Addition-Isomerization Polymerization of Chiral Phosphaalkenes

Jun 1, 2016 - *E-mail [email protected] (D.P.G.)., *E-mail [email protected] (G.R.D.). Cite this:Macromolecules 49, 11, 4067-4075 ...
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Addition-Isomerization Polymerization of Chiral Phosphaalkenes: Observation of Styrene−Phosphaalkene Linkages in a Random Copolymer Spencer C. Serin, Gregory R. Dake,* and Derek P. Gates* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *

ABSTRACT: These studies provide the first evidence for styrene−phosphaalkene connectivities in a phosphaalkene copolymer. The synthesis and structural characterization of new phosphaalkene−oxazolines, ArPC(Ph)(3-C6H4Ox) [1a,b, Ar = Mes (1a), Mes* (1b), Ox = CNOCH(iPr)CH2], are reported. The radical-initiated homo- and copolymerization of 1a with styrene affords P-functional poly(methylenephosphine) (4a: Mn = 5300 g mol−1, PDI = 1.2) and poly(methylenephosphine-co-styrene) (5a: Mn = 4000 g mol−1, PDI = 1.1). Multinuclear NMR spectroscopic analyses of 4a and 5a provided evidence for the predominance of an addition-isomerization mechanism for the radical polymerization of 1a. In addition, signals could be assigned to CHPh−P(CHPhAr) (i.e., S−1a) and ArCH2−CH2 (i.e., 1a−S) linkages in copolymer 5a. With a monomer feed ratio of 1a:S (1:2, 33 mol % 1a) the inverse gated 13C{1H} NMR spectrum suggested an incorporation of 19 mol % 1a in copolymer 5a. Polymers 4a and 5a were further functionalized to Au(I)-containing macromolecules [4a·AuCl: Mn = 13 000, PDI = 1.2; 5a·AuCl: Mn = 7500, PDI = 1.1].

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INTRODUCTION Functional macromolecules featuring heavier p-block elements in the main chain are of current interest due to their unique chemical, physical, and electronic properties when compared to their organic analogues.1 For example, the presence of phosphorus within the backbone introduces new oxidation states, geometries, bonding environments, and ligand properties that are not found for carbon. Hence, the prospect to utilize organophosphorus polymers as ligands for transition metals opens the door to numerous exciting possibilities for materials with interesting catalytic, magnetic, or sensing properties. Driven by these opportunities, the past decade has witnessed tremendous growth of organophosphorus polymer chemistry.2 Notwithstanding this growth, methods to incorporate P atoms into long chains remain limited to condensation and ringopening polymerization and pose considerable synthetic challenges. Inspired by the striking parallels between PC and CC bonds in molecular chemistry,3 we have been exploring the addition polymerization of phosphaalkenes by analogy to olefin polymerization (Scheme 1). We have demonstrated that MesPCPh2 (A in Scheme 1) polymerizes in the presence of radical or anionic initiators to afford poly(methylenephosphine) (PMP in Scheme 1).4 The anionic polymerization of A or B is living and permits access to phosphaalkene−olefin block copolymers.5,6 PMPs represent a fascinating class of functional polymer as illustrated by the following examples: random copolymers of A with styrene have been employed as macromolecular ligands in Pd catalysis;7 block copolymers of A © XXXX American Chemical Society

Scheme 1. Isolobal Analogy between Olefins and Phosphaalkenes As Applied to Addition Polymerization (Top); Selected Examples of Polymerizable Phosphaalkene Monomers (Bottom)

with isoprene self-assemble as gold(I) complexes in block selective solvents;8 polymers derived from chromophorecontaining monomers C and D show “turn-on” fluorescence with oxygen;9 polymers from ferrocene-containing monomer E show interesting redox activity;10 and polymers of A are effective flame retardants.11 Most recently, we have shown that the copolymerization of the enantiomerically pure phosphaalkene F12 with styrene13 will give a macromolecular P, N ligand for chelation of rhodium(I) atoms.14 Herein, we report the synthesis, characterization, and polymerization of a new class of enantiomerically pure Received: March 31, 2016 Revised: May 10, 2016

A

DOI: 10.1021/acs.macromol.6b00667 Macromolecules XXXX, XXX, XXX−XXX

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was added dropwise to a cooled (−78 °C) THF solution of either MesP(Li)SiMe3 or Mes*P(Li)SiMe3 [formed in situ from MesP(SiMe3)2 or Mes*PH(SiMe3), respectively]. The reaction mixture was warmed to room temperature whereupon an aliquot was removed for analysis by 31P{1H} NMR spectroscopy. In all cases, the signal assigned to ArP(Li)SiMe3 [δ = −187 (Mes); −127 (Mes*)] was replaced by two new signals in a range consistent with those expected for the E and Z isomers of a phosphaalkene (δ = 1a: 237.6, 237.4; 1b: 246.9, 246.5). The desired phosphaalkene 1a (yield = 47%) was isolated and characterized as the Z-isomer after precipitation with n-pentane. In contrast, air- and moisture-stable 1b (yield 64%) was isolated as an E/Z mixture (E/Z = 58:42) after column chromatography (SiO2). Monomers 1a and 1b were subjected to full characterization by 31P, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, and for 1b elemental analysis. Phosphaalkene 1a undergoes rapid E/Z isomerization in solution (CDCl3); at ambient temperature (ca. 22 °C) the equilibrium was observed to be 0.6:1.0 in favor of the Z-isomer. Signals could be unequivocally assigned to either the Z- or E-isomer by using 1 H−1H NOESY and 1H−1H COSY NMR spectroscopy. This stereoisomerization is commonly observed for phosphaalkenes, especially when less sterically hindered substituents are employed on the phosphorus atom (e.g., Mes).9,20 Recrystallization from n-pentane afforded the isolation of crystals of exclusively the Z-isomer that were analyzed by X-ray crystallography. Phosphaalkenes 1a and 1b were recrystallized from hexanes and n-pentane, respectively, to afford pale green crystals of each compound suitable for X-ray diffraction. The molecular structures of Z-1a and Z-1b are shown in Figure 1. In each case, support for the retention of the S configuration of the oxazoline moiety and enantiomeric purity was obtained from the Flack parameter [Z-1a: 0.08(8); Z-1b: 0.03(8)] being close to zero. The PC bond lengths of each phosphaalkene [Z-1a: 1.693(2) Å; Z-1b: 1.694(6) Å] are at the long end of the range typically found for C-substituted phosphaalkenes (1.61−1.71 Å)21 but are shorter than the PC bonds in inversely polarized phosphaalkenes (e.g., RPC(NR2)2, 1.70−1.76 Å).22 Overall, the metrical parameters are consistent with those observed previously for triaryl-substituted phosphaalkenes such as ArP CPh2 (Ar = Mes or Mes*) and related systems.15,12,23

