L-8, and I-R-8) initiating sites were prepared from the reaction of the peripheral hydroxyl moieties with αbromoisobutyryl bromide (BiBb) in the presence of a base [triethylamine (TEA) or pyridine]. Polymerization Kinetics and Star−Star Coupling vs Time. To systematically investigate the experimental parameters that contribute to the onset of star−star coupling, initial polymerizations using tert-butyl acrylate (t-BuA) as monomer and I-833 (Scheme 1) as initiator were performed. Conditions that are compliant with high-f stars should also apply to star polymers with a lower number of arms, since controlled growth of well-defined star polymers is considered to be more challenging as f increases. Guided by previously reported experimental conditions for Cu(0)-mediated CRP, 0.05 equiv of CuBr2 and 0.18 equiv of Me6TREN per initiating site were used as deactivator and ligand, respectively.27,30 Cu(0) was supplied to the reaction by an acid etched wire wrapped around a magnetic stir bar. Trifluoroethanol (TFE) or a mixture of toluene:i-PrOH (4:1, v/v) was employed as solvents, and a degree of polymerization (DPn) of 40 per arm was targeted. Figure 2 summarizes the results obtained from a kinetic experiment performed in TFE where molecular weight distributions and monomer conversions (X) were probed with SEC and NMR, respectively. In agreement with the extensive literature on Cu(0)-mediated CRP,9,10,36 lowdispersity polymers (Đ ≈ 1.05) were obtained throughout the course of the reaction. Despite the fast polymerization rate, the
a
f = the number of initiating sites per core. The abbreviations (gray) of the resulting initiators are given next the corresponding alcohol precursor.
reaction is controlled up to high monomer conversion as evident from the semilogarithmic plot (Figure 2d) and linear relationship between DPn and X (Figure 2c). Performing the reaction in toluene:i-PrOH, another common solvent system for Cu(0)-mediated CRP and of interest when targeting highly hydrophobic monomers,30 produced similar results (Supporting Information, section S7), although the polymerization was noticeably slower. This observation was previously attributed to a lower disproportionation rate of Cu(I)Br in more apolar media.30 While the results described in the following text are focused on TFE as a solvent, it should be noted that equivalent data was obtained using toluene:i-PrOH and can be found in the Supporting Information. B
DOI: 10.1021/acs.macromol.8b02375 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. (a) Size exclusion chromatograms as a function of time for the polymerization of tert-butyl acrylate (t-BuA) using I-8 as initiator in trifluoroethanol (TFE) ([R-Br]0/[t-BuA]0/[CuBr2]0/[Me6TREN]0 = 1/40/0.05/0.18). (b) Monomer conversion and polydispersity index (PDI) as a function of the polymerization time. (c) Degree of polymerization (DPn) as determined by nuclear magnetic resonance (NMR) spectroscopy versus the monomer conversion. (d) Semilogarithmic curve based on the data plotted in panel b.
Figure 3. Evolution of molecular weight distributions as a function of time for the polymerization of t-BuA using (a) I-4 and (b) I-8 as initiator in trifluoroethanol (TFE) ([R-Br]0/[t-BuA]0/[CuBr2]0/[Me6TREN]0 = 1/40/0.05/0.18).
agreement with previous reports.9,10,36 The push toward higher conversions was accompanied by an increase in the contribution of star−star coupling (Figure 3b). Shortly after the polymerization reaches its maximum conversion (X = 99%, as determined by NMR), the relative contribution of coupled side-products increases significantly as evident from the appearance of high-molecular-weight shoulders in the measured chromatograms. Consequently, reaction times seem to
The maximum monomer conversion obtained during these kinetic experiments was limited to approximately 90%, due to introduction of oxygen during sample withdrawal causing the termination of the reaction at lower monomer conversion. No signs of star−star coupling are observed, even with the highresolution SEC employed throughout this study. However, when the number of samples was limited, the polymerization was able to reach quantitative monomer conversions in C
DOI: 10.1021/acs.macromol.8b02375 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Overview of Synthesized Star Polymers and Optimized Polymerization Timesa entry
initiator
monomer(s)
DPn,target/arm
reaction time [h]
Xb [%]
DPn,NMR/armb
MW × 10−3c [g mol−1]
PDId [−]
%couplinge [%]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21f 22f 23f 24f
I-4 I-4 I-4 I-4 I-6 I-6 I-6 I-6 I-8 I-8 I-8 I-8 I-8 I-8 I-8 I-L-8 I-R-8 I-8 I-8 I-8 I-8 I-8 I-8 I-8
t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA t-BuA MA HA 2-EHA t-BuA/2-EHA 2-EHA/t-BuA t-BuA/HA t-BuA/2-EHA/t-BuA/2-EHA
20 40 80 160 20 40 80 160 10 20 40 80 160 320 640 40 40 40 40 40 40/40 40/40 40/40 20/20/20/20
4 6 4 4 2 4 4 4 4 2.5 3 3 5 7 23 5 4 4 6 6 3/6 8/8 3/5 2.5/6/3/4
99 99 99 95 98 98 94 93 98 98 99 98 96 87 12 98 97 99 99 99 98/95 98/96 98/99 99/97/92/60
20 40 79 152 20 39 75 149 10 20 40 78 154 278 78 39 39 37 40 38 39/38 39/38 39/40 20/19/18/12
10 20 40 80 15 30 60 120 10 20 40 80 160 290 80 40 40 30 50 60 100 100 90 100
1.04 1.04 1.03 1.04 1.03 1.03 1.03 1.04 1.04 1.04 1.04 1.03 1.03 1.08 1.08 1.06 1.05 1.11 1.04 1.03 1.04 1.04 1.05 1.05
