An Addition–Isomerization Mechanism for the Anionic Polymerization

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

An Addition−Isomerization Mechanism for the Anionic Polymerization of MesPCPh2 and m‑XylPCPh2 Benjamin W. Rawe, Andrew M. Priegert, Shuai Wang, Carl Schiller, Sonja Gerke, and Derek P. Gates* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *

ABSTRACT: We report that the anionic polymerization of Pmesityl and m-xylyl-substituted phosphaalkenes follows an unusual addition−isomerization mechanism. Specifically, the polymerization of ArPCPh2 [Ar = Mes (1a), m-Xyl (1b)] involves the hindered nucleophilic anion intermediate, Ⓟ− P(Ar)−CPh2−, which undergoes a proton migration from the oCH3 of the Mes/m-Xyl moiety to the −CPh2 moiety to afford a propagating benzylic anion. This mechanism is supported by the preparation of model compounds MeP(CHPh2)-4,6-Me2C6H2− 2-CH2−CPh3 (2a) or MeP(CHPh2)-6-MeC6H3−2-CH2−CPh3 (2b), which were both crystallographically characterized. Polymerization of 1a or 1b in THF solution using n-BuLi (2 mol %) revealed 1H and 13C NMR signals assigned to −CH2− and −CHPh2 groups consistent with an addition−isomerization polymerization mechanism to afford poly(methylenephosphine) 3a or 3b. A large kinetic isotope effect (≤23) was determined for the n-BuLi-initiated polymerization of 1a-d9 compared to 1a in THF at 50 °C, consistent with C−H (or C−D) activation as the rate-determining step. This C−H activation step was modeled using DFT computations which revealed that the intramolecular proton transfer from the o-CH3 of the Mes moiety to the −CPh2 moiety has an activation energy (Ea = +18.5 kcal mol−1). For comparison, this computational value was quite close to the experimentally measured activation energy of propagation ArPCPh2 in THF [Ea = 14.0 ± 0.9 kcal mol−1 (1a), 15.6 ± 2.8 kcal mol−1 (1b)].

■

INTRODUCTION The addition polymerization of the CC bond of an olefin is perhaps the most important and general method for the synthesis of macromolecules. Although propagation typically occurs as a series of head-to-tail addition reactions, side reactions such as termination, chain transfer, and branching can greatly affect the resultant polymer architecture and properties. Consider the polymerization of the simplest olefin, ethylene, which can undergo irregular isomerization processes during polymerization as a result of C−H bond activation. Thus, depending on the initiator type and polymerization conditions, linear, branched, hyperbranched, and cyclic microstructures are possible for polyethylene (Scheme 1).1 Similar irregular Htransfers resulting in complex microstructures can occur for the anionic polymerizations of some vinyl organosilanes,2−4 α(aminomethyl)acrylates,5 and acrylonitriles.6 In contrast, regioregular isomerizations during propagation are exceedingly rare. Perhaps the most well-established is the anionic polymerization of acrylamides, H2CCHC(O)NHR, to afford

highly regioregular polypeptides, [−CH2CH2C(O)N(R)−]n, rather than the head-to-tail poly(vinylacrylamide), [CH2CH{C(O)NHR}]n.7−13 The field of functional polymers featuring inorganic elements in the main chain has grown dramatically due to their unique properties and potential applications.14−18 As part of a broad program to synthesize phosphorus-containing macromolecules,19 we have been interested in extending addition polymerization to PC bonds. In particular, the close analogy between phosphaalkenes and olefins in molecular chemistry20 has inspired us to develop the addition polymerization of the monomer MesPCPh2 (A, R = Ph) and related phosphaalkenes.21−28 On the basis of NMR spectroscopic investigations, the resultant poly(methylenephosphine)s were originally assigned a microstructure that was consistent with a simple head-to-tail propagation analogous to that observed for polyolefins (i.e., B, x = 0 in Scheme 2). Recently, we discovered that the nitroxide-mediated radical polymerization of A (R = Ph) gives a polymer with a fascinating and unexpected microstructure that is presumably derived from regioregular C−H activation processes (i.e., B, x ≫ y).25 Follow-up studies revealed that this novel addition−isomerization mechanism of propagation for phosphaalkenes might be more general.21−24

Scheme 1. Polymerization of Ethylene To Give Polymers of Many Different Structures through Side Reactions

Received: January 16, 2018 Revised: March 13, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 2. Addition−Isomerization Polymerization of Phosphaalkene A (Mes =2,4,6-Trimethylphenyl) Affords Poly(methylenephosphine) B with a C−H Activated Microstructure

Herein, we report our detailed studies of the highly regioregular addition−isomerization mechanism of anioninitiated polymerization for the phosphaalkenes MesP=CPh2 (1a) and m-XylP=CPh2 (1b). Specifically, we describe experimental and computational investigations of model compounds and polymers derived from the aforementioned monomers to show that the key step involves the unexpected proton transfer from the o-CH3 of the P-aryl moiety to the −CPh2 anion.

Figure 1. 31P NMR (162 MHz, toluene, 298 K) spectra of the reaction mixture during the conversion of 3a from 1a; (a) after addition of MeLi to 1a; (b) after addition of Trt-Cl; (c) after stirring at room temperature for 72 h.

■

RESULTS AND DISCUSSION Model Compounds. We have previously reported the synthesis of several model compounds, prepared by the addition of electrophiles to carbanion MesP(Me)−C(Li)Ph2 (made from the addition of MeLi + 1a).29 In each case, the electrophile (E+) added selectively at the −CPh2 carbanion to afford simple addition products, MesP(Me)−CPh2E [E = H, Me, P(NEt2)2, SiMe3, and SiMe2H]. Although these results provided support for the proposed head-to-tail addition polymerization of MesP=CPh2 (1a), the electrophiles that were used were relatively small compared to monomer 1a. In light of our recent findings, we hypothesized that treating ArP(Me)−C(Li)Ph2 (R = m-Xyl or Mes) with a far bulkier electrophile (e.g., E = CPh3) may disfavor simple addition chemistry and mimic a C−H activation−addition pathway that has been observed for the polymerization of 1a (Scheme 3). In accordance with our previously published procedure,29 MesP(Me)−C(Li)Ph2 was prepared in situ by treating monomer 1a (1 equiv) with MeLi (1 equiv). The progress of reaction was monitored by 31P{1H} NMR spectroscopy, which revealed a major signal assigned to MesP(Me)−C(Li)Ph2 (δ = −47, Figure 1a). Subsequently, the reaction mixture was cooled