phosphaalkene (1). The radical-initiated polymerization of PMes-containing monomer 1a affords homopolymer 4a or copolymer 5a with styrene (S). Importantly, multinuclear NMR spectroscopy permitted the confirmation of an additionisomerization mechanism for the polymerization of 1a and provided the first direct evidence for 1a−S linkages in a phosphaalkene copolymer.

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RESULTS AND DISCUSSION Monomer Synthesis and Characterization. Given that oxazoline-substituted phosphaalkene F does not form homopolymers while phenyl-substituted A does, we hypothesized that a phenylene−oxazoline phosphaalkene such as 1a may be more likely to homopolymerize. Using the well-known phospha-Peterson reaction, monomer 1a should be accessible from MesPLi(SiMe3) and ketone 3 (Scheme 2).13,15−17 Scheme 2. Synthesis of Enantiomerically Pure PhenylBridged Phosphaalkene−Oxazolines

Oxazoline 3 was prepared from commercially available 3benzoylbenzoic acid in 55% yield via amide 2 following procedures analogous to the preparation of naphthyl−imidazolines18 and phenylene−oxazolines.18,19 Ketone 3 was fully characterized using 1H and 13C{1H} NMR spectroscopy, mass spectrometry [HRMS (3·H + ): m/z 294.1494 (found); 294.1495 (calcd)], and elemental analysis. Compound 3 3 −1 showed an optical rotation [α]22 dm−1 (c D = −35.6° cm g = 2.4 × 10−1, CH2Cl2). Elaboration of 3 to phosphaalkenes 1a and 1b was accomplished using the aforementioned phospha-Peterson reaction15 as the PC bond forming step. The general synthetic procedure is as follows: a solution of ketone 3 in THF

Figure 1. Molecular structures of (i) Z-1a and (ii) Z-1b. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Z-1a: P(1)−C(1) 1.693(2), P(1)−C(20) 1.834(2), C(1)−C(2) 1.485(2), C(1)−C(8) 1.481(2), C(1)−P(1)−C(20) 104.31(8), C(2)−C(1)−P(1) 118.5(1), C(8)−C(1)−P(1) 126.3(1), C(2)−C(1)−C(8) 115.2(2). Z-1b: P(1)−C(1) 1.694(6), P(1)−C(20) 1.857(5), C(1)−C(2) 1.483(7), C(1)−C(8) 1.500(8), C(1)−P(1)−C(20) 102.2(2), C(2)−C(1)−P(1) 117.8(4), C(8)− C(1)−P(1) 125.7(4), C(2)−C(1)−C(8) 116.5(5). B

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Macromolecules Homo- and Copolymerization of Monomer 1a. The homopolymerization of phosphaalkene 1a was performed in the melt (160 °C, 16 h) in the presence of the radical initiator 1,1′-azobis(cyclohexanecarbonitrile) (VAZO 88, 1 mol %) (Scheme 3). A significant increase in viscosity was observed

homopolymerization. Characteristic PMP signals were again observed in the 31P NMR spectrum between 20 and −20 ppm. Dissolution of the sample in THF and precipitation with MeOH (×3) afforded copolymer 5a as an off-white solid. GPCMALS analysis revealed a lower Mn and narrower PDI than PMP 4a (4000 g mol−1 and 1.1, respectively). Copolymer 5a 3 −1 dm−1 (c = 1.1 × had an optical rotation [α]22 D = −29.3° cm g −1 10 , CH2Cl2). Microstructures of Polymers 4a and 5a. Given our recent observation of an addition-isomerization mechanism for the radical-initiated homopolymerization of MesPCPh2 (A)24 and copolymerization of phosphaalkene F with styrene,14 a detailed investigation of the microstructure of polymers 4a and 5a was undertaken. The 31P NMR spectrum of homopolymer 4a in CDCl3 shows broad resonances centered at 4 (minor), −8 (major), and −48 ppm (minor), each potentially having some additional fine structure (see Figure 2a). By comparison, the homopolymer of A displays a broad resonance at −10 ppm with some fine structure. We previously speculated that the fine structure results from tacticity effects and presume that the additional complexity for 4a may arise from the chiral Ox moiety and the chiral carbon center adjacent to P. The 31P NMR spectrum of phosphaalkene−styrene copolymer 5a displays similar broad signals centered at 6 (major), −8 (major), and −44 ppm (minor) (see Figure 2b). Other than a slight increase in the intensity of the signal at 6 ppm, the incorporation of styrene in 5a does not significantly change the 31P NMR spectroscopic features when compared to 4a. The 13C{1H} and 1H NMR spectra of both 4a and 5a exhibit signals attributed to the expected substituents: oxazoline [13C: δ ≈ 160 (NC), 73 (CH), 70 (CH2), 19, 18 (2 × CH3); 1H: δ = 4.6−3.7 (CH and CH2), and 1.0, 0.90 (2 × CH3)], mesityl [13C: δ ≈ 145−126 (Ar−C), 23 (o-CH3), 21 (p-CH3); 1H: δ = 8.1−6.1 (Ar−H), 2.3−1.8 (o,p-CH3)] as well as the phenyl and arylene groups. These assignments were supported by edited 1 H−13C{1H} HSQC NMR experiments (edHSQC), where the experiment is collected with multiplicity editing during the selection step permitting 13C{1H} NMR signals to be classified as either CH/CH3 or CH2 groups. Importantly, these experiments also permitted the assignment of PCH (13C: δ = 52.1; 1H: δ = 4.9) and ArCH2 (13C: δ = 33.6; 1H: δ = 3.5) within homopolymer 4a. These signals are denoted by crosscorrelations in Figure 3i and are phased positive (blue, CH) and negative (red, CH2), respectively. These values are in close agreement with our previous detailed analysis of the micro-

