Iodosylbenzene-Pseudohalide-Based Initiators for Radical

Article pubs.acs.org/joc

Cite This: J. Org. Chem. 2017, 82, 11806-11815

Iodosylbenzene-Pseudohalide-Based Initiators for Radical Polymerization Rajesh Kumar, Yakun Cao, and Nicolay V. Tsarevsky* Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, Texas 75275, United States S Supporting Information *

ABSTRACT: Iodosylbenzene reacts with various (pseudo)halides (trimethylsilyl azide or isocyanate or potassium azide, cyanate, and bromide) to yield unstable hypervalent iodine(III) compounds, PhIX2 (X = (pseudo)halide), that undergo rapid homolysis of the hypervalent I−X bonds and generate (pseudo)halide radicals, which can initiate the polymerization of styrene, (meth)acrylates, and vinyl esters. Polymers are formed containing (pseudo)halide functionalities at the α-chain end but, depending on the termination mechanism and the occurrence of transfer of (pseudo)halide groups from the initiator to the propagating radicals, also at the ω-chain end. With slowly polymerizing monomers (styrene and methyl methacrylate) and initiators that were generated rapidly at high concentrations and were especially unstable, the reactions proceeded via a “dead-end” polymerization mechanism, and only low to moderate monomer conversions were attained. When the initiator was generated more slowly and continuously throughout the polymerization (using the combination of iodosylbenzene with the poorly soluble potassium (pseudo)halide salts), typically higher conversions and higher molecular weights were reached. The presence of (pseudo)halide functionalities in the polymers was proved by elemental analysis, IR, and NMR spectroscopy. The azide-containing polymers underwent click-type coupling reactions with dialkynes, while the (iso)cyanate-containing polymers reacted with diamines to afford high-molecular-weight polymers with triazole- and urea-type interchain links, respectively.

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INTRODUCTION Ever since their discovery in 1886 by Willgerodt,1 hypervalent iodine compounds have continued to attract the attention of synthetic organic, theoretical, and materials chemists, and have found numerous applications, especially in organic synthesis, which are described in several monographs2−6 and review papers.7−14 Owing to the unique reactivity of hypervalent iodine(III) compounds of the type ArIL2 (Ar = aryl; L = ligand, such as (pseudo)halide, carboxylate, etc.), namely their ability to participate in both radical (e.g., bond homolysis with the formation of iodoarenes ArI and radicals L•)15,16 and ionic reactions (e.g., ligand exchange with nucleophiles Nu−, leading to compounds of the types ArILNu and eventually ArINu2), these compounds are of importance in the synthesis of functional macromolecules. Uses of ArIL2 in polymer science and technology that have already been reported include (i) initiators of radical polymerization,17−23 (ii) reagents for postpolymerization modifications (polymer-analogous reactions),24−26 and (iii) structural elements or building blocks of complex macromolecules.27,28 The ligand exchange reactions of the acyloxy groups in (diacyloxyiodo)arenes with (pseudo)halide anions X− or the reactions between iodosylarenes ArIO and trimethylsilyl (pseudo)halides TMSX are convenient routes to (di(pseudo)haloiodo)arenes ArIX2, which are typically unstable and © 2017 American Chemical Society

decompose in situ with the formation of (pseudo)halogen radicals, which can react with numerous substrates, including compounds with unsaturated carbon−carbon bonds or easy to abstract hydrogen atoms. In addition to chlorination reactions in the presence of (dichloroiodo)arenes (typically not prepared by ligand exchange but by chlorination of the corresponding iodoarene), numerous synthetically useful chemical transformations have been reported involving, for example, azides and thiocyanates as the pseudohalogens. Very limited number of ligand exchange reactions involving λ3-iodanes of the type ArIL2 and nucleophiles, followed by homolytic cleavage (upon heating or irradiation with light) of the hypervalent bonds of the newly formed iodanes have been reported that have application in the polymerization of radically polymerizable monomers to yield directly functional polymers. For example, it was shown21 that the exchange of the acetoxy groups in (diacetoxyiodo)benzene with methacryloyloxy groups (i.e., reaction with methacrylic acid) is an easy way to prepare directly, without the need of isolation of the reaction products, branched polymers, owing to the fact that both the produced [(acetoxy)(methacryloyloxy)iodo]benzene and Special Issue: Hypervalent Iodine Reagents Received: August 2, 2017 Published: October 3, 2017 11806

DOI: 10.1021/acs.joc.7b01945 J. Org. Chem. 2017, 82, 11806−11815

Article

The Journal of Organic Chemistry

Scheme 1. Polymerization of Vinyl Monomers Initiated by the Iodosylbenzene-Pseudohalide System and Formation of Pseudohalide-Capped Polymers

Table 1. Polymerization of Sty, MMA, VAc, and MA Initiated by Various PhIO-Pseudohalide Systems at 30 °C # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

monomer

pseudohalide

Sty

TMSN3

MMA

TMSNCO TMSN3

KN3 TMSNCO

KCNO

VAc

KBr TMSN3

MA

TMSNCO TMSN3

KN3 TMSNCO

KCNO KBr

[M]0/[in]0a

solvent

100 100 100 25 100 100 500 100 500 25 100 100 500 100 500 100 100 100 100 25 100 100 500 100 500 25 100 100 500 100 500 100