to −78 °C, and a toluene solution of Trt-Cl (Trt = triphenylmethyl, 1.02 equiv) was added. The initially red solution immediately turned orange and was accompanied by a white precipitate, presumably LiCl, as the reaction mixture was warmed to room temperature. The 31P NMR spectrum of an aliquot removed from the reaction mixture revealed that the signal assigned to MesP(Me)−C(Li)Ph2 had been consumed and was replaced by two new signals at −18 and −38 ppm (ca. 1:3 ratio; Figure 1b). Neither of these signals was consistent with the protonated species Mes(Me)P−CHPh2 (δ = −24 in CD2Cl2).29 Remarkably, upon standing for 3 days, these intermediate signals were slowly consumed and quantitatively replaced by a singlet resonance at −26 ppm (Figure 1c). Virtually identical observations were made when monitoring the reaction starting with m-xylyl-monomer 1b. In this case, the final product was observed at −24 ppm. The crude products for both reactions were isolated and analyzed by 1H NMR spectroscopy. Importantly, a doublet resonance was detected that was assigned to a ArP−CHPh2 proton [Ar = Mes: δ = 4.73, 2JHP = 4 Hz, 1H; Ar = m-Xyl: δ = 4.67, 2JHP = 4 Hz, 1H]. A similar resonance was observed in the previously prepared model compound Mes(Me)P−CHPh2 [δ = 4.87, 2JHP = 5 Hz] which aided this assignment.29 Importantly, this suggests that the trityl moiety was not attached at the −CPh2 position. Two strongly coupled signals, integrating for 1H each, were also observed in each spectrum. These signals were indicative of diastereotopic Ar−CHaHb−R protons [Ar = Mes: δ = 5.11 and 3.72, 2JHH = 15 Hz; Ar = mXyl: 5.01 and 3.66, 2JHH = 15 Hz)]. In each case, the more downfield resonance of the −CHaHb− moiety was further split by coupling to phosphorus [4JHP = 10 Hz, in both cases] as confirmed by recording the 1H{31P} NMR spectrum. The appearance of resonances corresponding to CHPh2 and Ar− CH2−R protons was consistent with the proposed structures 2a and 2b. This hypothesis was confirmed by an X-ray crystallographic analysis of compounds of 2a and 2b (Figure 2). Remarkably, each molecular structure revealed the presence of a −CH2−

Scheme 3. Synthesis of 2a,b from Phosphaalkenes 1a,ba

Both 2a and 2b feature o-CH2−Trt linkages that are derived from the C−H activation of an o-Me proton from the P−aryl moiety. The simple addition product was not isolated in each case. a

B

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

spectroscopy on a relatively low field spectrometer (300 MHz for 1H NMR).27,28 The present work was performed at higher field (600 MHz for 1H NMR) and incorporated the results of two-dimensional NMR experiments. Polymers 3a and 3b were synthesized by the addition of n-BuLi (2.0 mol %, 1.6 M in hexanes) to a THF solution of the corresponding phosphaalkene (Scheme 4). The progress of each reaction was monitored Scheme 4. Synthesis of Polymers 3a,b from Phosphaalkenes 1a,b

by 31P NMR spectroscopy. In each case, the sharp signal assigned to monomer (1a: δ = 234; 1b: δ = 231) was replaced by a single broad signal at higher field, consistent with the desired polymer (3a: δ = −10; 3b: δ = −11). Each polymer was isolated as a pale-yellow powder after repeated precipitations from concentrated THF solutions into hexanes. The molecular weights of polymers 3a and 3b were determined by gel permeation chromatography (GPC) equipped with a multiangle light scattering detector (Table 1). Analogous to previous reports of the living anionic

Figure 2. Molecular structures of 2a (top) and 2b (bottom). Thermal ellipsoids are shown at the 50% probability level; all hydrogen atoms except H29 are omitted for clarity.

CPh3 linkage resulting from activation of the CH3 moiety of the Mes and m-Xyl substituents. Although the two products crystallized in different space groups (2a: P-1; 2b: P21/n), the metrical parameters are in close agreement with one another. The new C−C bond (C8−C9) lengths are slightly longer [2a: 1.576(6) Å; 2b: 1.587(2) Å] than typical carbon−carbon single bonds (∼1.54 Å),30 indicative of the strain induced by the bulky trityl group. Moreover, the C6−C8−C9 bond angle [2a: 117.2(3)°; 2b: 118.2(1)°] is significantly more obtuse than the predicted sp3 geometry (109.5°), likely a consequence of steric repulsion. Although its structure was not observed experimentally, we were interested in using density functional theory (DFT) to investigate whether the simple addition product from this reaction, MesP(Me)−CPh2Trt, was plausible. A minimized geometry was obtained; however the large substituents around the −CPh2Trt carbon meant that the calculated −CPh2−Trt bond length was found to be 1.707 Å, comparable to the range of calculated bond lengths for the (currently) experimentally elusive compound (Ph3C)2 (1.702−1.791 Å) that has also been computed by DFT previously.31 Strikingly, the energy of the DFT-minimized structure of MesP(Me)−CPh2Trt was 46.0 kcal mol−1 higher in energy than the computed energy of oCH3-activated 2a. This energy difference is rather substantial considering that the dissociation energy of Csp3−Csp3 bonds range from 77.8 to 90.2 kcal mol−1.32 Thus, we conclude that extreme steric hindrance of MesP(Me)−CPh2Trt prevents its formation in favor of the observed TrtCH2(Ar)P(Me)−CPh2H (2a). The following sections will probe whether an analogous mechanism is observed during the anionic polymerization of 1a and 1b. Polymer Microstructure. Given the aforementioned results, we embarked on a detailed spectroscopic examination of the microstructure of polymers 3a and 3b, respectively. We had previously investigated 3a by 31P, 1H, and 13C{1H} NMR

Table 1. Summary of Molecular Weight Data Obtained for the Anionic Polymerization of Phosphaalkenes 1a, 1b, and 1a-d9 entry monomer 1 2 3

1a 1b 1a-d9

[M]:[I]a

Mn calcdb [g mol−1]

Mp measd/Mn measdc [g mol−1]

Đ

50:1 50:1 50:1

15900 15200 16400

16700/13800 12000/8800 9000/10000

1.10 1.17 1.08

a [ArP=CPh2]:[n-BuLi]. bCalculated using the monomer-to-initiator ratio. cAbsolute molecular weights were determined using triple detection GPC. The dn/dc values for 3a (0.239) and 3b (0.245) were determined by direct measurement using a differential refractometer (Figures S13 and S14).

