Article pubs.acs.org/Organometallics
Experimental and Theoretical Study of the Living Polymerization of N-Silylphosphoranimines. Synthesis of New Block Copolyphosphazenes Silvia Suárez Suárez,† David Presa Soto,† Gabino A. Carriedo,*,† Alejandro Presa Soto,*,† and Anne Staubitz*,‡ †
Department of Organic and Inorganic Chemistry, IUQOEM, University of Oviedo, Julián Clavería, 33006 Oviedo, Spain Otto-Diels-Institute for Organic Chemistry, University of Kiel, Otto-Hahn-Platz 3/4, 24118 Kiel, Germany
‡
S Supporting Information *
ABSTRACT: The sequential living polymerization of N-silylphosphoranimines for the synthesis of polyphosphazene-b-polyphosphazene diblock copolymers (PP-b-PP) has been studied both experimentally and theoretically. For the experiments, BrMe2PN−SiMe3, [Cl3PNPCl3][X] (X = PCl6−, Cl−), Cl3PN−SiMe3, ClMe2PN−SiMe3, and [Me3PNPMe2Cl]+ were used as representative model reagents. Density functional theory (DFT) calculations in the gas phase adjusted for solvent effects on ClMe2PN− SiMe3, [Cl3PNPCl3]+, Cl3PN−SiMe3, and ClMe2PN−SiMe3 confirmed the experimental observations. The results have shown the necessity of starting with monoend-capped initiators to avoid the formation of triblock chains. It was also demonstrated that the nature of the nucleophilic N-silylphosphoranimines and the electrophilic cationic end groups of the living polyphosphazenes strongly affects the polymerization reaction, imposing limits to its synthetic potential. Thus, good electron donor N-silylphosphoranimines, i.e. XR2PN−SiMe3, react better with electron-deficient cationic end groups such as N−PCl3+, probably by molecular orbital (MO) control. The results led to the designed synthesis of well-defined PP-b-PP block copolymers with narrow molecular weight distributions of formula [N P(Ph)(Me)]n-b-[NP(OCH2CF3)2]m and [NP(Ph)(Me)]n-b-[NP(O2C12H8)]m, which are excellent candidates for micellation studies.
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INTRODUCTION The presence of elements other than carbon in polymeric chains (inorganic polymers) can generate interesting and useful properties such as flexibility at low temperatures, thermal, radiative, and oxidative stability, flame retardancy, new electrical and optical features, and novel chemical reactivity.1a,b Since the early studies of F. Gordon A. Stone in this area,1c−e the synthesis of those materials remains an important challenge. Within the large variety of monomeric precursors containing main-group elements in the structure, the phosphoranimines R3PNR′, discovered in 1919 by Staudinger and Meyer,2 have contributed to significant advances in this field. The chemistry of these molecules has been the subject of intense study.3 Phosphoranimines undergo a variety of reactions such as 1,2additions,4 cyclodimerizations,5 aza-Wittig,6 and catalytic metathesis7 and have been used as ligands in main-group8 and transition-metal chemistry.9 Moreover, they are important monomeric precursors for a large variety of polyphosphazenes, [NPX2]n, a class of inorganic polymers consisting of alternating phosphorus and nitrogen atoms in the backbone. The first known example, [NPCl2]n (poly(dichlorophosphazene)), was prepared in the late 1890s as a cross-linked, hydrolytically unstable elastomer.10 Since then, various synthetic methods to obtain non-cross-linked [N © 2012 American Chemical Society
PCl2]n (1) have been achieved, on the basis of the ring-opening polymerization (ROP) of hexachlorocyclotriphosphazene [N PCl2]3 (2), either at ca. 200−250 °C (Chart 1, eq 1)11 or at room temperature in the presence of trialkylsilyl carboranes as initiators.12 Non-cross-linked [NPCl2]n (1) proved to be an essential intermediate to synthesize numerous polyphosphazene derivatives [NPX2]n, by macromolecular substitution of the chlorine atoms by oxygen or nitrogen donor nucleophiles (X).13 However, some polyphosphazenes, mainly those bearing alkyl or aryl substituents, are inaccessible by these strategies, because treatment of [NPCl2]n (1) with aryl or alkyl Grignard or lithium reagents leads to decomposition of the polymers. Another route to access this pivotal polymer is the use of phosphoranimines. Thus, heating (N-phosphoryl)trichlorophosphoranimine Cl3PNP(O)Cl2 affords [N PCl2]n (1) at ca. 250 °C by condensation polymerization which proceeds with elimination of P(O)Cl3 (Chart 1, eq 2).14 Furthermore, the introduction of phosphoranimines as monomers allowed the successful preparation of poly(alkyl/ Special Issue: F. Gordon A. Stone Commemorative Issue Received: October 20, 2011 Published: January 24, 2012 2571