Scheme 3. Synthesis of Poly(methylenephosphine) 4a and Poly(methylenephosphine-co-styrene) 5a

suggestive of polymerization. 31P NMR spectroscopic analysis of a THF solution of the crude polymer revealed that the signals assigned to phosphaalkene 1a had been partially consumed (δ31P = 237.4, 237.6; ca. 40%) and were replaced by several broad signals assigned to 4a (range: 20 to −20 ppm; ca. 60%). The latter signals are consistent with those previously reported for PMPs.4,9,10 Despite repeated trials involving larger amounts of initiator (5 mol % VAZO 88), longer polymerization times (t > 24 h), and higher melt temperatures (Tp > 180 °C), higher monomer-to-polymer conversions were not attainable. Yellow polymer 4a was isolated in pure form by precipitation of the crude reaction mixture in THF with MeOH (×3). GPC-MALS analysis of polymer 4a revealed a monodisperse distribution with modest molecular weight (Mn = 5300 g mol−1; PDI = 1.2). The Mn value suggests a degree of polymerization of ca. 12. Polymer 4a had an optical rotation 3 −1 [α]22 dm−1 (c = 1.4 × 10−1, CH2Cl2). D = −19.9° cm g In addition to the homopolymer of 1a, poly(methylenephosphine-co-styrene) 5a was prepared in an analogous fashion. Phosphaalkene 1a, styrene (1:2 loading ratio, respectively), and VAZO 88 (1 mol %) were flame-sealed in a Pyrex tube in vacuo and heated at 160 °C for 14 h. Again, an increase in viscosity was observed suggestive of polymerization. 31 P NMR spectroscopic analysis revealed a larger amount of phosphaalkene had been consumed (ca. 93% conversion) with respect to

Figure 2. 31P{1H} NMR spectra (162 MHz, 298 K) in CDCl3 of (a) homopolymer 4a, (b) copolymer 5a, (c) 4a·AuCl, and (d) 5a·AuCl. C

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Figure 3. 1H−13C{1H} edHSQC NMR spectra (400 MHz for 1H, CDCl3, 298 K) of (i) homopolymer 4a and (ii) copolymer 5a, (iii) 1H−1H NOESY NMR spectrum (400 MHz, CDCl3, 298 K) of copolymer 5a, and (iv) 1H−1H COSY NMR spectrum (400 MHz, CDCl3, 298 K) of copolymer 5a. For (i) and (ii), the ordinate shows the 13C{1H} NMR spectrum and the abscissa shows the 1H NMR spectrum. The dashed lines show the cross peaks permitting the assignment of the −PCHPh(3-C6H4Ox) and the −ArCH2− (formerly Mes) in polymers 4a and 5a. The edHSQC spectra were collected with multiplicity editing during the selection step permitting assignments of CH/CH3 (blue) vs CH2 (red) moieties.

structure of poly(A) [PCH (13C: δ = 52.4; 1H: δ = 4.8) and ArCH2 (13C: δ = 33.0; 1H: δ = 3.6)].24 Therefore, we conclude that the microstructure of 4a results from an additionisomerization polymerization mechanism where propagation occurs through the o-Me moiety of the former Mes group (i.e., x ≫ y in 4a). Our previous studies of phosphaalkene−styrene random copolymers involving monomers A and F did not reveal NMR spectroscopic evidence for styrene−phosphaalkene linkages. Therefore, we embarked on additional NMR spectroscopic experiments on copolymer 5a to gain additional insight into these fascinating copolymers. Since we did not observe the nonisomerized form of 4a (i.e., y in Scheme 3), there are four possible environments for a phosphaalkene unit within copolymer 5a (Figure 4 shows its nearest neighbors). Given that the feed ratio of styrene to phosphaalkene is 2:1 and that phosphaalkene incorporation is typically lower than the feed ratio,7,14 then microstructure (i: S−1a−S) should statistically be the most prominent. Since styrene is in excess and the

Figure 4. Most likely environments for a central phosphaalkene unit (in blue) within copolymer 5a (Ar = 3-C6H4Ox). Each central unit has two nearest neighbors.

D

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its 1H NMR spectrum (δ = 3.4, 2.9)24 with respect to the analogous polystyrene model compound PhCH2−CH2CH(Ph)CH3 [δ = 2.5)].29 This further demonstrates the effect of the adjacent atom to the ArCH2−E unit of 5a (i.e., E = P vs C). The absence of the expected second signal for the ArCH2 (i.e., microstructures iia and iii) may reflect the preference for the putative propagating radical from 1a (i.e., ArCH2•) to add to styrene (e.g., r1a = k1a/kS ≪ 1). Moreover, it must be noted that it is very difficult to detect the broad ArCH2 signals in either the 1 H or the 13C{1H} NMR spectrum; thus, structure types iia and iii may be present but not observed. This may be due to the fact that the ArCH2 is near several racemic centers in both the main chain and side chain of 5a. Confident that the 13C{1H} NMR spectrum had been suitably assigned, integration of the styrenic signals relative to those from 1a was performed to estimate the phosphaalkene incorporation (mol % 1a) within copolymer 5a. Inverse-gated 13 C{1H} NMR spectroscopy has been employed to estimate monomer incorporation within organic copolymers.30,31 The inverse gated 13C{1H} NMR spectrum of 5a is shown in Figure 5 along with assignments and integrals. The aliphatic styrene