polymerization of 1a, 26,27 a narrow molecular weight distribution was observed, and the calculated and measured values of Mn for 3a were in close agreement (Mn calcd = 15 900 g mol−1; Mn measd = 13 800 g mol−1; Đ = 1.10). In contrast, the measured Mn value obtained for 3b was somewhat lower than expected from the [M]:[I] ratio (Mn calcd = 15 200 g mol−1; Mn measd = 8800 g mol−1). The peak molecular weight value (Mp) measured for polymer 3b was considerably closer to that predicted for given the [M]:[I] ratio of 50:1 used for the polymerization (Mp measd = 12 000 g mol−1). In addition, the dispersity (Đ = 1.17) was slightly higher than are generally accepted for a living polymerization, and tailing at the low MW end of the GPC chromatogram was observed. These observations suggest that some quenching of the propagating anion, perhaps due to trace impurities in the monomer or solvent, may have occurred during the slow polymerization of 1b. Nonetheless, the polymer sample obtained was suitable for NMR spectroscopic analysis. C

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

than predicted from the [M]:[I] ratio (Mn calcd = 16 400 g mol−1; Mn measd = 9000 g mol−1; Đ = 1.08). As expected, the 31P{1H} and 13C{1H} NMR spectra of 3a-d9 closely resembled those obtained for 3a. The 1H NMR spectrum of 3a-d9 lacked the prominent resonances attributable to aliphatic protons (Figure 4c) which are prevalent in the 1H

Recording several 1D and 2D NMR spectra of each polymer in CDCl3 solution helped to elucidate the microstructures of polymers 3a,b. For example, the 1H−13C HSQC NMR spectrum of 3a is shown in Figure 3. Importantly, cross-

Figure 3. 1H−13C HSQC NMR spectrum (600 MHz for 1H, CDCl3, 298 K) of polymer 3a. The ordinate axis shows the 13C APT NMR spectrum, and the abscissa axis shows the 1H NMR spectrum.

correlations were observed that permitted the assignment of signals to aromatic C−H groups and Ar−CH3 groups. In addition, cross-correlations were observed that could only be assigned to the presence of −CHPh2 [ δH = 4.8 (3a), 5.0 (3b); δC = 52 (3a), 52 (3b)] and −CH2− moieties [ δH = 3.6 (3a), 3.5 (3b); δC = 32 (3a), 32 (3b)]. The 13C APT NMR spectrum was also consistent with these assignments. It should be noted that the appearance of the NMR spectra recorded for 3a and 3b closely matched the spectra of polymers previously obtained by the radical initiated polymerization of 1a and 1b.25 To further interrogate the structure of these polymers, we sought to prepare a polymer from phosphaalkene 1a-d9. This monomer had an identical structure to 1a but featured orthoand para-CD3 substituents on the mesityl ring (rather than CH3 in 1a) and was synthesized from mesitylene-d9.33 Our initial attempts to polymerize 1a-d9 at room temperature using 2 mol % of n-BuLi as an anionic initiator (1.6 M in hexanes, reaction solvent = glyme) were unsuccessful. Upon addition of the anionic initiator the reaction mixture turned dark red immediately, and this color persisted for several hours. When an aliquot removed from the reaction mixture was analyzed by 31P{1H} NMR spectroscopy after 3.5 days, no broad resonances indicative of polymer formation were observed. Nevertheless, the dark red color of the reaction suggested that initiation had been successful to form the anionic species, [BuMesP−CPh2]−. In an effort to increase the rate of propagation, the polymerization was repeated, under otherwise identical conditions, at 50 °C, and the reaction progress was monitored by 31P{1H} NMR spectroscopy. After 7 days, the spectrum suggested that 90% of 1a-d9 had been consumed to afford 3a-d9. The reaction was subsequently terminated with methanol (3 drops), and the polymer was isolated as an off-white powder after precipitation from hexanes. The isolated 3a-d9 (yield = 10%) was analyzed by GPC to reveal a narrow dispersity but a lower molecular weight

Figure 4. (a) 1H NMR spectrum (600 MHz, CDCl3, 298 K) of polymer 3a. (b) 1H NMR spectrum (600 MHz, CDCl3, 298 K) of polymer 3b. (c) 1H NMR spectrum (600 MHz, CDCl3, 298 K) of polymer 3a-d9. (d) 2H{1H} NMR spectrum (92 MHz, CHCl3, 298 K) of polymer 3a-d9. ∗ = CDCl3

NMR spectrum for protonated polymers 3a,b (Figures 4a and 4b, respectively). Instead, only resonances corresponding to the aromatic protons are observed for 3a-d9. A 2H{1H} NMR spectrum (Figure 4d) of 3a-d9 was recorded which showed a prominent resonance centered at 2.1 ppm which was assigned to CD3 deuterium atoms. A shoulder, centered at 2.9 ppm, was assigned to Ar−CD2−P moieties. In addition, very broad resonance was observed centered at ca. 4.4 ppm that was assigned to CDPh2 deuterons within the polymer. The integrated ratio of the resonances at 2.1 and 2.9 ppm compared to that at 4.4 ppm was ca. 8:1. Importantly, no signals for aromatic deuterons were observed. Overall, these results suggest that a C−D activation step occurred in the polymerization reaction and that 3a-d9 is best represented by the microstructure shown in Scheme 5 (where x ≫ y). Polymers 3a, 3b, and 3a-d9 likely result from a regioregular addition−isomerization mechanism involving a C− H activation of ortho-methyl protons of the mesitylene group. The following section will investigate the energetics of such a transformation. D

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 5. Anionic Polymerization of the Deuterated Phosphaalkene 1a-d9 To Yield Polymer 3a-d9

Activation Energies (Kinetics and DFT). The polymerization of 1a and 1b could be performed at room temperature, but 1a-d9 would only polymerize at reasonable rates at ca. 50 °C. Thus, it was hypothesized that C−H/D activation was the rate-limiting step in the anionic polymerization of 1a/b. A firstorder rate plot showing the data collected for the anionic polymerization of 1a, 1b, and 1a-d9 at ca. 50 °C is shown in Figure 5. Clearly, the rate of propagation is considerably slower Figure 6. Results of DFT computations, showing computed mechanisms to dimeric structures III and IV. Energies are electronic energies/free energies in kcal mol−1. aOnly electronic energies are provided due to computational expense of frequency calculation on a large system.