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polymers have polydispersities (PDI = 1.1−1.4) much narrower than those resulting from thermal ROP. Moreover, this living polymerization provides access to polyphosphazene block copolymers by sequential monomer addition.19 Since the development of this living polymerization, many different polyphosphazene block copolymers have been obtained.1a,20 Because the preparation of PP-b-PP block copolymers by living polymerization with sequential monomer addition is highly promising to give well-defined materials which are capable of self-assembly, further improvements are urgently required. For example, most of the organo-phosphoranimines (XR2PNSiMe3; X = F, Cl, Br; R = alkyl, aryl, alkoxy) polymerize only partially, even after long reaction times (typically 4−7 days), giving low-molecular-weight (Mn) oligomers and uncontrolled amounts of unreacted monomers. Moreover, when used as a second monomer, only a few units are incorporated into the second block. Optimal reaction conditions (time, temperature, solvent, etc.) have not been well established to date. In fact, reported polymerization rates for some organophosphoranimines (maintaining all the other conditions constant) may vary from 2 days to 2 week,s21 and frequently, the polymers have high PDIs.18b,21 PP-b-PP block copolymers are very interesting candidates for self-assembly studies. Previous work on the self-assembly of diblock copolymers in solution has shown that the presence of crystalline core-forming blocks promotes the formation of nanostructures with very low interfacial curvature morphologies.22 On the other hand, polyphosphazene block copolymers are among the most versatile classes of macromolecules because properties such as hydrophobicity, hydrophilicity, and crystallinity can be easily tuned by changing the block substituents.23 Therefore, they present ideal candidates for micellation studies. However, so far, only a few examples of self-assembly studies, in solution or in the solid state, involving polyphosphazene block copolymers have been reported.24 Herein we wish to report an experimental and theoretical study aimed at a better understanding of the sequential Nsilylphosphoranime polymerization for the synthesis of PP-b-PP block copolymers. The results lead to the designed synthesis of well-defined PP-b-PP block copolymers of the formula [N P(Ph)(Me)]n-b-[NP(OCH2CF3)2]m (15b1, n = 20, m = 30; 15b2, n = 20, m = 100; 15b3, n = 60, m = 200) and [N P(Ph)(Me)]n-b-[NP(O2C12H8)]m (16b1, n = 35, m = 50; 16b2, n = 60, m = 245; 16b3, n = 70, m = 20), which bear crystalline core-forming blocks ([NP(OCH2CF3)2] or [N P(O2C12H8)]). According to preliminary work, in progress in our laboratory, those materials are very promising for selfassembly studies.
Chart 1. Different Routes to Polyphosphazenes
aryl)phosphazenes for the first time by thermal (100−180 °C) condensation of (CF3CH2O)R2PNSiMe3 (Chart 1, eq 3), thus considerably widening the range of accessible polyphosphazenes.15 Matyjaszewski and co-workers described an anionic polymerization of (CF3CH2O)3PNSiMe3 promoted by fluoride ions through fluoride attack on the silicon center with elimination of F−SiMe3 and formation of the reactive anionic intermediate (CF3CH2O)3PN− (Chart 1, eq 4).16 However, apart from the room temperature synthesis of poly(alkyl/aryl)phosphazenes using organic phosphites as initiators,17 most of these methods require elevated temperatures and give low yields of polymers with broad molecular weight distributions. Arguably the most useful route to the controlled synthesis of [Cl2PN]n (1) involves the ambient temperature living cationic polymerization of trichloro-N-trimethylsilylphosphoranimine, Cl3PNSiMe3 (3) (Scheme 1). The polymerization of 3, initiated by PCl5, gives [Cl3PN−(Cl2PN)n−PCl3][PCl6] (1[PCl6]) with molecular weights easily controlled by altering the ratio of monomer to initiator (3 and PCl5).18 The
Scheme 1. Living Cationic Polymerization of Cl3PN−SiMe3 (3) Initiated by PCl5
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Scheme 2. Reaction of [Cl3PNPCl3][Cl] (4[Cl]) with 1 equiv of BrMe2PNSiMe3 (5) in CH2Cl2 at Room Temperaturea
a
Also, some insoluble unreacted initiator 4[Cl] was observed.