phosphaalkene has the most diagnostic NMR signals, the environments for a central styrene unit are not considered here (e.g., S−S−S, 1a−S−S, etc.). Of course, the aforementioned microstructures (i−iii) do not take into account the additional complexity arising from tacticity. The following section will discuss our NMR spectroscopic evidence for the type of connectivities found in copolymer 5a. 1 H−13C{1H} HSQC NMR spectroscopy has been demonstrated to be a valuable tool for detailed backbone analysis of organic polymers.25−27 Analogous to that described above for homopolymer 4a, cross-correlations were observed that were attributed to PCH and ArCH2 moieties in 5a [see Figure 3(ii): 13 C: δ = 50.9; 1H: δ = 4.7 (PCH) and 13C: δ = 33.6; 1H: δ = 2.2 (ArCH2)]. Strikingly, a new and more intense cross-correlation was observed in the region of the PCH moiety [13C: δ = 48.4; 1 H: δ = 3.6]. Moreover, the 1H chemical shift attributed to the ArCH2 moiety of copolymer 5a is found considerably upfield from that in homopolymer 4a (δ = 2.2 vs 3.6). We hypothesize that these striking differences between the spectra of homopolymer 4a and copolymer 5a are a consequence of the microstructure of copolymer 5a. For example, microstructure i where the phosphaalkene is surrounded by two styrenes would be expected to have significant differences in the chemical shifts of the PCH and ArCH2 groups, both being influenced by the adjacent styrene. In contrast, microstructure iia in 5a has a similar chemical environment to homopolymer 4a for its ArCH2 while the PCH group would be quite different. In microstructure iib, the reverse is expected. Microstructure iii, being the lowest in probability, might be expected to display similar ArCH 2 and the PCH chemical shifts to the homopolymer. Based on the above, a mixture of all three microstructures of 5a is expected to show at least two distinct ArCH2 and PCH environments. In the case of the PCH group, two distinct signals are observed in each of the 1H and 13C{1H} NMR spectra [13C: δ = 50.9, 48.4; 1H: δ = 4.7, 3.6] while one is observed for 4a [13C: δ = 52.1; 1H: δ = 4.9]. We postulate that the more intense upfield signals are attributable to S−1a linkages in microstructures i and iia [e.g., −CHPh−P(CHPhAr)]. α,β-Disubstituted polymers of the type [−CHR−CH(CHR′2)−]n show similar upfield shifts in 1H and 13C NMR signals of the CH unit that have been attributed to γ-gauche interactions between R and the CH.28 In 5a, similar interactions may be present between the PCH and S (i.e., R = Ph). To probe this linkage further, an 1H−1H NOESY NMR spectrum of 5a was recorded [Figure 3(iii)]. Importantly, an NOE correlation was observed between the proton of the PCH and the CHPh of S along with the expected NOE correlations involving the oxazoline. Additional evidence for various types of S−PCH connectivities in copolymer 5a was gained from the analysis of the 1H−1H COSY NMR spectrum. Namely, two cross-correlations between the backbone protons of S and the PCH proton are observed [δ = 3.6 (PCH) and 2.4 (SCHPh); 3.8 (PCH) and 1.9 (SCHPh)]. The aforementioned data marks the first time we have observed direct spectroscopic evidence for phosphaalkene−styrene linkages in copolymers. In contrast to the PCH unit, only one signal is observed for the ArCH2 in the 1H−13C{1H} edHSQC NMR spectrum of 5a (13C: δ = 33.6; 1H: δ = 2.2). This resonance is shifted upfield with respect to homopolymer 4a (δ = 3.5) which presumably arises from the predominance of microstructures i and iib. For comparison, the diastereotopic CH2 of model compound PhCH2−P(Mes) (CHPh2) exhibits a similar downfield shift in

Figure 5. Inverse-gated 13C{1H} NMR spectrum (101 MHz, 298 K) in CDCl3 of poly(methylenephosphine-co-styrene) 5a. Assignments were made with the aid of 13C{1H}-APT and 1H-13C{1H} edHSQC NMR experiments.

carbons (δ13C: 39−47, −CH2CHPh−) integrate to 8.4 relative to either aliphatic carbon of the oxazoline in 1a (δ13C: 70, 73; both integrate to 1.0). Therefore, the styrene to phosphaalkene ratio in 5a is 4.2:1 [i.e., z:(x + y)] which corresponds to the formulation of 5a [(x + y) = 0.19n, z = 0.81n where x ≫ y]. To place this into context, the Mn of 4000 g mol−1 determined by GPC-MALS for 5a corresponds to ca. 5 phosphaalkene units and ca. 20 styrene units. Chemical Functionalization of Polymers 4a and 5a. We have previously shown that the phosphine moieties of homo- and copolymers of phosphaalkene A are effective ligands for transition metals.7,9,14,32 Thus, CH2Cl2 solutions of either 4a or 5a were treated with Au(tht)Cl (1 equiv per P, tht = tetrahydrothiophene). In each case, the reaction solutions were precipitated with n-pentane to afford off-white solids that were separated by centrifugation and dried in vacuo. The 31P{1H} NMR spectra of the products are shown in Figure 2 and are consistent with that expected for complexes 4a·AuCl and 5a· AuCl. In particular, the complex features of the broad signals are retained in each case [4a·AuCl: δ = 41.0 (minor), 23.5 (major); 5a·AuCl: δ = 41.9 (major), 24.7 (major)]. These signals are downfield shifted by ca. 35 ppm compared to the chemical shifts for the uncomplexed homo- and copolymers. By comparison, the gold(I) complex of the homopolymer from monomer A is shifted downfield by ca. 32 ppm from its uncomplexed form.32 Interestingly, complexed polymers 4a· AuCl and 5a·AuCl display modest air stability with all work-up procedures being conducted in air atmosphere. GPC-MALS E