DMAc PhCl DMAc DMAc DMAc PhCl DMAc DMAc DMAc DMAc DMAc PhCl DMAc DMAc DMAc DMAc DMAc PhCl DMAc DMAc DMAc PhCl DMAc DMAc DMAc DMAc DMAc PhCl DMAc DMAc DMAc DMAc

conversion [%] (time [min]) 12 10 10 53 20 21 12 60 82 77 55 63 16 78 86 70 31 32 30 81 72 91 61 75 35 88 92 94 36 70 67 25

(30) (30) (600) (1200) (30) (180) (120) (1740) (3000) (1200) (900) (1440) (600) (1740) (2400) (2820) (30) (30) (600) (30) (30) (30) (30) (180) (120) (30) (600) (480) (180) (180) (300) (300)

Mn,app [g mol−1]; Đb 8700; 2.21 2200; 2.17 4700; 2.01 2100; 5.44 6500; 4.89 5100; 5.32 4500; 3.20 149 300; 2.32 98 800; 3.59 24 400; 3.28 19 000; 2.70 35 900; 2.21 27 400; 2.11 87 500; 2.47 122 800; 2.60 31 600; 2.20 4800; 1.90 6300; 1.76 5900; 2.16 3800; 3.30 17 100; 1.91 30 800; 1.69 3300; 3.27 67 300; 2.04 210 100; 2.11 15 800; 2.20 33 900; 3.12 142 000; 1.91 93 600; 1.96 75 400; 2.06 172 500; 2.32 29 200; 2.22

a

Molar/concentration ratio of monomer to initiator, where the initiator consisted of PhIO-pseudohalide (1:2 (n/n)). bApparent number-average molecular weight and molecular weights distribution dispersities determined by SEC calibrated with linear polySty standards.

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Figure 1. Kinetics (a) and evolution of molecular weights and molecular weight distribution dispersities (b) for the polymerization of Sty, MMA, VAc, and MA initiated by PhIO-TMSN3 (1:2 (n/n); 1 mol % vs monomer) at 30 °C in DMAc.

arenes to various carbon-centered radicals is documented in the literature and some rate constants have been determined.33 It could therefore be reasonably assumed that similar transfer reactions are likely to take place from the hypervalent iodine(III)-based initiator PhIX2 generated via the reaction of iodosylbenzene with (pseudo)halides, to the propagating polymeric radicals. All mentioned reactions and the corresponding rate coefficients are presented in Scheme 1. In addition to serving as radical precursors,15,16 hypervalent iodine(III) compounds can generate a number of cationic species, and in order to ensure that the polymerizations reported in this study were indeed radical (and not cationic), three of the monomers employed (MMA, VAc, and MA) were selected for their inability to undergo cationic polymerization. Several different PhIO-(pseudo)halide-based initiators were studied, all at 30 °C, including TMSN3 and KN3, TMSNCO and KOCN, and KBr (in all cases the molar ratio of PhIO and (pseudo)halide was 1:2), in two solvents of rather different polarityDMAc and PhCl. The trimethylsilyl pseudohalides were soluble in the reaction mixtures and upon their addition to the mixtures of monomer, solvent, and PhIO, homogeneous solutions were formed within a short time period. The potassium (pseudo)halides have limited solubility in DMAc (especially in the presence of the weakly polar monomers), and even lower solubility in the rather nonpolar PhCl. All reaction mixtures, in which potassium salts were used as components of the initiating system, remained heterogeneous throughout the polymerization. All polymerization data is summarized in Table 1. The polymerizations of MMA, in which PhIO-KX-based initiators were employed, were slower but reached higher monomer conversions and higher polymer molecular weights were attained compared to those, in which PhIO-TMSX-based initiators were used (Table 1). This is clearly seen by examining entries 5 and 8 (1 mol % of azide-based initiator vs MMA), 7 and 9 (0.2 mol % of azide-based initiator vs MMA), 11 and 14 (1 mol % of (iso)cyanate-based initiator vs MMA), or 13 and 15 (0.2 mol % of (iso)cyanate-based initiator vs MMA). Similar trends were observed in the polymerizations of the more rapidly polymerizing monomer, MA, especially with regards to reaction rates and molecular weights of the polymers, as can be

(dimethacryloyloxyiodo)benzene serve as both initiators of polymerization and monomers (i.e., as inimers). The generation of azide radicals by a ligand exchange reaction between (diacetoxyiodo)benzene and NaN3 and their use in the synthesis of linear and branched polymers with azide functionalities at the chain ends were also reported.23 The main goals of this work were (i) to explore alternative hypervalent iodine(III)-based sources of azide and other (pseudo)halide radicals that could be employed to initiate polymerization, and (ii) to examine systematically the scope and limitations of (pseudo)halide radical-initiated polymerizations of various monomers.