kcal mol−1) which suggests that microstructure y of polymer 3 is less probable than microstructure x. Next, a pathway was considered to account for the formation of microstructure x of polymer 3 (Scheme 4). Benzyllic anion II, the postulated intermediate, is only slightly higher in energy than −CPh2 anion I (ΔE° = +9.5 kcal mol−1, ΔG° = +12.1 kcal mol−1) but is significantly less sterically hindered. In contrast to the formation of dimer III from I and 1a, the formation of dimer IV from II and 1a is thermodynamically favorable (ΔE° = −25.3 kcal mol−1). Moreover, there is a strong thermodynamic preference for product IV compared to head-to-tail product III (ΔE° = −20.2 kcal mol−1), and the overall formation of IV from I and 1a is favorable (ΔE° = −15.8 kcal mol−1). On the basis of the aforementioned experimental data and this computational data, we conclude that the addition− isomerization mechanism of propagation for the anionic polymerization of 1a is favored over a simple addition mechanism on both energetic and steric grounds. In other words, the −CPh2 group is simply too sterically encumbered to permit significant head-to-tail enchainment (i.e., I + 1a → III). With the energy of the most plausible intermediates determined, it remained to computationally examine the transition states in the two-step transformation of I to IV. The deuterium labeling study described above suggests that the intramolecular proton transfer step (I → II) would be ratedetermining with a transition state (TS‡I−II). This is consistent with the fact that n-BuLi adds rapidly to 1a to afford I (i.e., low activation energy), and the same would be expected for the addition of the benzylic anion II to 1a. The activation barrier was found to be moderate (TS‡I−II: ΔE° = +21.2 kcal mol−1, ΔG° = +17.3 kcal mol−1, and Ea = +18.5 kcal mol−1). This activation barrier is similar to that determined experimentally for 1a (Ea = +14.0 ± 0.9 kcal mol−1).26 For comparison, kinetic experiments on the polymerization of 1b with n-BuLi (2 mol %) in glyme were performed at five different temperatures. From the Arrhenius plot (Figure S15) the activation energy was determined to be +15.6 ± 2.8 kcal mol−1, which is similar to the barrier obtained for the

Figure 5. Plot showing the linear portion of ln [M]0/[M] vs time plots for the anionic polymerization of 1a,b and 1a-d9 with 2% n-BuLi at ∼50 °C in glyme. Rates of propagation kp (L mol−1 h−1) = 153 ± 20 (1a), 152 ± 23 (1b), 6.6 ± 1.2 (1a-d9).

for the deuterated monomer relative to the protonated monomers. These measurements suggest a kinetic isotope effect, kH/kD, of ≤23, consistent with the postulated addition− isomerization mechanism to form 3a (microstructure x). Given the extremely slow rate of propagation for 1a-d9, the rate constant is likely underestimated given that trace impurities will have a great effect on the measured kp. Therefore, the kinetic isotope effect is likely overestimated. Nonetheless, the data does show that C−H/C−D bond cleavage is likely a major factor in the rate-determining step of polymerization. Additional insight into the reaction mechanism was gained by developing a model for the first propagation step in the polymerization of 1a using DFT (Figure 6). The geometries of all species were optimized using the B3LYP functional and 631+G(d) basis set. Single point energy calculations on the optimized structures were performed using the B97D3 functional and 6-311++G(d,p) basis set, in addition to a solvation correction (in THF, ε = 7.52). Further computational details and coordinates of minimized geometries can be found in the Supporting Information. Two reaction pathways were considered. The simplest was the head-to-tail addition of −CPh2 anion I to monomer 1a to afford dimer III. This mechanism was thermodynamically unfavorable (ΔE° = +4.4 E

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

103 Å (1000−75 000 g mol−1), a Wyatt Optilab T-rEx (refractive index detector, λ = 658 nm, 40 °C), a Wyatt miniDAWN (laser light scattering detector, 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. 1.5 mg mL−1). The dn/dc values for 3a (0.239) and 3b (0.254) were determined by direct measurement using a Wyatt Optilab T-rEx refractive index detector (λ = 658 nm). Hexanes and toluene were deoxygenated with nitrogen and dried by passing through a column containing activated basic alumina. THF and glyme were freshly distilled from sodium/benzophenone ketyl before use. Methanol was degassed prior to use. Benzophenone (Aldrich) was sublimed prior to use. MeLi (1.6 M in diethyl ether) and n-BuLi (1.6 M in hexanes) were purchased from Aldrich, and the exact concentration was determined by titration with N-benzylbenzamide (Aldrich) before use.34 Chlorotrimethylsilane and trityl chloride were purchased from Aldrich and were used as received. 1a35,36 and XylP(SiMe3)237 were prepared according to adapted literature procedures. 1a-d9 was prepared in the same manner, however, starting from mesitylene-d9 which was prepared according to a literature procedure.33 Kinetic studies were performed following a literature procedure.26 X-ray Crystallography. 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 and refined using the riding model. All data sets were corrected for absorption effects (SADABS), Lorentz, and polarization effects. All calculations were performed using the SHELXT crystallographic software package from Bruker AXS.38 Additional crystal data and details of data collection and structure refinement are included in the Supporting Information (Table S1). All crystallographic data have been deposited with the Cambridge Structural Database: 1811958 (2a) and 1058778 (2b). Computational Details. Density functional theory calculations were performed in Gaussian 09 (Revision D.01).39 Initial geometry optimizations were performed using the 6-31+G(d) basis set and the B3LYP functional (denoted as BS1).40,41 Frequency calculations were used to confirm the identity of minima (no negative eigenvalues) and transition states (one negative eigenvalue) using BS1. Single point energy calculations were used to calculate enthalpy for the geometries using the 6-311++G(d,p) basis set on all atoms and the B97D3 functional. Additionally, solvation corrections were implemented using the polarizable continuum model42 using tetrahydrofuran (ε = 7.43) as solvent. Frequency calculations were used to obtain free energies (G°) where possible (systems fewer than 70 atoms). Single point energy calculations were rerun with the wB97XD and CAM-B3LYP43 functionals but showed a marginally higher transition state energy for TS‡I−II. Optimizing the ground state geometries with these functionals using the 6-31+G(d) basis set had little to no change in substrate conformation. Synthetic Details. Preparation of 1b. This compound has been reported previously from the dehydrochlorination of m-XylP(Cl)CHPh2 (m-Xyl = 2,6-dimethylphenyl).44 The present work employed a different procedure. To a stirred solution of m-XylP(SiMe3)2 (6.35 g, 22.5 mmol) in THF (30 mL), MeLi (15 mL, 24 mmol, 1.6 M in Et2O) was added. The resulting yellow solution was heated to 55 °C for 1.5 h. Upon cooling to −78 °C, a solution of benzophenone (4.10 g, 22.5 mmol) in THF (20 mL) was added, and the reaction mixture turned dark red. After stirring for 30 min the reaction mixture was warmed to room temperature. At this time an aliquot was removed from the reaction mixture and analyzed by 31P NMR spectroscopy, which showed a single resonance at 231 ppm. The reaction was stirred for an additional 30 min, over which time it was cooled to −78 °C, and chlorotrimethylsilane (3.7 mL, 29 mmol, ρ = 0.86 g mL−1) was added. Upon warming of the reaction vessel to room temperature the solution turned from a dark red to a yellow color over a period of 30 min. All volatiles were removed in vacuo, and to the resulting oil, hexanes (3 × 30 mL) was added, followed by filtration and solvent removal in vacuo. The crude product was distilled under vacuum and subsequently