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spectra were referenced to the deuterated solvent. edHSQC spectra were collected using the standard pulse sequence (pulprog: HSQCedetdp; 1J coupling constant: 145 Hz) taken from the Bruker software library. Inverse-gated 13C{1H} collected using 90° pulse (pulprog: zgig; 1J coupling constant: 145 Hz) and d1 = 3 s. Mass spectra were recorded on a Kratos MS 50 instrument in EI mode (70 eV). Elemental analyses were performed by Mr. Derek Smith in the UBC Microanalysis Facility. The optical rotations were measured at a concentration in g per 100 mL, and their values (average of 10 measurements) were obtained on a Jasco P-1010 polarimeter. Polymer molecular weights were determined by gel permeation chromatography−multiangle light scattering (GPC-MALS) using an Agilent liquid chromatograph equipped with an Agilent 1200 series isocratic pump, Agilent 1200 series standard autosampler, Phenomenex Phenogel 5 mm narrow bore columns (4.6 × 300 mm) 104 Å (5000−500 000 g mol−1), 500 Å (1000−15 000 g mol−1), and 103 Å (1000−75 000 g mol−1), a Wyatt Optilab T-rEx differential refractometer (λ = 658 nm, 40 °C), a Wyatt TriStar miniDAWN (laser light scattering detector at λ = 690 nm), and a Wyatt ViscoStar viscometer. A flow rate of 0.5 mL min−1 was used and samples were dissolved in THF (ca. 2 mg mL−1). The molecular weights were determined using 100% mass recovery methods from Astra software version 5 with each polymer sample being run at least three times to ensure reproducibility of the calculated refractive index increment (dn/ dc). Synthesis of (S)-PhCOC6H4(CNOCH(i-Pr)-CH2) (3) [in Ambient Moisture and Atmosphere]. To a suspension of 3-benzoylbenzoic acid (5.00 g, 22.1 mmol) in CH2Cl2 (50 mL) was added a few drops of DMF and a septum affixed to the flask with an outlet to an external oil bubbler. (COCl)2 (2.25 mL, 3.36 g, 26.5 mmol) was added dropwise over 30 min, and the reaction mixture was stirred until the bubbling subsided (ca. 2 h). Volatiles were removed by rotary evaporation to give the crude acid chloride. To a cooled (0 °C) solution of L-valinol (2.47 mL, 2.30 g, 22.1 mmol) in Et3N (15.5 mL, 11.2 g, 111 mmol) and dry CH2Cl2 (100 mL) was added a solution of acid chloride in CH2Cl2 (50 mL) in one batch. The reaction mixture was warmed to room temperature and stirred 1 h whereupon it was quenched with saturated aqueous NaHCO3 (100 mL), and the organic layer was separated. The organic layer was washed with 1 M HCl (100 mL), separated, and dried over Na2SO4. Volatiles were removed by rotary evaporation to give crude amide 2. 1H NMR (300 MHz, CDCl3): δ = 8.20 (t, JHH = 2 Hz, 1H), 8.03 (dt, JHH = 8, 1 Hz, 1H), 7.87 (dt, JHH = 9, 1 Hz, 1H), 7.80−7.73 (m, 2H), 7.65−7.45 (m, 4H), 6.62 (d, JHH = 9 Hz, 1H), 4.01−3.92 (m, 1H), 3.85−3.75 (m, 1H), 2.32 (br s, 1H), 2.08−1.97 (m, 1H), 1.03 (d, JHH = 6 Hz, 3H), 1.01 (d, JHH = 6 Hz, 3H). To a solution of the previously synthesized amide 2 (22.1 mmol) in CH2Cl2 (100 mL) was added Ts-Cl (8.43 g, 44.2 mmol) followed by Et3N (15.4 mL, 11.2 g, 111 mmol). The reaction mixture was heated to reflux overnight. The reaction mixture was successively washed with water (3 × 100 mL), saturated CuSO4 (2 × 100 mL), and brine (100 mL). The organic layer was separated and dried over Na2SO4. Volatiles were removed by rotary evaporation to give crude ketone 3. Recrystallization in MeOH gave the title compound 3 as a white solid (3.54 g, 55%). 1H NMR (400 MHz, CDCl3): δ = 8.34 (t, JHH = 1 Hz, 1H), 8.20 (dd, JHH = 6, 1 Hz, 1H), 7.91 (m, 1H), 7.81 (dd, JHH = 8, 1 Hz, 2H), 7.64−7.59 (m, 1H), 7.57−7.48 (m, 3H), 4.47−4.39 (m, 1H), 4.19−4.09 (m, 2H), 1.92−1.83 (m, 1H), 1.04 (d, JHH = 7 Hz, 3H), 0.93 (d, JHH = 6 Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3): δ = 195.8, 162.3, 137.7, 137.0, 132.5, 132.2, 131.8, 129.9, 129.4, 128.2, 3 −1 dm−1 128.2, 128.1, 72.6, 70.2, 32.7, 18.8, 18.0. [α]22 D = −35.6° cm g −3 −3 (c = 2.4 × 10 g cm , CH2Cl2). HRMS calcd for C19H20NO2 (M + H+): 294.1494. Found: 294.1495. Anal. Calcd for C19H19NO2: C, 77.79; H, 6.53; N, 4.77. Found: C, 77.51; H, 6.60; N, 4.59. Synthesis of (S)-MesPCPhC6H4(CNOCH(i-Pr)-CH2) (1a). To a solution of MesP(SiMe3)2 (1.70 g, 5.73 mmol) in THF (10 mL) was added methyllithium (1.6 M in Et2O, 3.6 mL, 5.73 mmol). The reaction mixture was heated to 55 °C for 1−2 h. 31P NMR analysis of an aliquot removed from the reaction mixture suggested quantitative formation of MesP(Li)SiMe3 (δ = −187). The reaction mixture was

analysis of the isolated polymers in THF solution revealed the molecular weight of complexed 4a·AuCl (Mn = 13 000 g mol−1, PDI = 1.2) and 5a·AuCl (Mn = 7500 g mol−1, PDI = 1.1). In each case, a considerable increase in mass is observed upon coordination to the heavy gold atom (cf. 4a: Mn = 5300 g mol−1, PDI = 1.2; 5a: Mn = 4000 g mol−1, PDI = 1.1). It must be noted that for the homopolymer 4a the increase in Mn is much higher than that calculated (ca. 8200 g mol−1). By comparison, the molecular weight of the homopolymer derived from MesPCPh2 (Mn = 38 900 g mol−1) showed a larger than expected increase in molecular weight upon coordination to gold(I) [Mn = 66 100 g mol−1 (calcd) vs 71 600 g mol−1 (measured)].32 It is possible that these are real effects and may result from Au···Au interactions (i.e., [Au]:[P] ratios >1.0) or due to errors in the determination of dn/dc using 100% mass recovery methods. Further experiments with a range of polymers are necessary to fully understand these observations.