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RESULTS AND DISCUSSION 1. Polymerizations. The reactions of both TMSX and KX (X = (pseudo)halide) with either (diacetoxyiodo)benzene or PhIO29 are known to yield compounds of the type PhIX2, which, depending on the nature of the group X, may be very unstable and decompose rapidly with the formation of (pseudo)halide radicals X•. These radicals, especially when present at high concentration, may couple to the corresponding (pseudo)halogen X2, which may be unstable and participate in further “side” reactions. For instance, when X = N3, nitrogen is released very rapidly,30 probably through the formation of the intermediate N6,31 and when X = SCN, the initially produced dithiocyangoen (SCN)2 undergoes oligomerization or polymerization with the formation of orange-colored oligomeric or polymeric products.32 However, in the presence of large amounts of unsaturated compounds (monomers), the radicals X• may also initiate polymerization. The initiation step (with a rate coefficient ki) in these cases, yields a monomeric radical (i.e., with a degree of polymerization DP = 1), which can react further with the monomer and propagate. Eventually, the polymeric radicals, each containing an X group at the α-chain end, terminate by coupling (ktc) or disproportionation (ktd), thus affording “dead” polymer chains with two or one X endgroups, respectively. In addition, the propagating radicals may terminate with the radicals X• (kt,X) or potentially abstract an X group from ArIX2 (i.e., take part in transfer reactions (ktr)), both of which produce polymer chains with X groups at the ωchain ends. Transfer of chlorine atoms from (dichloroiodo)11808


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Figure 2. Kinetics (a) and evolution of molecular weights and molecular weight distribution dispersities (b) for the polymerization of MMA initiated by various PhIO-pseudohalide combinations (1:2 (n/n); 1 mol % vs monomer) at 30 °C in DMAc.

observed, although it was less significant. The molecular weights of the polymers prepared with the PhIO-TMSX initiators were dramatically lower than those of polymers synthesized using the corresponding PhIO-KX systems, most likely due to efficient termination and/or transfer to the initiator (Scheme 1) when PhIX2 was present at large concentrations (in the homogeneous mixtures). Interestingly, the PhIO-KBr-based initiator also afforded polymers (as opposed to vicinal dibromo-compounds) with reasonably high molecular weights using either MMA (Table 1, entry 16) or MA (entry 32) as the monomer. These reactions in all likelihood afforded polymers with one or two alkyl bromide end groups, but due to the fact that many other approaches are known for the synthesis of Br-capped macromolecules, for instance by atom transfer radical polymerization35 or by transfer to CBr4, these materials were not analyzed or studied further. In summary, in many of the studied polymerizations, particularly those of monomers with relatively low propagation rate coefficients, such as Sty and MMA, and/or those that were initiated by particularly unstable with respect to homolysis of the hypervalent I−X bonds (i.e., rapidly dissociating) initiators, such as the PhIO-TMSN3 system, the limiting monomer conversions were relatively low. For example, when the ratio of monomer to PhIO-TMSN3 initiator was 100, the conversions of Sty did not exceed 10−12% before the polymerizations stopped (Table 1, entries 1 and 2) and these of MMA were of the order of 20% (entries 5 and 6). Polymerization systems involving slowly polymerizing monomers and rapidly decomposing radical initiators are often named “dead-end polymerizations” and their kinetics was described in the late 1950s.36,37 The limiting conversion, i.e., the maximal conversion observed at “infinite” time (convmax) depends upon the ratio of the propagation and the square root of the termination rate coefficients (both monomer-dependent) as well as other reaction parameters, such as the rate coefficient of dissociation (kd) and the initial concentration of the initiator ([in]0), and the initiation efficiency ( f):

ascertained by inspecting entries 21 and 24 (1 mol % of azidebased initiator vs MA), 23 and 25 (0.2 mol % of azide-based initiator vs MA), 27 and 30 (1 mol % of (iso)cyanate-based initiator vs MA), or 29 and 31 (0.2 mol % of (iso)cyanate-based initiator vs MA). These results can be explained with the lower solubility of potassium pseudohalides compared to the corresponding TMSX compounds, which leads to slower but more “uniform” and continuous generation of the actual initiator, PhIX2 (for further discussion, vide inf ra). The nature of the solvent did not affect the polymerization rates and the monomer conversions (compare, for instance, entries 1 and 2 (Sty), 5 and 6 (MMA), 17 and 18 (VAc), or 21 and 22 (MA) for the PhIO-TMSN3-based initiating systems, or 11 and 12 (MMA), or 27 and 28 (MA) for the PhIO-TMSNCO-based initiator), as expected for radical mechanism of polymerization. The solvent had a somewhat more pronounced impact on the molecular weights, particularly in the polymerizations of MA, where the reactions conducted in DMAc yielded polymers with lower molecular weights than those in PhCl (compare entries 21 and 22 or 27 and 28). This can be explained by the more pronounced transfer reactions with the former solvent. Although the transfer coefficients of polyacrylate radicals to DMAc and PhCl are not available in the literature, it is known34 that for polystyrene radicals the transfer to DMAc (transfer coefficient CDMAc = ktr,DMAc/kp = 4.6 × 10−4 at 60 °C) is more efficient than transfer to PhCl (CPhCl = ktr,PhCl/kp = 0.133−1.5 × 10−4 at 60 °C). Detailed polymerization kinetics data of all four studied monomers using the PhIO-TMSN3 initiating system in DMAc are presented in Figure 1a. With the notable exception of the most rapidly polymerizing monomer, MA, all polymerizations stopped at relatively low monomer conversions. The polymer molecular weights (Figure 1b), again, with the exception of that of polyMA (Mn,app = 17 100 g mol−1) were relatively low (5 × 108 (80 °C) 1 × 109