polymerization of 1a and to that determined computationally. Furthermore, Eyring plots for the polymerization of 1a and 1b (Figure S16) were used to determine the enthalpy and entropy of activation for the polymerization. High enthalpies of activation were obtained for both polymerizations (1a: ΔH‡ = +13.4 kcal mol−1; 1b: ΔH‡ = +18.8 kcal mol−1), whereas the entropies of activation were close to zero (ΔS‡ = −0.0072 kcal K−1 mol−1; 1b ΔS‡ = +0.0111 kcal K−1 mol−1). These experimentally determined values are well matched with those obtained computationally (ΔH‡ = +17.9 kcal mol−1 and ΔS‡ = +0.0192 kcal K−1 mol−1). Hence, we conclude that the nearzero entropy of activation reflects the fact that the ratedetermining C−H activation step is intramolecular rather than intermolecular. We note that this experimental result is consistent with the proposed addition−isomerization mechanism involving TS‡I−II.

■

SUMMARY We have employed a multifaceted approach to elucidate an unexpected microstructure and a highly unusual addition− isomerization mechanism for the anion-initiated polymerization of phosphaalkenes (ArPCPh2, 1a,b). Specifically, 1- and 2dimensional NMR spectroscopic experiments provided spectroscopic evidence for the presence of −CHPh2 and Ar−CH2− moieties within the resultant polymers 3a and 3b. Their formation requires that the o-CH3 group of the P−Ar substituent be activated and a proton transferred in a highly regioregular fashion during propagation. These data were complemented through the preparation of model compounds and deuterium-labeling experiments that demonstrated a C− H/C−D activation of the o-CH3 moiety is feasible. Kinetic studies of the n-BuLi-initiated polymerization of 1a and 1a-d9 in THF revealed a large kinetic isotope effect (≤23). These measurements confirmed that C−H/D activation of the o−Me group of the P−Ar substituent is the rate-determining step. Moreover, the experimentally determined activation energy for propagation was close to that modeled using DFT computations. Therefore, we conclude that the anionic polymerization of P-Mes and P-Xyl phosphaalkenes (1a and 1b, respectively) occurs via a unique addition−isomerization mechanism of propagation. Future studies will explore the potential fine-tuning of this novel mechanism, perhaps uncovering methods to favor microstructure x or microstructure y, and provide access to unprecedented hybrid functional homo- and copolymers.

■

EXPERIMENTAL SECTION

Materials and Methods. All manipulations of air- and/or watersensitive compounds were performed under a nitrogen atmosphere using standard Schlenk or glovebox techniques. 1H, 31P, and 13C NMR spectra were recorded on Bruker Avance 300, 400, or 600 MHz spectrometers at room temperature unless otherwise specified. Chemical shifts are reported relative to residual CHCl3 (δ = 7.26 for 1H) and C6D5H (δ = 7.16 for 1H); 85% H3PO4 as an external standard (δ = 0.0 for 31P) or CDCl3 (δ = 77.0 for 13C, δ = 7.26 for 2 H). Mass spectra were acquired using a Kratos MS 50 instrument in EI mode (70 eV). Elemental analyses were performed in the University of British Columbia Department of Chemistry Microanalysis Facility. Polymer molecular weights were determined by triple detection gel permeation chromatography (GPC-MALS) using an Agilent liquid chromatograph equipped with an Agilent 1200 series isocratic pump, an Agilent 1200 series standard autosampler, Phenomenex Phenogel 5 μm narrow bore columns (4.6 × 300 mm) 104 Å (5000−500 000 g mol−1), 500 Å (1000−15 000 g mol−1), and F

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Preparation of 3a. Procedure followed an adapted literature procedure.26 To a stirred solution of 1a (5.00 g, 15.8 mmol) in THF (20 mL) at room temperature, n-BuLi (0.21 mL, 0.32 mmol, 1.5 M in hexanes) was added. The conversion of 1a (234 ppm) to 3a (−10 ppm) was monitored by 31P{1H} NMR. Upon complete conversion (after ∼16 h), the reaction mixture was quenched with 3−4 drops of degassed methanol and added dropwise to hexanes (3 × 100 mL) to precipitate the product. After filtration, the product was isolated as a pale-yellow powder. Yield: 3.85 g (77%). GPC-LLS (THF): Mn = 13 800 g mol−1, Đ = 1.10, dn/dc = 0.239. 31P{1H} NMR (162 MHz, CDCl3): δ −10 ppm (br). 1H NMR (600 MHz, CDCl3) δ 7.74−5.94 (br, Ar−H), 5.33−4.27 (br, P−CHPh2), 3.78−2.98 (br, −CH2−), 2.86−1.54 ppm (br, Ar−CH3). 13C{1H} NMR (151 MHz, CDCl3) δ 148.0−146.2 (br), 145.1−141.6 (br), 140.3−136.9 (br), 132.2−127.4 (br), 127.1−125.1 (br), 54.1−49.0 (br, P−CHPh2), 33.8−26.3 (br, −CH2−), 23.6−21.6 (br, Ar−CH3), 21.3−20.0 (br, Ar−CH3). Preparation of 3b. To a stirred solution of 1b (0.273 g, 0.904 mmol) in THF (1.5 mL), n-BuLi (11.5 μL, 18.1 μmol, 1.57 M in hexanes), 16 μL, was added. The conversion of 1b (231 ppm) to 3b (−11 ppm) was monitored by 31P{1H} NMR. Upon complete conversion (after ∼22 h), the reaction mixture was quenched with 3−4 drops of degassed methanol and added dropwise to hexanes (3 × 50 mL) to precipitate the product. After filtration, the product was isolated as a pale yellow powder. Yield: 0.199 g (72%). GPC-LLS (THF): Mn = 8800 g mol−1, Đ = 1.17. dn/dc = 0.254. 31P{1H} NMR (162 MHz, CDCl3): δ − 11 ppm (br). 1H NMR (600 MHz, CDCl3): δ 7.91−6.41 (br, Ar−H), 5.37−4.57 (br, P−CHPh2), 4.11−3.39 (br, −CH2−), 3.22−1.54 (br, Ar−CH3). 13C{1H} NMR (151 MHz, CDCl3): δ 148.8−145.2 (br), 144.9−140.4 (br), 130.9−126.9 (br), 126.5−125.0 (br), 56.1−55.9, 53.0−50.0 (br, P−CHPh2), 36.3−28.5 (br, −CH2−), 25.9−24.1 (br, Ar−CH3), 23.9−21.4 (br, Ar−CH3). Representative Procedure for the Kinetic Studies of Polymerization. All kinetic measurements followed an identical procedure, and a representative example for 1b will be given here. All reactions were setup in a predried vial in the glovebox. To a stirred solution of 1b (0.364 g, 1.21 mmol) in glyme (3.0 mL) was added nBuLi (15.4 μL, 1.57 M in hexanes, 0.0236 mmol). The reaction was stirred for 1 min, and after this time an aliquot of the reaction mixture was transferred to a NMR tube. The sample was loaded into the NMR spectrometer (T = 298.6 K), and a 31P NMR spectrum was recorded every 15 min (72 scans per NMR experiment) until the reaction was deemed complete. In order to permit reliable integration of the obtained 31P NMR spectra, a relaxation time of 3 s and a 30° tip angle were used. When the reaction was deemed complete by NMR spectroscopy, the reaction mixture was removed from the glovebox, and 5 drops of degassed methanol was added to the reaction mixture to terminate the polymerization reaction. The polymer was purified by precipitation of a THF solution into hexanes (3 × 50 mL), yield 30%. The isolated polymers had identical features in their 31P and 1H NMR spectra to that reported above. Each polymerization was repeated at least once more at each temperature to ensure reproducible data. Preparation of 3a-d9. To a stirred solution of 1a-d9 (0.250 g, 0.790 mmol) in glyme (2 mL) at 50 °C, n-BuLi (10 μL, 0.016 mmol, 1.6 M in hexanes) was added. The conversion of 1a-d9 (234 ppm) to 3a-d9 (−11 ppm) was monitored by 31P{1H} NMR over the course of 7 days. Once the reaction appeared to reach maximum conversion (∼90%), the mixture was quenched with 3 drops of degassed MeOH and precipitated with hexanes (3 × 30 mL). After filtration, the product was isolated as a pale yellow powder. Yield: 25 mg (10%). GPC-LLS (THF): Mn = 11 000 g mol−1, Đ = 1.09. 31P{1H} NMR (162 MHz, CDCl3): δ −11 (br). 1H NMR (600 MHz, CDCl3): δ 7.92−6.23 (br, Ar−H). 2H{1H} NMR (92 MHz, CHCl3): δ 6.21−3.68 (br, P−CDPh2), 3.53−0.46 (br, Ar−CD3). 13C{1H} NMR (151 MHz, CDCl3): δ 148.1−146.7 (br), 144.9−141.7 (br), 140.2−136.7 (br), 131.2−129.1 (br), 127.3−126.1 (br), 53.9−49.8 (br, P−CDPh2), 32.6−26.3 (br, −CD2−), 23.4−21.5 (br, Ar−CD3), 21.1−19.7 (br, Ar−CD3).