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SUMMARY We have described the synthesis and structural characterization of a novel class of phosphaalkene monomer bearing chiral oxazoline substituents (1a,b). Unlike the previously reported Pmesityl phosphaalkene−oxazoline (F), the new monomer 1a can be homopolymerized using radical methods of initiation to afford polymers of modest molecular weight. In both the homoand copolymerization of 1a an addition-isomerization mechanism of propagation predominates. The presence of chiral centers at both the P- and C-centers of the former PC bond, as well as the enantiomerically pure oxazoline moiety, leads to fascinating NMR spectroscopic properties of polymers derived from 1a. Of particular interest is the random copolymer of 1a and styrene. For the first time, we have provided definitive spectroscopic evidence for styrene−phosphaalkene linkages in a phosphaalkene−styrene copolymer. Specifically, 1H−13C{1H} edHSQC and 1H−1H COSY/NOESY NMR spectroscopy on 5a give evidence for CHPh−P(CHPhAr) (i.e., S−1a) and ArCH2−CH2 (i.e., 1a−S) linkages. Finally, we have shown that both new polymers derived from 1a are amenable to the formation of coordination complexes with metals. Future investigations will focus on further exploring the fascinating microstructures, properties, and potential applications of these and related functional P-containing polymers.

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EXPERIMENTAL SECTION

Materials and Methods. Unless stated otherwise, all manipulations were performed under a nitrogen atmosphere using standard Schlenk or glovebox techniques. Dichloromethane (CH2Cl2), and diethyl ether (Et2O) were deoxygenated with nitrogen and dried by passing through a column containing activated alumina. Tetrahydrofuran (THF) was dried over sodium/benzophenone ketyl and distilled prior to use. Methanol was deoxygenated with nitrogen prior to use. 1,1′-Azobis(cyclohexanecarbonitrile) (VAZO 88) was purchased from Aldrich and recrystallized from EtOH prior to use. Styrene was purchased from Aldrich and freshly distilled from CaH2 prior to use. CDCl3 and CD2Cl2 were purchased from Cambridge Isotope Laboratories Inc. and deoxygenated with nitrogen prior to use. Compounds L-valinol,33 MesP(SiMe3)2,34 Mes*PH2,35 and Au(tht)Cl36 were synthesized according to literature procedures. 3-Benzoylbenzoic acid, SOCl2, Et3N, 4-toluenesulfonyl chloride (Ts-Cl), methyllithium (1.6 M solution in Et2O), and trimethylsilyl chloride (TMS-Cl) were purchased from Sigma-Aldrich and used as received. 1 H, 13C{1H}, and 31P NMR spectra were recorded at 298 K on Bruker Avance 300 or Avance 400 spectrometers. H3PO4 (85%) was used as an external standard (δ = 0) for 31P NMR spectra. 1H NMR spectra were referenced to residual protonated solvent, and 13C{1H} NMR F