9.8 × 10−3 ∼0.05 0.18 0.46

1 5 18 47

a

Calculated using the Arrhenius equation with pre-exponential factors and activation energies provided in ref 38. bRate coefficient for termination between monomeric radicals.39

the data in that table, it is to be expected that the limiting conversion will increase in the order Sty < MMA < VAc < MA, which was indeed observed (Table 1 and Figure 1a). The value of the initiation efficiency ( f), a number between zero and unity, is the fraction of radicals produced by the initiator that actually initiate polymerization, as opposed, for instance, to undergoing “side” (i.e., not producing polymers) reactions in the solvent cage prior to addition to the monomer. When the coupling of the radicals formed by the dissociation of the initiator does not yield (reform) the original initiator, or at least another molecule that can dissociate back to the initiating radicals, as are the majority of cases discussed in this work (because X2 is different from PhIX2 and not all coupling products X2 are able to dissociate back to the initiating radicals X•, e.g., when X = N3), the initiation efficiency is always lower than unity and may or may not depend on the monomer concentration.40,41 While determination of the initiation efficiencies of [di(pseudo)haloiodo]arenes is beyond the scope of this work, they may be relatively low, considering the very fast “side” reactions that some of the (pseudo)halide radicals participate in. For example, the coupling of two azide radicals generated by decomposition of the unstable PhI(N3)2, yielding nitrogen (most likely,31 after the initial formation of the intermediate coupling product N6), is characterized by rate coefficients in the range 3−4.5 × 109 M−1 s−1.30 The molecular weights of the polymers are related to the kinetic chain length, which also depends upon the ratio of the propagation and termination rate coefficients.41 However, the data in Table 2 is not suitable for prediction of molecular weights, because reactivity of the propagating radicals in transfer reaction should also be taken into account. For example, the polymers derived from VAc (Table 1, entries 17− 19) had low molecular weights, due to the very high reactivity of the corresponding polymeric radical in transfer reactions to initiator, monomer and/or polymer, and solvent. 2. Composition and Structure of the Polymers. In order to prove the presence of pseudohalide chain end(s) (and possibly pendant) groups in some of the polymers described in the previous section, it was essential to prepare low molecular weight materials. The PhIO-TMSN3 initiating system was studied in detail because it yielded polymers with azide groups, which are easy to detect, even at low amount, by IR

Figure 3. IR spectra (films cast on KBr plates) and nitrogen contents of polymers prepared using the PhIO-TMSN3 initiating system (1 mol % (Sty and VAc) or 4 mol % (MMA and MA) vs monomer).

polymers contained nitrogen, which is consistent with the presence of azide at the chain end(s) and possibly the backbone. Among the azide-containing polymers, the amount of nitrogen was the largest in the polySty(N3)x sample, which suggested that in that case, the azide-capped polymer was possibly further azidated by the azide radicals that were still being generated in the system. Indeed, it has been shown24 that the similar system (diacetoxyiodo)benzene-TMSN3 is useful for the direct azidation of polySty, and, depending on the reaction conditions, as much as 1 out of 11 monomer repeat units per chain could be azidated. Cyanates and isocyanates are also relatively easy to detect by IR spectroscopy. Both groups absorb IR light in the 2280−2240 cm−1 region.42,44 The spectra of polymers prepared from Sty and VAc using the PhIO-TMSNCO initiating system are 11810