recrystallized from hexanes to afford a crystalline yellow solid. Yield: 2.86 g (42%). 31P{1H} NMR (162 MHz, CDCl3): δ 231 (s); 1H NMR (400 MHz, CDCl3): δ 7.62−6.91 (m, 13H; Ar−H), 2.36 (s, 6H, Ar− CH3). 13C{1H} NMR (101 MHz, CDCl3): δ 193.8 (d, 1JCP = 43 Hz, PC), 144.8 (d, JCP = 24 Hz), 143.2 (d, JCP = 14 Hz), 140.6 (d, JCP = 7 Hz), 140.0 (d, JCP = 44 Hz), 129.1 (s), 129.0 (s), 128.7 (m), 128.6 (s), 128.4 (s), 127.9 (s), 127.7 (s), 127.6 (s), 127.5 (s), 127.4 (s), 22.5 (d, JCP = 9 Hz, Ar-CH3). Elemental analysis calcd (%) for C21H19P: C 83.44; H 6.29; found: C 83.65, H 6.36. Preparation of 2a. To a stirred solution of 1a (0.25 g, 0.79 mmol) in toluene (3 mL) at −78 °C, MeLi (1.6 M in diethyl ether, 0.50 mL, 0.80 mmol) was added. Upon warming, the solution became a deep red color. 31P{1H} NMR analysis of this solution showed complete consumption of the phosphaalkene starting material indicated by the absence of a resonance corresponding to the starting material (234 ppm). Instead, a broad singlet (−46 ppm) could be observed, assigned to Mes(Me)P−CPh2Li. The reaction mixture was once again cooled to −78 °C, at which time a solution of trityl chloride (0.18 g, 0.81 mmol) in toluene (1 mL) was added. Upon warming, the reaction became cloudy and orange. 31P{1H} NMR analysis of this solution showed an absence of the resonance assigned to Mes(Me)P−CPh2Li and instead two broad singlet resonances (−18 and −38 ppm) in approximately a 1:3 ratio. After 3 days of stirring at room temperature, 31P{1H} NMR analysis showed complete conversion to a new broad singlet resonance (−26 ppm). The reaction mixture was filtered through glass microfiber paper, and all volatiles were removed under vacuum to give the crude product as an orange-red solid. Crude yield: 0.32 g (87% when taking into consideration amounts removed for NMR analysis). In a separate reaction done on a larger scale (1.52 g, 4.8 mmol of MesPCPh2), immediately after the addition of trityl chloride and warming the reaction was filtered, and an aliquot was removed. From this aliquot, after slow evaporation of the toluene, crystals suitable for X-ray analysis were isolated. 31P{1H} NMR (121 MHz, CD2Cl2): δ −26 (s). 1 H NMR (400 MHz, CD2Cl2): δ 7.51 (d, JHH = 7 Hz, 2H, Ar−H), 7.34−7.11 (m, 23H, Ar−H), 6.76 (s, 1H, Ar−H), 6.43 (m, 1H, Ar− H), 5.10 (dd, 2JHH = 15 Hz, 4JHP = 10 Hz, 1H, −CHH−), 4.73 (d, 2JHP = 5 Hz, 1H, P−CHPh2), 3.72 (d, 2JHH = 15 Hz, 1H, −CHH−), 2.56 (s, 3H, o-CH3), 1.90 (s, 3H, p-CH3), 0.89 (d, 3H, 2JHP = 6 Hz, P− CH3). 13C{1H} NMR (101 MHz, CD2Cl2): δ 147.7 (s), 146.7 (s), 143.7 (s), 143.6 (d, JCP = 13 Hz), 143.0 (d, JCP = 12 Hz), 138.1 (s), 132.7 (d, JCP = 24 Hz), 130.9 (s), 130.7 (s), 130.0 (s), 129.7 (d, JCP = 5 Hz), 129.5 (s), 129.4 (s), 129.3 (s), 129.2 (s), 129.2 (s), 129.0 (s), 128.9 (s), 128.9 (s), 128.6 (s), 128.0 (s), 127.0 (d, JCP = 2 Hz), 126.7 (d, JCP = 2 Hz), 126.4 (s), 59.0 (s, CPh3), 52.2 (d, 3JCP = 16 Hz; −CH2−), 44.3 (d, JCP = 34 Hz, P−CHPh2), 23.5 (d, 3JCP = 5 Hz; oCH3), 21.1 (s, p-CH3), 9.7 (d, 3JCP = 10 Hz, P−CH3). MS (ESI-TOF): m/z: 575.5 [M + H]+. Elemental analysis calcd (%) for C42H39P: C, 87.77; H, 6.84. Found: C, 87.52; H, 6.80. Preparation of 2b. Procedure adapted from that for 2a, using phosphaalkene 1b (1.51 g, 5.00 mmol) in toluene (30 mL), MeLi (1.6 M in diethyl ether, 3.4 mL, 5.4 mmol), and trityl chloride (1.62 g, 5.81 mmol) in toluene (5 mL). A notable difference from the preparation of 2a is that immediately after addition of the trityl chloride solution just one intermediate is observed (−36 ppm) when monitoring the reaction progress by 31P{1H} NMR spectroscopy. Yield: 2.21 g (79%). 31 1 P{ H} NMR (162 MHz, CDCl3): δ −24. 1H NMR (400 MHz, CDCl3): δ 7.49 (d, JHH = 8 Hz, 2H, Ar−H), 7.34−7.05 (m, 23H, Ar− H), 6.90 (d, JHH = 8 Hz, 1H, Ar−H), 6.75 (t, JHH = 8 Hz, 1H, Ar−H), 6.61 (m, 1H, Ar−H), 5.01 (dd, 2JHH = 15 Hz, 4JHP = 10 Hz, 1H, −CHH−), 4.67 (d, 2JHP = 3 Hz, 1H, P−CHPh2), 3.66 (d, 2JHH = 15 Hz, 1H, −CHH−), 2.56 (s, 3H, Ar−CH3), 0.90 (d, 2JHP = 6 Hz, 3H; P−CH3). 13C{1H} NMR (101 MHz, CDCl3): δ 147.1 (s), 142.1 (d, JCP = 26 Hz), 143.3 (s), 142.8 (s), 142.7 (s), 142.1 (d, J = 12 Hz), 136.1 (s), 135.9 (s), 130.7 (s), 130.3 (s), 129.6 (s) 129.5 (s), 129.2 (s), 129.1 (s), 128.7 (s), 128.5 (s), 128.5 (s), 128.3 (d, JCP = 6 Hz), 128.1 (s), 127.5 (s), 126.6 (s), 126.5 (s), 126.2 (s), 125.9 (s), 58.6 (s, −CPh3), 51.8 (d, 1JCP = 34 Hz, P−CHPh2), 44.0 (d, 3JCP = 17 Hz, −CH2−), 23.4 (d, 3JCP = 5 Hz, Ar−CH3), 9.7 (d, 1JCP = 21 Hz, P− CH3). MS (EI, 70 eV): m/z: 560.0 [M+]. Elemental analysis calcd (%) for C41H37P: C, 87.86 H, 6.61. Found: C, 87.94; H, 6.75. G