DOI: 10.1021/acs.macromol.6b00667 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules cooled to −78 °C and treated with a solution of oxazoline 3 (1.68 g, 5.73 mmol) in THF (5 mL). After warming to room temperature, analysis of an aliquot removed from the reaction mixture by 31P NMR spectroscopy revealed two singlet resonances which are consistent with phosphaalkene (E/Z-1a) (δ = 238.4, 238.2). The reaction mixture was quenched with excess TMS-Cl (ca. 3 mL), the solvent evaporated in vacuo, and the product extracted into hexanes (3 × 10 mL). After filtration, the hexanes was removed in vacuo. The product was dissolved in 5 mL of n-pentane and stirred at 0 °C until a precipitate formed. The solid was filtered and dried in vacuo to give the product as a yellow solid (1.15 g, 47%). Single crystals of Z-1a suitable for X-ray diffraction analysis were obtained by slow evaporation of a CDCl3 solution of E/Z-1a. 31P{1H} NMR (121 MHz, CDCl3): δ = 237.6 (Z), 237.4 (E). 1H NMR (400 MHz, CDCl3, Z-isomer): δ = 7.68 (m, 1H), 7.54 (m, 3H), 7.35 (m, 3H), 7.11 (d, JHH = 7.7 Hz, 1H), 7.00 (d, JHH = 8.0 Hz, 1H), 6.71 (br s, 1H), 6.70 (br s, 1H), 4.32 (m, 1H), 3.99 (m, 2H), 2.32 (s, 3H), 2.27 (s, 3H), 2.16 (s, 3H), 1.70 (m, 1H), 1.00 (d, JHH = 6.9 Hz, 3H), 0.93 (d, JHH = 6.9 Hz, 3H). 1H NMR (400 MHz, CDCl3, E-isomer): δ = 8.20 (s, 1H), 8.02 (d, JHH = 8 Hz, 1H), 7.59− 7.57 (m, 1H), 7.40−7.36 (m, 1H), 7.08−7.05 (m, 3H), 6.89 (d, JHH = 7 Hz, 2H), 6.74 (s, 2H), 4.45−4.37 (m, 1H), 4.18−4.12 (m, 2H), 2.29 (overlapping s, 6H), 2.23 (s, 3H), 1.94−1.86 (m, 1H), 1.07 (d, JHH = 7 Hz, 3H), 0.96 (d, JHH = 7 Hz, 3H). 13C NMR assigned only for major Z-isomer: 13C{1H} NMR (101 MHz, CDCl3): δ = 192.8 (d, JPC = 44.5 Hz), 162.9, 144.1 (d, JPC = 23.0 Hz), 143.1 (d, JPC = 15.3 Hz), 140.4 (d, JPC = 6.1 Hz), 140.0 (d, JPC = 1.0 Hz), 138.3, 136.0 (d, JPC = 43.0 Hz), 131.0 (d, JPC = 6.1 Hz), 128.9 (d, JPC = 4.6 Hz), 128.6 (d, JPC = 7.7 Hz), 128.2, 128.2, 127.5, 127.5, 127.3 (d, JPC = 3.1 H), 127.1 (d, JPC = 7.7 Hz), 72.4, 69.9, 32.7, 22.2 (d, JPC = 9.2 Hz), 22.1 (d, JPC = 7.7 Hz), 21.0, 18.7, 18.0. MS (EI): m/z 428, 427 [30, 100, M+], 351, 350 [12, 44, M−Ph+]. HRMS (EI): m/z 427.2065 (calcd for C28H30NOP 427.2066). Synthesis of (S)-Mes*PCPhC6H4(CNOCH(i-Pr)-CH2) (1b). To a cooled solution (−78 °C) of Mes*PH2 (0.898 g, 3.23 mmol) in THF (40 mL) was added n-butyllithium (1.5 M in hexanes, 2.8 mL, 4.2 mmol). The reaction mixture was warmed to room temperature, stirred for 1 h, and cooled to −78 °C. TMS-Cl (0.53 mL, 0.46 g, 4.2 mmol) was added; the reaction mixture was warmed room temperature and stirred for 1 h. The reaction mixture was cooled to −78 °C and treated with a solution of oxazoline 3 (0.759 g, 2.58 mmol) in THF (15 mL). After warming to room temperature, analysis of an aliquot removed from the reaction mixture by 31P NMR spectroscopy revealed a singlet resonance that is consistent with phosphaalkene (E/Z-1b) (δ = 246.9, 246.3). The reaction mixture was quenched with excess TMS-Cl (ca. 1 mL) and the solvent evaporated in vacuo. At this point the product could be handled under ambient moisture and air. The product was dissolved in hexanes (ca. 3 mL) and filtered through a small plug of Celite. Volatiles were removed using rotary evaporation, the green residue was purified by column chromatography (SiO2, gradient Hex:EtOAc (50:1) to Hex:EtOAc (10:1)), and both stereoisomers of 1b were isolated as a green solid (0.92 g, 64%). Single crystals of Z-1b suitable for X-ray diffraction analysis from a saturated n-pentane solution of E/Z-1b. Z-1b: 31P{1H} NMR (162 MHz, CDCl3): δ = 246.9. 1H NMR (400 MHz, CDCl3): δ = 7.60 (d, JHH = 7.6 Hz, 1H), 7.42 (m, 2H), 7.34 (m, 3H), 7.25 (br s, 1H), 7.22 (br s, 2H), 6.88 (t, JHH = 7.9 Hz, 1H), 6.51 (d, JHH = 7.9 Hz, 1H), 4.32 (t, JHH = 8.2 Hz, 1H), 3.99 (m, 2H), 1.78 (m, 1H), 1.51 (s, 9H), 1.49 (s, 9H), 1.28 (s, 9H), 1.03 (d, JHH = 6.7 Hz, 3H), 0.89 (d, JHH = 6.7 Hz, 3H). 13C{1H} NMR (101 MHz, CDCl3): δ = 180.1 (d, JPC = 45.4 Hz), 163.2, 154.4, 150.2, 145.5 (d, JPC = 27.8 Hz), 142.7 (d, JPC = 16.1 Hz), 135.7, 135.1, 132.1 (d, JPC = 5.9 Hz), 129.1 (d, JPC = 7.3 Hz), 128.6, 128.4, 128.3, 128.0, 126.9, 126.8, 126.5, 121.7 (d, JPC = 14.6 Hz), 72.7, 70.2, 38.2, 34.9. 33.4, 33.4, 33.0, 31.4, 19.2, 18.3. E-1b: 31P{1H} NMR (162 MHz, CDCl3): δ = 246.5. 1H NMR (400 MHz, CDCl3): δ = 7.98 (d, JHH = 7.6 Hz, 1H), 7.95 (s, 1H), 7.52 (d, JHH = 7.6 Hz, 1H), 7.39 (t, JHH = 7.6 Hz, 1H), 7.31 (s, 2H), 6.97 (t, JHH = 7.3 Hz, 1H), 6.86 (t, JHH = 8.2 Hz, 2H), 6.37 (d, JHH = 7.6 Hz, 2H), 4.40 (m, 1H), 4.13 (m, 2H), 1.88 (m, 1H), 1.48 (s, 18H), 1.36 (s, 9H), 1.04 (d, JHH = 6.7 Hz, 3H), 0.94 (d, JHH = 6.7 Hz, 3H).