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value, as seen in Figure S7 is 136.3 ppm, and the experimental chemical shift value of the ipso-carbon atom in phenyl azide is 139.9 ppm47). In general, by comparing experimental 1H and 13C NMR spectra of the polymers derived from MMA (Figure S2), VAc (Figure S3), and MA (Figure S4), synthesized by using the PhIO-TMSN3 initiating system, with predicted chemical shifts values (Figure S8 for polyMMA(N3)x, Figure S9 for polyVAc(N3)x, and Figure S10 for polyMA(N3)x) confirm the presence of azide groups at the α-chain ends, i.e., initiation by azide radicals. For instance, in polyMMA(N3)x (Figure S2) the presence of azide groups at the α-terminus was easily ascertained, in addition to possible azidation of some of the methyl groups attached to the backbone (3.4−3.8 and 59.2− 60.6 ppm for the methylene protons and carbon atoms adjacent to azide groups, based on the predictions shown in Figure S8) but it was difficult to detect the presence of ω-end azide groups. In the case of polyVAc(N3)x (Figure S3) partial azidation of the pendant acetyl group probably also took place, as can be concluded from the presence of a small peak in the 1H NMR spectrum at 4.1 ppm, close to the predicted (Figure S10) and the experimental value of ca. 3.1−4.1 ppm, determined48 for poly(vinyl azidoacetate) copolymers. The corresponding 13C peak (predicted at 50.9 ppm) could not be seen, probably due to the very low degree of pendant group azidation (note also the very low intensity of the mentioned proton peak at 4.1 ppm). In the case of polyMA(N3)x (Figure S4) azide groups were present not only at the α-terminus (2.5−2.7 and 52.4− 53.1 ppm respectively for the protons and carbon directly adjacent to the azide functionality) but also at some of the ωchain ends (3.7−3.9 (the reported value for a low-molecularweight analogue, methyl 2-aziopropionate, is 3.96 ppm46) and 60.2 ppm for the protons and carbon atoms, respectively), as a result of termination of a propagating radical with an azide radical and/or potential transfer23 of an azide group from the unstable initiator PhI(N3)2 to the propagating polyacrylate radical (Scheme 1). The presence of isocyanate groups at the ω-end in polySty(NCO)x (Figure S5) could be readily confirmed (4.0−4.4 and 57 ppm for the corresponding protons and carbon atoms, based on comparisons with the predictions on Figure S7). In addition, isocyanate groups were present in some of the pendant benzene rings, as confirmed by the presence of a peak at 137.5 ppm in the 13C spectrum (Figure S5b), i.e., close to the predicted value of 134.6 ppm (Figure S7) and the experimental value49 for the ipso-carbon atom in PhNCO, 133.6 ppm. It should be noted that according to the NMR spectra and their comparison with the predictions, no cyanate groups were present in polySty(NCO)x (note that the predicted value for a cyanate group attached to the pendant phenyl group is 149 ppm (Figure S7) and also that the ipsocarbon in PhOCN has a resonance of 153.5 ppm49). In polyVAc(NCO)x (Figures S6 and S10), the presence of cyanate (in addition to isocyanate) groups could not be excluded. 3. Modifications (Coupling Reactions) of (Pseudo)Halide-Containing Polymers. The presence of azide groups in the polymers prepared by using the PhIO-TMSN3 initiating system makes them suitable substrates for various types of modifications, due to the diverse reactions, in which azides can participate.50−57 Of particular interest is the use of azidecontaining polymers as building blocks of complex functional macromolecules, e.g., by azide−alkyne click chemistry.58−65 The azide-containing polymers (Table 3, entries 1−4) derived

shown in Figure 4. The (iso)cyanate absorbance is clearly seen, especially in the case of polySty(NCO)x, suggesting that the

Figure 4. IR spectra (films cast on KBr plates) of polymers prepared using the PhIO-TMSNCO initiating system (1 mol % vs monomer) in PhCl.

mentioned hypervlent iodine initiator can be successfully employed for the direct synthesis of (iso)cyanate-functionalized polymers, which could find applications in the synthesis of polyureas or polyurethanes. Careful inspection of the region of wavenumbers above ca. 3200 cm−1 indicated that, although precautions were taken to keep the polymers dry, some hydrolysis (yielding, for instance, amine groups attached to the polymers) did occur. The presence of less (iso)cyanate groups than expected based on the elemental analysis (i.e., wt % of N), could and indeed did have an effect on the efficiency of the coupling reactions with diamines (vide inf ra). The polymers were further characterized by NMR spectroscopy (Supporting Information, Figures S1−S6). In the 1H NMR spectrum of polySty(N3)x (Figure S1a), the peak at 3.9− 4.2 ppm can be attributed to the methine proton attached to the azide-capped ω-chain end carbon atom.45 In the lowmolecular-weight analogues of ω-capped with azide group polySty, namely 1-phenylethyl azide, this peak appears at 4.60 ppm,46 i.e., very close to the predicted value of 4.71 ppm for what is formally a dimer of Sty with an azide group at the ωchain end (Figure S7). The peaks in the 3.1−3.4 ppm region are most likely due to the methylene and methine protons at the azide-containing α-Sty unit of the polymer chain (Figure S7), formed by initiation of the Sty polymerization by an azide radical. The 13C spectrum of the polymer (Figure S1b) further confirms the presence of azide groups at both the α- and the ωends of some of the chains. Specifically, the peak at 57.5 ppm could be attributed to the azide-capped α-carbon atom of the backbone (as well as, plausibly and based on the predictions, to a midbackbone carbon atom attached to an azide group). On the basis of previously reported results on azidation of polySty,24 it was expected that some of the pendant aromatic rings would also be azidated. Indeed, a peak was observed in the 13C NMR spectrum at 137.8 ppm, which is likely due to an aromatic carbon atom attached to an azide group (the predicted 11811


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Table 3. Molecular Weights and Molecular Weight Distribution Dispersities of Pseudohalide-Containing Polymers Prepared by Using the PhIO-TMSN3 or PhIO-TMSNCO Initiating Systems before and after Modification with Difunctional Coupling Agentsa

a

#

polymer

elemental analysis (wt % N)

1 2 3 4 5 6

polySty(N3)x polyMMA(N3)x polyVAc(N3)x polyMA(N3)x polySty(NCO)x polyVAc(NCO)x

1.12 0.74 0.83 0.86 3.12 2.26

Mn,app [g mol−1]; Đ 4200; 8800; 8100; 9200; 5800; 4200;

2.04 4.89 2.77 4.53 2.58 2.10

coupling reaction Pg2O (+CuBr), DMF, r.t. Pg2O (+CuBr), DMF, r.t. Pg2O (+CuBr), DMF, r.t. Pg2O (+CuBr), DMF, r.t. H2N-(CH2)3-NH2, THF, r.t. H2N-(CH2)3-NH2, THF, r.t.