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

isocyanate and acrylamide derivatives. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 465−472. (10) Guaita, M.; Camino, G.; Trossarelli, L. Initiation of the Sodium tert-Butoxide Catalyzed Hydrogen Transfer Polymerization of Nsubstituted Acryamides in Aprotic Solvents: N-Methylacrylamide and N-Phenylacrylamide. Makromol. Chem. 1970, 131, 309−311. (11) Wexler, H. Migrational Polymerization of Methacrylamide. Makromol. Chem. 1968, 115, 262. (12) Yokota, K.; Shimizu, M.; Yamashita, Y.; Ishii, Y. Hydrogen Migration Polymerization of N-Substituted Acrylamides. Makromol. Chem. 1964, 77, 1−6. (13) Breslow, D. S.; Hulse, G. E.; Matlack, A. S. Synthesis of PolyBeta-Alanine from Acrylamide - a Novel Synthesis of Beta-Alanine. J. Am. Chem. Soc. 1957, 79, 3760−3763. (14) Priegert, A. M.; Rawe, B. W.; Serin, S. C.; Gates, D. P. Polymers and the p-block elements. Chem. Soc. Rev. 2016, 45, 922−953. (15) Allcock, H. R. The expanding field of polyphosphazene high polymers. Dalton Trans. 2016, 45, 1856−1862. (16) Carrera, E. I.; Seferos, D. S. Semiconducting Polymers Containing Tellurium: Perspectives Toward Obtaining High-Performance Materials. Macromolecules 2015, 48, 297−308. (17) Jäkle, F.; Fernández, E.; Whiting, A. Recent Advances in the Synthesis and Applications of Organoborane Polymers. Top. Organomet. Chem. 2015, 49, 297−325. (18) Zhou, J.; Whittell, G. R.; Manners, I. Metalloblock Copolymers: New Functional Nanomaterials. Macromolecules 2014, 47, 3529−3543. (19) Bates, J. I.; Dugal-Tessier, J.; Gates, D. P. Phospha-organic chemistry: from molecules to polymers. Dalton Trans. 2010, 39, 3151−3159. (20) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy; Wiley: West Sussex, England, 1998. (21) Rawe, B. W.; Brown, C. M.; MacKinnon, M. R.; Patrick, B. O.; Bodwell, G. J.; Gates, D. P. A C-Pyrenyl Poly(methylenephosphine): Oxidation “Turns On” Blue Photoluminescence in Solution and the Solid State. Organometallics 2017, 36, 2520−2526. (22) Serin, S. C.; Dake, G. R.; Gates, D. P. Addition-Isomerization Polymerization of Chiral Phosphaalkenes: Observation of StyrenePhosphaalkene Linkages in a Random Copolymer. Macromolecules 2016, 49, 4067−4075. (23) Serin, S. C.; Dake, G. R.; Gates, D. P. Phosphaalkene-Oxazoline Copolymers with Styrene as Chiral Ligands for Rhodium(I). Dalton Trans. 2016, 45, 5659−5666. (24) Rawe, B. W.; Chun, C. P.; Gates, D. P. Anionic Polymerisation of Phosphaalkenes Bearing Polyaromatic Chromophores: Phosphine Polymers Showing “Turn-On” Emission Selectively with Peroxide. Chem. Sci. 2014, 5, 4928−4938. (25) Siu, P. W.; Serin, S. C.; Krummenacher, I.; Hey, T. W.; Gates, D. P. Isomerization Polymerization of the Phosphaalkene MesP=CPh2: An Alternative Microstructure for Poly(methylenephosphine)s. Angew. Chem., Int. Ed. 2013, 52, 6967−6970. (26) Noonan, K. J. T.; Gates, D. P. Studying a Slow Polymerization: A Kinetic Investigation of the Living Anionic Polymerization of P=C Bonds. Macromolecules 2008, 41, 1961−1965. (27) Noonan, K. J. T.; Gates, D. P. Ambient-Temperature Living Anionic Polymerization of Phosphaalkenes: Homopolymers and Block Copolymers with Controlled Chain Lengths. Angew. Chem., Int. Ed. 2006, 45, 7271−7274. (28) Tsang, C. W.; Yam, M.; Gates, D. P. The Addition Polymerization of a P=C Bond: A Route to New Phosphine Polymers. J. Am. Chem. Soc. 2003, 125, 1480−1481. (29) Gillon, B. H.; Noonan, K. J. T.; Feldscher, B.; Wissenz, J. M.; Kam, Z. M.; Hsieh, T.; Kingsley, J. J.; Bates, J. I.; Gates, D. P. Molecular Studies of the Initiation and Termination Steps of the Anionic Polymerization of P=C Bonds. Can. J. Chem. 2007, 85, 1045− 1052. (30) Zavitsas, A. A. The Relation Between Bond Lengths and Dissociation Energies of Carbon-Carbon Bonds. J. Phys. Chem. A 2003, 107, 897−898.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00100. NMR spectra of synthesized compounds, Arrhenius and Eyring plots for polymerization of 1a and 1b; X-ray crystallographic parameters for 2a and 2b; coordinates of computational structures; kinetics data for polymerization of 1a and 1b (PDF) Crystal data for 2a (CCDC 1811958) and 2b (CCDC 1058778) (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.P.G). ORCID