Figure 1. 31P{1H} NMR spectrum of the reaction of 4[Cl] with 1 equiv (a) and 2 equiv (b) of 5. The assignment of the signals of each compound was performed by comparison with similar compounds found in the literature.18b,c,19a,21,25
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RESULTS AND DISCUSSION
even more pronounced when two chemically different terminal sites are present in the chains, affecting the exact nature of the products. In order to determine the differential reactivity of various phosphazene terminal groups, we decided to investigate the reaction of the active N-silylphosphoranimine BrMe2P NSiMe3 (5), with [Cl3PNPCl3][X] (4[X]) (X = PCl6−, Cl−).26 The initial reaction would be expected to be the formation of the linear trimer [Cl3PN−Cl2PN−PMe2Br][X] (6[X]) (X = PCl6−, Cl−; Scheme 2), having two different active ends. When 1 equiv of the N-silylphosphoranimine 5 was added to a stirred suspension of 4[Cl] in CH2Cl2 at room temperature, a
1. Nature of the Copolymers Formed in the Sequential Monomer Additions. The general methodology to synthesize PP-b-PP block copolymers starts with the living cationic polymerization of 3 initiated by PCl5 (Scheme 1) and is followed by the addition of a second N-silylphosphoranimine XR2PNSiMe3 (X = F, Cl, Br). However, recent studies on the polymerization of Cl3PNSiMe3 (3) demonstrated that there are different reactivities of the two propagation sites, depending on the chain lengths of the growing polymer, and that, therefore, sufficiently long chains with two identical end groups will grow bidirectionally.25 The difference in reactivity is 2573
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Scheme 3. Schematic Representation of the Calculated Reaction Modelsa
a
The values of ΔG are given in kcal/mol in the gas phase and in CH2Cl2 (in brackets).
Supporting Information)27 at the B3LYP28 level of theory with standard split-valence polarized 6-31G(d) basis sets on all elements. This computational method has already been successfully employed for very similar species.29 The species [Cl3PN−PCl3+] (4 = ElCl) was used to represent the end group of the living polymer [NPCl2]n (1[PCl6]), and the species [Me3PN−PMe2Cl+] (ElMe) was used as a model for N−PMe2Cl+ chain ends. For the two phosphoranimines and the cationic species, different conformers were modeled and those structures with the lowest Hartree−Fock energies were chosen. As all reactions involve charged species, polarizable continuum model (PCM) calculations were carried out to ascertain that the calculated trends were maintained in the presence of solvents, in this case dichloromethane. The solvent-corrected free energies, ΔGDCM, were slightly less negative, because the extended delocalization of the positive charge within the cationic species makes the solvation effects less pronounced. The thermodynamic parameters (Scheme 3) showed that the addition of the electron-rich monomer ClMe2PN−SiMe3 (9 = NuMe) to the electron-deficient N−PCl3+ acceptor (ElCl) (ΔG = −44.9 kcal/mol) (eq 5) is much more favorable than to the N−PMe2Cl+ (ElMe) center (ΔG = −21.0 kcal/mol, eq 6). It is also evident that the addition of the more electron deficient Cl3PN−SiMe3 (3 = NuCl) is less favorable, but those to the N-PCl3+ acceptor (ElCl) would be preferable (eqs 8 and 7, respectively). The thermodynamic data were supported by analyzing the bonding in the nucleophilic and electrophilic centers of the reagents. The optimized geometries of the nucleophiles NuCl (3) and NuMe (9) (Figures SI 1 and SI 2, respectively; Supporting Information) showed that Cl3PN−SiMe3 (3 = NuCl) had an almost linear geometry (P−N−Si angle = 179.8°) which did not correspond to those of typical imines, which are bent at the nitrogen atom (Figure 2). This linear structure is highly unusual. The closely related phosphoranimines X3P NR with X = Cl, F, H, OH and R = H, F, OH have been predicted to display a bent structure.30 The molecular orbital