Mn,app [g mol−1]; Đ (after coupling) 13 000; 13 600; 19 900; 12 500; 14 800; 13 600;

4.87 3.60 2.92 3.97 14.98 13.21

Samples were collected and analyzed after 15 h.

Figure 5. Evolution of SEC traces of polySty(N3)x (a) and polyVAc(N3)x (b) prepared by using the PhIO-TMSN3 initiating system (1 mol % vs monomer in PhCl) during CuBr-catalyzed click coupling reactions with Pg2O in DMF at r.t. The last two numbers in each of the rows in the legend indicate the apparent number-average molecular weight and the molecular weight distribution dispersity, respectively.

Figure 6. Evolution of SEC traces of polySty(NCO)x (a) and polyVAc(NCO)x (b) prepared by using the PhIO-TMSNCO initiating system (1 mol % vs monomer in PhCl) during coupling reactions with 1,3-propylenediamine in THF at r.t. The last two numbers in each of the rows in the legend indicate the apparent number-average molecular weight and the molecular weight distribution dispersity, respectively.

The SEC traces of two of the azide-containing polymers, polySty(N3)x (entry 1 in Table 3), and polyVAc(N3)x (entry 2 in Table 3). along with the evolution of these traces during the Cu(I)-catalyzed reaction of the polymers with Pg2O in DMF over a 15-h period are shown in Figure 5. Upon click coupling, the entire molecular weight distribution shifted toward higher apparent molecular weights (shorter elution times or smaller elution volumes), although the “tailing” toward the low molecular weight region on the SEC traces of the coupling

from Sty, MMA, VAc, and MMA were reacted with a dialkyne, Pg2O (using an equimolar ratio of azide groups, determined by elemental analysis, and acetylene groups), in the presence of CuBr as the catalyst of the 1,3-cycloaddition. In all cases, the click coupling reactions led to the formation of polymers with higher molecular weights than the starting materials, indicating that the majority of macromolecules contained at least one azide functionality, as expected from the reaction mechanism represented in Scheme 1. 11812


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pendant backbone functionalities) is proved by elemental analysis, IR spectroscopy, and by conducting coupling reactions with propargyl ether (in the case of azide-containing polymers) or 1,3-propylenediamine (in the case of (iso)cyanate-containing polymers). The PhIO-(pseudo)halide-based initiators reported in this work provide a straightforward one-step methodology for the direct (i.e., not requiring postpolymerization modifications) synthesis of functionalized macromolecules.

products suggested that there were some unreacted chains of the original polymer. In the case of click coupling of polySty(N3)x, at longer reaction times (ca. 21 h), the reaction mixture became very viscous and it was difficult to withdraw samples from it; after dilution in the SEC solvent, THF, the solution could not be passed through a 0.2 μm PTFE syringe filter without applying high force, indicating that by that point, cross-linking reactions had commenced. This suggests that at least some of the chains contained more than two azide groups, as expected from the reported24 reaction between polySty and azide radicals, which yields backbone- and pendant-groupazidated polymer. The SEC results serve as another proof (in addition to elemental analysis, IR, and NMR spectra) of the existence of azide groups in the polymers. Cyanates66 and isocyanates67,68 are also reactive functionalities, with numerous applications in organic syntheses and materials science. One of the most important (and largevolume) applications is the synthesis of polyurethanes and polyureas. The polymers synthesized by using the PhIOTMSNCO initiating system contained (as seen in the IR spectra in Figure 4) (iso)cyanate groups and were therefore suitable candidates for reactions with diols or diamines. If more than two (iso)cyanate groups were present on average per chain, the reactions were expected to yield first highly branched and eventually network polymers. If the number of functionalities per chain were smaller, than simple coupling (i.e., increase in the apparent molecular weight) would be observed. The results presented in Table 3 (entries 5 and 6 for samples collected after 15-h stirring at r.t. in THF) clearly confirm that the (iso)cyanate groups were attached to the polymer chains. Samples were collected at 5 and 15 h, and the changes in the shapes of the molecular weight distributions, compared to the precursors, are shown in Figure 6. The very significant broadening of the molecular weight distributions was the result of some unreacted polymer (possibly due to the fact that some of the (iso)cyanate groups were hydrolyzed, as indicated by the IR spectra shown in Figure 4). However, although a fraction of the (iso)cyanate groups in the polymers were “lost”, which altered the ratio between (iso)cyanate and amine groups (originally set to the ideal molar ratio for reaching high molecular weights of 1:1), branching and eventually gelation still occurred (in ca. 35 h) with the polySty(NCO)x.