Derek P. Gates: 0000-0002-0025-0486 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for support of this work in the form of Discovery and Research Tools grants to D.G. The authors also thank Prof. Pierre Kennepohl for valuable discussions and guidance in performing the DFT calculations, Dr. Paul Xia for help with 2H NMR experiments, and Dr. Spencer Serin for performing X-ray diffraction experiments on crystals of 2a and 2b.

■

REFERENCES

(1) Odian, G. G. Principles of Polymerization, 4th ed.; WileyInterscience: Hoboken, NJ, 2004. (2) Yang, J. X.; Liu, S. C.; Zhu, F. H.; Huang, Y. W.; Li, B.; Zhang, L. New Polymers Derived from 4-Vinylsilylbenzocyclobutene Monomer with Good Thermal Stability, Excellent Film-Forming Property, and Low-Dielectric Constant. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 381−391. (3) Ganicz, T.; Stanczyk, W. A.; Gladkova, N. K.; Sledzinska, I. Vinylsilanes as monomers for side chain polymer liquid crystals. Macromolecules 2000, 33, 289−293. (4) Gan, Y. D.; Prakash, S.; Olah, G. A.; Weber, W. P.; HogenEsch, T. E. Anionic synthesis of narrow molecular weight distribution poly(trimethylvinylsilane) (PTMVS), polystyrene-PTMVS block copolymers, and poly(phenyldimethylvinylsilane). Conversion of poly(phenyldimethylvinylsilane) into poly(fluorodimethylvinylsilane). Macromolecules 1996, 29, 8285−8288. (5) Baraki, H.; Habaue, S.; Okamoto, Y. Stereospecific Anionic Polymerization and Novel Hydrogen-Transfer Polymerization of α(Aminomethyl)acrylates Having Unprotected Amino Group. Polym. J. 1999, 31, 1260−1266. (6) Ono, H.; Hisatani, K.; Kamide, K. NMR Spectroscopic Study of Side Reactions in Anionic Polymerization of Acrylonitrile. Polym. J. 1993, 25, 245−265. (7) Iwamura, T.; Adachi, K.; Chujo, Y. Synthesis of organic-inorganic polymer hybrids utilizing in-situ anionic hydrogen-transfer polymerization of acrylamide. Polymer 2016, 92, 13−17. (8) Iwamura, T.; Tomita, I.; Suzuki, M.; Endo, T. Hydrogen-transfer polymerization behavior of N-acylacrylamide. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 430−435. (9) Iwamura, T.; Tomita, I.; Suzuki, M.; Endo, T. Hydrogen-transfer polymerization of vinyl monomers derived from 4-methylbenzoyl H

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (31) Schreiner, P. R.; Chernish, L. V.; Gunchenko, P. A.; Tikhonchuk, E. Y.; Hausmann, H.; Serafin, M.; Schlecht, S.; Dahl, J. E. P.; Carlson, R. M. K.; Fokin, A. A. Overcoming Lability of Extremely Long Alkane Carbon-Carbon Bonds Through Dispersion Forces. Nature 2011, 477, 308−311. (32) Characteristic Bond Lengths in Free Molecules. CRC Handbook of Chemistry and Physics, 84th ed.; CRC: Boca Raton, FL, 2003. (33) Chen, T.-S.; Wolinska-Mocydlarz, J.; Leitch, L. C. Synthesis of Deuteriomethyl Aromatic Hydrocarbons by Exchange with Dimethylsulfoxide-D6. J. Labelled Compd. 1970, 6, 285−288. (34) Burchat, A. F.; Chong, J. M.; Nielsen, N. Titration of alkyllithiums with a simple reagent to a blue endpoint. J. Organomet. Chem. 1997, 542, 281−283. (35) Becker, G.; Uhl, W.; Wessely, H.-J. Acyl- und Alkylidenphosphane. XVI. (Dimethylaminomethyliden)- und (Diphenylmethyliden)phosphane. Z. Anorg. Allg. Chem. 1981, 479, 41−56. (36) Yam, M.; Chong, J. H.; Tsang, C. W.; Patrick, B. O.; Lam, A. E.; Gates, D. P. Scope and Limitations of the Base-Catalyzed PhosphaPeterson P=C Bond-Forming Reaction. Inorg. Chem. 2006, 45, 5225− 5234. (37) Becker, G.; Mundt, O.; Rossler, M.; Schneider, E. Bildung und Eigenschaften von Acylphosphanen. VI. Synthese von Alkyl- und Arylbis (trimethylsilyl)- sowie Alkyl und Aryltrimethylsilyphosphanen. Z. Anorg. Allg. Chem. 1978, 443, 42−52. (38) Sheldrick, G. SHELXT - Integrated space-group and crystalstructure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (40) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (41) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (42) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094. (43) Yanai, T.; Tew, D. P.; Handy, N. C. A new hybrid exchange− correlation functional using the Coulomb-attenuating method (CAMB3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (44) van der Knaap, T. A.; Klebach, T. C.; Visser, F.; Bickelhaupt, F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M. Synthesis and Structure of Aryl-Substituted Phospha-Alkenes. Tetrahedron 1984, 40, 765−776.

I

DOI: 10.1021/acs.macromol.8b00100 Macromolecules XXXX, XXX, XXX−XXX