clear solution was formed within 30 min (Scheme 2). After 2 h 31 1 P{ H} NMR spectroscopy (Figure 1a) revealed the total consumption of 5 (δ 10.4 ppm) and the formation of [Cl3P N−Cl2PN−PMe2Br][Cl] (6[Cl]) (δ 61.1 ppm, d; 8.8 ppm dd; −12.0 ppm, d; 88% by integration), [BrMe2PN− (Cl2PN)2−PMe2Br][Cl] (7[Cl]) (δ 56.5 ppm, m; −12.1 ppm, m; 7% by integration), and [BrMe2PN−(Cl2PN)2− Me2PN−PMe2Br][Cl] (8[Cl]) (δ 47.7 ppm, m; 23.2 ppm m; 4.09 ppm, m; −2.4 ppm, m; −25.4, m; 5% by integration). Therefore, although the formation of the trimeric species (6[Cl]) is preferred, the appearance of longer oligomers, 7[Cl] and 8[Cl], generated by the bidirectional chain growth, could not be avoided, even when only 1 equiv of 5 was used. This is clear evidence for a bidirectional polymerization even at the first stage of the reaction. When 2 equiv of the phosphoranimine 5 was reacted with 4[Cl] in CH2Cl2 at room temperature for 2 h, 31P{1H} NMR spectroscopy (Figure 1b) revealed the presence of 6[Cl] (1% by integration), 7[Cl] (47% by integration), 8[Cl] (40% by integration), and other signals that might correspond to longer oligomers (2% by integration). Therefore, an excess of the Nsilylphosphoranimine 5 reacted with the two active ends of the initiator 4[Cl]. This showed that, during the propagation step, the end group −PCl3+ (present in the initiator 4[Cl] and in the oligomer 6[Cl]) reacts more quickly with monomer 5 than does the −PMe2Br+ end group. As shown below, these results could be supported by theoretical calculations and led to the conclusion that the sequential monomer addition to living samples of [NPCl2]n (1[PCl6]) (prepared by living cationic polymerization of the Cl3PN−SiMe3 (3) initiated by PCl5) will give triblock copolymers (note that, as shown in Scheme 1, both ends of the polymer chain of 1[PCl6] are living). In order to better understand the factors affecting the sequential addition of phosphoranimines, theoretical calculations of the four possible phosphoranimine-cationic end group reactions shown in Scheme 3 were performed using the Gaussian 09 program (see Experimental Section and 2574
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PMe2Cl+] (ElMe) therefore does not present an easy trajectory of attack by the HOMO of nucleophiles, of either the phosphoranimine Cl3PN−SiMe3 (3 = NuCl) or ClMe2P N−SiMe3 (9 = NuMe). Therefore, the MO analysis explains why the most favorable addition was found between ClMe2P N−SiMe3 (9 = NuMe, the better donor) and [Cl3PN−PCl3+] (4 = ElCl, the better acceptor), followed by the addition of ClMe2PN−SiMe3 (9 = NuMe) to [Me3PN−PMe2Cl+] (ElMe) and of Cl3PN−SiMe3 (3 = NuCl) to [Cl3PN− PCl3+] (4 = ElCl) (see Scheme 3). For a more detailed discussion of the MO, see Supporting Information. As a consequence of these preferential reactivities in the reactions of living noncapped [NPCl2]n (1[PCl6]) with alkylphosphoranimines, the first step would give a trimer with two different end groups (Scheme 3, eq 5) and the second would prefer to react with the N−PCl3+ sites. From this point, the growing of the polymer chain would proceed by the only possible (less favorable) addition to newly formed N−PMe2Cl+ sites (Scheme 3, eq 6), leading to bidirectional growth and formation of triblock copolymers. It should be pointed out, however, that the exact nature of the attacking nucleophile in the mechanism of the living polymerizations of N-silylphosphoranimines is not still known and will require further experimental and theoretical studies, especially to explain the possible role of the chloride anions, which are always present in the reaction mixture. A detailed analysis of these processes will be the subject of a future publication.31 However, the experimental and theoretical studies discussed so far show that the synthesis of well-defined PP-b-PP block copolymers by sequential addition of two or more N-silylphosphoranimine monomers is subjected to some limitations that may be explained by the nature of the nucleophile (N-silylphosphoranimine) and the electrophilic centers (the living end of polymeric chain) involved. If the presence of triblock chains is to be avoided, the PP-b-PP copolymers might be better synthesized from mono-endcapped living samples of [NPCl2]n (1[PCl6]), [R3PN− (Cl2PN)n−PCl3][PCl6] (to avoid bidirectional growth), and good donor N-silylphosphoranimines. On the other hand, the polymerization of Cl3PN−SiMe3 (3 = NuCl) on the living polymer formed with ClMe2PN−SiMe3 (9 = NuMe) would be much less favorable (Scheme 3, eq 8). In fact, all our attempts to prepare [NPMe2]n-b-[NPCl2]m, polymerizing
Figure 2. Structure (left) and the two degenerate HOMOs (right) of Cl3PN−SiMe3 (3 = NuCl).