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EXPERIMENTAL SECTION

Materials. Methyl methacrylate (MMA, 99%, Aldrich), methyl acrylate (MA, 99%, Aldrich), styrene (Sty, 99%, Acros), and vinyl acetate (VAc, 99%, Acros) were purified before the experiments by passing the neat monomer through a column filled with basic alumina, which absorbs the phenolic polymerization inhibitor present in commercial samples. Iodosylbenzene (PhIO) was synthesized using a procedure described in the literature,29 which is based on the hydrolysis of (diacetoxyiodo)benzene (98%, Acros) with 3 M aqueous NaOH (pellets, 97+%, Sigma-Aldrich, were employed to prepare the solution), followed by washing with chloroform (99% extra pure, Acros). Trimethylsilyl azide (TMSN3, 94%, Alfa Aesar), trimethylsilyl isocyanate (TMSNCO, 85%, Sigma-Aldrich), trimethylsilyl bromide (TMSBr, 97%, Sigma-Aldrich), KN3 (99.9%, Sigma-Aldrich), KOCN (96%, Sigma-Aldrich), KBr (99%, Acros), propargyl ether (Pg2O, 98%, Aldrich), CuBr (99.99%, Aldrich), 1,3-diaminopropane (99%, Acros), and the solvents, including anhydrous N,N-dimethylacetamide (DMAc, 99.5%, Acros), anhydrous ether (98%, EMD Millipore), petroleum ether (technical grade, EMD Millipore), N,N-dimethylformamide (DMF, 98%, EMD Millipore), and tetrahydrofuran (THF, 99%, Fisher) were used as received. Chlorobenzene (PhCl, 99%, Acros) was dried over anhydrous sodium sulfate prior to use. The deuterated solvent, CDCl3, (99.8% D, Cambridge Isotope Laboratories) contained a small amount of tetramethylsilane (TMS) as a chemical shift reference. Analyses. To monitor the progress of the polymerization reactions, samples were withdrawn periodically using a nitrogen-purged syringe equipped with a Teflon-coated needle. Part of each sample was diluted with CDCl3 (containing a small amount of tetramethylsilane as the chemical shift reference) for NMR analysis (determination of conversion), which was carried out on a Bruker Avance DRX (400 MHz) spectrometer. Another part of the sample was diluted with THF and filtered through an Acrodisc 0.2 μm PTFE syringe filter, and the solution was subjected to size exclusion chromatography (SEC) analysis. Molecular weights (number-average (Mn) and weight-average (Mw)) and molecular weight distribution dispersities (Đ = Mw/Mn) were determined by SEC on a Tosoh EcoSEC system equipped with a series of 4 columns (TSK gel guard Super HZ-L, Super HZM-M, Super HZM-N, and Super HZ2000) and using THF as the eluent (30 °C) and a refractive index detector. The SEC instrument was calibrated using a series of linear polySty standards. Elemental analyses were carried out at Midwest Microlab, IN. Infrared (IR) spectra were collected on a Thermo Scientific Nicolet iS10 FT-IR Spectrometer. The samples were prepared by dissolving 20 mg of polymer in 1 mL of chloroform, followed by casting a film on a KBr plate by slow evaporation of the solvent (achieved by covering the salt plate with the polymer solution with a beaker). 1H (64 scans) and 13C NMR spectra (10 000−15 000 scans) of the purified polymers (ca. 0.2 g in 0.6 mL of CDCl3 containing tetramethylsilane) were acquired on the spectrometer mentioned above. Synthetic Procedures. Synthesis of Polymers Using the PhIOTMSX (X = N3 or NCO) Initiating System. In a 10 mL dry reaction tube, a magnetic stir bar was added followed by the monomer (1 mL, corresponding to 11.0 mmol (in the case of MA), 9.4 mmol (MMA), 8.7 mmol (Sty), or 10.9 mmol (VAc)) and PhIO (1 mol % vs monomer, i.e., 0.11 mmol (24.2 mg) in the polymerizations of MA, 0.094 mmol (20.7 mg) (MMA), 0.087 mmol (19.4 mg) (Sty), or 0.109 mmol (24.0 mg) (VAc)). The tube was capped with a prewashed with acetone rubber septum, secured with electric tape, and was then wrapped with aluminum foil to prevent the exposure of the

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CONCLUSIONS The combination of iodosylbenzene with various (pseudo)halides, including trimethylsilyl azide or cyanate as well as potassium azide, isocyanate, or bromide, affords unstable hypervalent iodine(III) compounds, most likely, PhIX2 (X = (pseudo)halide), which rapidly decompose in situ to the corresponding (pseudo)halide radicals, even at moderate temperatures (30 °C). These radicals can initiate the polymerization of monomers such as styrene, acrylates and methacrylates, as well as vinyl esters, and (pseudo)halidecapped functional polymers are produced. In the cases of monomers with relatively low propagation rate coefficients (e.g., styrene and methyl methacrylate) and especially when the radical source (initiator) PhIX2 is generated rapidly at comparatively high concentrations and is particularly unstable (e.g., X = azide), limiting monomer conversions were observed, in accordance with “dead-end” polymerization mechanism. The presence of (pseudo)halide groups in the prepared polymers (in some cases, plausibly not only at the chain ends, but also as 11813