C{1H} NMR (101 MHz, CDCl3): δ = 179.2 (d, JPC = 45.5), 163.2, 154.5, 154.4, 150.7, 145.8 (d, JPC = 29.3 Hz), 142.1 (d, JPC = 16.9 Hz), 135.8, 135.2, 129.1 (d, JPC = 8.8 Hz), 128.1, 127.7 (d, JPC = 3.7 Hz), 127.1 (d, JPC = 2.2 Hz), 126.7, 126.4, 121.9 (d, JPC = 2.2 Hz), 72.6, 70.0, 38.1, 34.9, 33.1 (d, JPC = 7.3 Hz), 32.8, 31.4, 19.0, 18.0. MS (ESI): m/z 578, 577 [30, 85, M·Na+], 556, 555 [35, 100, M+]; HRMS (ESI): m/z 554.3549 (calcd for C37H48NOP 554.3552); Anal. Calcd for C37H48NOP: C, 80.25; H, 8.74; N, 2.53. Found: C, 80.60; H, 8.80; N, 2.53. Synthesis of Homopolymer 4a. Phosphaalkene 1a (2.0 g, 4.7 mmol) and VAZO 88 (11 mg, 0.047 mmol) were added to a Pyrex tube. The tube was flame-sealed in vacuo and, subsequently, heated at 160 °C in an oven equipped with rocking tray. Over a period of 14 h, the polymerization mixture became increasingly viscous. The tube was removed from the oven and cooled to ambient temperature, at which point the sample was solid, and broken in a nitrogen-filled glovebox. The contents were dissolved in THF (ca. 10 mL) in the glovebox and transferred to a Schlenk flask (250 mL). The solution was concentrated in vacuo and precipitated with MeOH (100 mL) to give a light-yellow solid. This process was repeated two times to give polymer 4a as a light-yellow powder that was in vacuo for 24 h (0.12 g, 6%). 31P{1H} NMR (162 MHz, CDCl3): δ = 4.0 (br s), −7.6 (br s). 1 H NMR (400 MHz, CDCl3): δ = 8.1−6.1 (br m, aryl H), 4.9 (br s, P−CH), 4.6−3.7 (br m, methane CH from oxazoline), 3.0−0.5 (br m, alkyl CH3 and isopropyl CH). 13C{1H} NMR (101 MHz, CDCl3): δ = 163.2, 146.6, 142.6, 138.1, 131.9, 128.4, 127.9, 126.3, 125.8, 72.5, 69.8, 52.1, 32.8, 23.2, 20.9, 19.1, 18.0; GPC-LLS (THF): Mn = 5300 g 3 −1 dm−1 (c = 1.4 mol−1; PDI = 1.2; dn/dc = 0.19; [α]22 D = −19.9° cm g × 10−3 g cm−3, CH2Cl2). Synthesis of Copolymer 5a. Styrene (1.2 g, 12 mmol), phosphaalkene 1a (2.5 g, 5.9 mmol), and VAZO 88 (43 mg, 0.18 mmol) were added to a Pyrex tube. The tube was flame-sealed in vacuo and, subsequently, heated at 160 °C in an oven equipped with rocking tray. Over a period of 14 h, the polymerization mixture became increasingly viscous. The tube was removed from the oven and cooled to ambient temperature, at which point the sample was solid, and broken in a nitrogen-filled glovebox. The contents were dissolved in THF (ca. 10 mL) in the glovebox and transferred to a Schlenk flask (250 mL). The solution was concentrated in vacuo and dissolved in a minimal amount of THF (ca. 5 mL). The THF solution was precipitated with degassed MeOH (50 mL) and collected by centrifugation. After decanting the solvent, the precipitation and centrifugation was repeated two additional times. The resulting solid was dried in vacuo for 24 h to give polymer 5a as an off-white powder (0.54 g, 14%). 31P{1H} NMR (162 MHz, CDCl3): δ = 5.9 (br s), −7.6 (br s). 1H NMR (400 MHz, CDCl3): δ = 8.1−6.0 (br m, aryl H), 4.5− 3.8 (br m, methane CH from oxazoline), 3.6 (br s, P−CH), 2.9−0.5 (br m, styrene CH2/CH, alkyl CH3 and isopropyl CH). 13C{1H} NMR (101 MHz, CDCl3): δ = 163.3, 145.2, 142.4, 138.3, 130.3, 127.9, 127.6, 125.6, 72.5, 69.9, 48.4, 43.8, 40.5, 32.8, 23.2, 20.9, 19.0, 18.0. GPC-LLS (THF): Mn = 4000 g mol−1; PDI = 1.1; dn/dc = 0.20; [α]22 D = −29.3° cm3 g−1 dm−1 (c = 1.1 × 10−3 g cm−3, CH2Cl2). Standard Procedure for Gold Coordination to PMP (4a·AuCl and 5a·AuCl). To a solution of either polymer 4a (20 mg, 0.047 mmol P) or 5a (43 mg, 0.047 mmol P) in CH2Cl2 (2 mL) was added Au(tht)Cl (15 mg, 0.047 mmol), and the reaction was stirred for 1 h. After 1 h the mixture was handled in ambient air and moisture and was precipitated in n-pentane (20 mL). The product was collected by centrifugation and the solvent decanted. The resulting solid was dissolved in CH2Cl2 (1 mL), precipitated in n-pentane (20 mL), and collected by centrifugation. The solvent was decanted, and the residual solid was dried in vacuo for 24 h to give the product as an off-white solid (4a·AuCl: 30 mg, 97%; 5a·AuCl: 36 mg, 67%). 4a·AuCl: 31P{1H} NMR (162 MHz, CDCl3): δ = 41.0, (br minor), 23.5 (br, major). 1H NMR (400 MHz, CDCl3): δ = 8.6−6.3 (br m, aryl H), 5.3 (br s, P−CH), 4.8−3.8 (br m, methane CH from oxazoline), 3.0−0.6 (br m, alkyl CH3 and isopropyl CH). GPC-LLS (THF): Mn = 13 000 g mol−1; PDI = 1.2; dn/dc = 0.086. 5a·AuCl: 31P{1H} NMR (162 MHz, CDCl3): δ = 41.9 (br, major), 24.7 (br, major). 1H NMR (400 MHz, CDCl3): δ = 8.2−6.1 (br m, 13

G

DOI: 10.1021/acs.macromol.6b00667 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules aryl H), 4.5−3.8 (br m, methane CH from oxazoline), 3.6 (br s, P− CH), 2.9−0.6 (br m, styrene CH2/CH, alkyl CH3 and isopropyl CH). GPC-LLS (THF): Mn = 7500 g mol−1; PDI = 1.1; dn/dc = 0.13. X-ray Crystallographic Studies. All single crystals were immersed in oil and mounted on a glass fiber. Data were collected on a Bruker X8 APEX II diffractometer with graphite-monochromated Mo Kα radiation. All structures were solved by direct methods and subsequent Fourier difference techniques. All non-hydrogen atoms were refined anisotropically with hydrogen atoms being included in calculated positions but not refined. All data sets were corrected for absorption effects (SADABS), Lorentz, and polarization effects. All calculations were performed using SHELXL-2014 crystallographic software package from Bruker AXS.37 Absolute configuration was confirmed on the basis of the refined Flack parameter.38 Compound Z1b crystallized with two inequivalent molecules of Z-1b in the asymmetric unit and three disordered t-Bu groups. Additional crystal data and details of data collection and structure refinement are given in the Supporting Information. All crystallographic data has been deposited with the Cambridge Structural Database: 1460770, 1460771.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00667. Structure of 1a (CIF) Structure of 1b (CIF) Figures S1−S24 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.P.G.). *E-mail [email protected] (G.R.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from NSERC in the form of Discovery, Strategic, and Research Tools & Instruments grants to D.P.G. and G.R.D. The authors thank Drs. Paul Xia and Maria Ezhova of the UBC-Chemistry NMR Facility for valuable discussions involving the NMR experiments.

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REFERENCES

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