(MO) analysis showed that the highest occupied molecular orbital (HOMO) was a pair of virtually degenerate orthogonal π orbitals, without strong polarization toward nitrogen. Cl3P N−SiMe3 (3 = NuCl) is therefore a relatively poor nitrogen nucleophile (for a more detailed discussion of the orbital involved and partial charges, see Supporting Information). In ClMe2PN−SiMe3 (9 = NuMe), the P−N−Si angle (154.6°) suggested a hybridization for the N atom of sp1.4, which was much closer to the sp2 hybridization expected for the imine sp2 structure (120°) than in the linear Cl3PN−SiMe3 (3 = NuCl) (Figure 3). ClMe2PN−SiMe3 (9 = NuMe) can therefore be considered a better nucleophile than Cl3PN− SiMe3 (3 = NuCl; for a more in-depth discussion see also Supporting Information). The study of the electrophilic parts of the reaction revealed that, in [Cl3PN−PCl3+] (4 = ElCl), with the acceptor phosphorus center surrounded by highly electron withdrawing chlorine atoms, the P−N−P angle was almost linear (178.5°) and the natural bond orbital (NBO) charge on both phosphorus atoms was +1.573 (see also Supporting Information). In [Me3PN−PMe2Cl+] (ElMe), however, the P−N−P angle was 147.2°, and the NBO charges on the two phosphorus atoms were +1.908 (for the Me3P) and +1.830 (for the chlorine-bonded phosphorus). Because of the angular structure, the nitrogen atom still had a very pronounced lone pair, which was found to donate into the σ* of the P−Cl bond. The symmetry of the LUMO of the acceptor center [Me3PN−
Figure 3. Structure (left) of ClMe2PN−SiMe3 (9 = NuMe) and the HOMO (top right) and HOMO-1 (bottom right). 2575
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Scheme 4. Synthesis of the PP-b-PP Block Copolymers 12b, 15b1−15b3, and 16b1−16b3
Table 1. Polymerization Conditions for the Preparation of Polymers 15a1−15a3 and 16a1−16a3a
15a1 15a2 15a3 16a1 16a2 16a3
[Ph3PNPCl3]+ (11), mmol (mg)
Cl3PNSiMe3 (3), mmol (g)
tpolym, h
ClPhMePN−SiMe3 (14), mmol (g)
tpolym, days
yield, %
0.060 (40) 0.060 (40) 0.060 (40) 0.148 (98) 0.074 (49) 0.074 (49)
1.80 (0.42) 6.00 (1.41) 12.00 (2.81) 3.96 (0.90) 14.80 (3.46) 1.90 (0.43)
15 30 30 15 30 17
1.20 (0.29) 1.20 (0.29) 3.60 (0.88) 3.67 (0.89) 3.76 (0.92) 4.70 (1.16)
3 4 4 4 4 7
48 50 60 56 60 40
a
All reactions were performed in CH2Cl2 for the polymerization of monomer 3 and in toluene for monomer 14. The polymerizations were performed at room temperature.