Article

The Journal of Organic Chemistry contents to light. The dry solvent (DMAc or PhCl; 1 mL) was then injected and the tube was placed in an ice−water cooling bath in order to minimize evaporation of the reaction components during the following purging with nitrogen. The reaction mixture was deoxygenated by purging with nitrogen, which was introduced using a Teflon-coated needle, for 10 min. The tube was immersed in a water bath at 30 °C. After this, TMSX (X = N3 or NCO; 2 equiv vs PhIO) was added using a micro syringe. The heterogeneous mixture rapidly became homogeneous. At timed intervals, samples (ca. 0.08 mL) were withdrawn from the reaction mixture with a nitrogen-purged syringe, equipped with a Teflon-coated needle. Part of the sample was diluted with CDCl3 (for NMR analysis) and part with THF (for SEC analysis). Similar experiments were carried out using different amounts of PhIO (0.2−4 mol % vs monomer) and TMSX (2 equiv vs PhIO) in DMAc. The polymers thus prepared contained one or more (x) (pseudo)halide groups X and are designated polyStyXx, polyMMAXx, polyVAcXx, and polyMAXx. PolySty(N3)x and polyVAc(N3)x were purified by reprecipitation of solutions of the polymer in methylene chloride into large excess of hexane, which was repeated three times, followed by drying. PolyMMA(N3)x and polyMA(N3)x were purified by dialysis against acetone using a membrane with molecular weight cutoff of 1000 Da (Spectrum Laboratories). The solvent was changed every 10−12 h and this was repeated six times, and the polymer was obtained by evaporation of the solvent. The corresponding (iso)cyanate-containing polymers were precipitated from methylene chloride solutions in pentane or hexane. Synthesis of Polymers Using the PhIO-KX (X = Br, N3, or OCN) Initiating System. In a 10 mL dry reaction tube, equipped with a magnetic stir bar, the monomer (1 mL, corresponding to 11.0 mmol in the case of MA or 9.36 mmol in the case of MMA), PhIO (1 mol % vs monomer), and potassium (pseudo)halide (bromide, azide, or isocyanate; 2 equiv vs PhIO) were mixed. The tube was sealed with an acetone-washed and dried rubber septum, which was secured with electric tape, and wrapped with aluminum foil. Anhydrous DMAc (1.0 mL) was then added through the septum and the mixture (cooled in an ice−water bath) was deoxygenated by purging with nitrogen (introduced with a Teflon-coated needle) for 10 min. Then, the tube was transferred to a water bath at 30 °C. Samples (ca. 0.08 mL) were withdrawn periodically from the mixture with a nitrogen-purged syringe equipped with a Teflon-coated needle for analysis, as described above. Similar experiments were carried out at a lower ratio of initiator to monomer (0.2 mol % of PhIO and 2 equiv of pseudohalide salt vs PhIO). The purifications of the final products were carried out as described above. Click Reactions of the Azide-Containing Polymers with Pg2O. The nitrogen contents in the polymers prepared using the PhIO-TMSN3 initiating system was first determined in order to know the amount of azide groups present in a given mass of polymer. In a 10 mL reaction tube containing a magnetic stir bar, the amount of azidecontaining polymer containing 600 μmol of N, i.e., 200 μmol of azide (0.75 g (in the case of polySty(N3)x), 0.98 g (polyMA(N3)x), 1.14 g (polyMMA(N3)x), or 1.00 g (polyVAc(N3)x)) and CuBr (2.8 mg, 20 μmol, 10 mol % vs azide groups), were added and the tube was capped with a prewashed with acetone rubber septum, which was then secured with electric tape. The tube was evacuated and backfilled with nitrogen five times. Deoxygenated DMF (1.0 mL) was injected with a nitrogenpurged syringe, and the mixture was stirred until solution was formed. Then, deoxygenated Pg2O (10.3 μL, 100 μmol, corresponding to 200 μmol of acetylene groups) was added using a nitrogen-purged micro syringe, and the solution turned yellow. The reaction mixture was stirred at r.t. and samples were taken in 5 and 15 h, which were then diluted with THF, filtered, and analyzed by SEC. Reactions of (Iso)Cyanate-Containing Polymers with 1,3Propylene Diamine. In a 10 mL reaction tube containing a magnetic stir bar, polymer prepared using the PhIO-TMSNCO initiating system, containing 600 μmol of N, i.e., 600 μmol of NCO groups (0.26 g of polySty(NCO)x, or 0.29 g of polyVAc(NCO)x) was added followed by THF (1 mL). The reaction mixture was stirred until the polymer dissolved and then 1,3-propylenediamine (25 μL, 300 μmol) was added. The reaction tubes were capped and the solutions were

stirred at r.t. Samples were withdrawn after 5 and 15 h, diluted with THF, filtered, and analyzed by SEC.

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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/acs.joc.7b01945. 1 H and 13C NMR spectra of azide- and (iso)cyanatecontaining polymers and predictions of 1H and 13C chemical shifts of low-molecular-weight compounds with structures resembling those of the polymers containing a pseudohalide functionality at various positions in the chain (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-214-768-3259. ORCID

Nicolay V. Tsarevsky: 0000-0001-6247-1359 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the National Science Foundation through a CAREER grant (CHE1455200) to NVT.

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REFERENCES

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