(with respect to 11[PCl6]) of the second phosphoranimine ClMe2PN−SiMe3 (9) to the remaining portion of monoend-capped 1[PCl6] gave after 72 h at room temperature a polymer of the formula [NP(Me)2]4-b-[NPCl2]102 (12a; Scheme 4), together with the undesired side products [N P(Me)2]3 (34.2 ppm) [NP(Me)2]4 (27.1 ppm), and [N P(Me)2]5 (22.3 ppm) (10% overall with respect to [N P(Me)2 ]n). The monomer incorporation in the block copolymer was found to be less than 10%. After macromolecular substitution with NaOCH2CF3 and several precipitations into water and hexanes (see Experimental Section) the substituted polymer 12b (Scheme 4) was isolated in moderate yield (40%). 31P{1H} NMR and 1H NMR spectroscopy confirmed the formula proposed for 12b, [NP(Me)2]4-b[NP(OCH2CF3)2]102 (Mn = 12.8 × 104 and PDI = 1.3, by GPC). Attempts to perform the polymerization at higher temperatures (35−45 °C), at different concentrations, or with different solvents (CH2Cl2, toluene, no solvent) did not significantly improve the incorporation of the monomer. However, it is known that methylphenylphosphoranimines polymerize more efficiently than the dimethyl derivatives.32 Therefore, we decided to synthesize the PP-b-PP block copolymers [NP(Ph)(Me)]n-b-[NPCl2]m (15a1−15a3 and 16a1−16a3; Scheme 4) by the reaction of mono-endcapped living samples of [NPCl2]n (1[PCl6]) with the phosphoranimine Cl(Me)(Ph)PN−SiMe3 (14). As expected, the incorporation of the [NP(Ph)(Me)] units was more successful and the resulting polymers could be used as precursors for crystalline-coil PP-b-PP block copolymers by macromolecular substitution of the [NPCl2] block by the nucleophilic substituents (−OCH 2 CF 3 and −OC 6 H 4− C6H4O−).
first ClMe2PN−SiMe3 and adding Cl3PN−SiMe3 (3) for the second block, were unsuccessful. 2. Synthesis of the Crystalline-Coil Block Copolymers [NP(Ph)(Me)]n-b-[NP(OCH2CF3)2]m (15b1, n = 20, m = 30; 15b2, n = 20, m = 100; 15b3, n = 60, m = 200) and [NP(Ph)(Me)]n-b-[NP(O2C12H8)]m (16b1, n = 50, m = 35; 16b2, n = 60, m = 245; 16b3, n = 70, m = 20). Taking into account all considerations above, it seemed clear that a good strategy to prepare the series of copolymers [NPR2]n-b[NPCl2]m was to start from mono-end-capped [NPCl2]n. Thus, we first attempted the preparation of [NP(Me)2]n-b[NP(OCH2CF3)2]m, which had previously been synthesized by reacting ClMe2PN−SiMe3 (9) with living [NPCl2]n (1[PCl6]) without an end cap.19a,21 For this purpose, the mono-end-capped initiator [Ph 3 PNPCl 3 ][PCl 6 ] (11[PCl6]) was formed by reacting Ph3PN−SiMe3 (10) with 2 equiv of PCl5 in CH2Cl2. 31P{1H} NMR spectroscopy revealed the total consumption of 10 (δ 1.5 ppm) and the quantitative formation of 11[PCl6] (δ 28.3 ppm, d; 1.0 ppm, d; −296.8 ppm) after 40 min. The product 11[PCl6] was used in situ without further purification (see Experimental Section). The reaction of 11[PCl6] with 100 equiv of the monomer Cl3PN−SiMe3 (3) was completed in 5 h (sharp signal at −17.2 ppm) to give mono-end-capped [NPCl2]n (1[PCl6]; n = 102), as determined by measuring the relative integration of the signals corresponding to the end groups (Ph3PN, 21.0 ppm; −PCl3+, 7.8 ppm). An aliquot of the reaction mixture was treated with NaOCH2CF3 to produce [NP(OCH2CF3)2]n (13) as a control reaction to give an air- and moisture-stable polymer, which allowed us to measure the molecular weight by conventional gel permeation chromatography (GPC), giving a value of Mn = 12.4 × 104 (PDI = 1.13).19a Addition of 30 equiv 2576
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Table 2. Characterization Details of the Block Copolymers 15b1−15b3 and 16b1−16b3
15b1 15b2 15b3 16b1 16b2 16b3
DP [NPCl2]n, 31P{1H} NMRa
104Mn NP(OCH2CF3)2]n, GPC (PDI)b
104Mn [NP(OR)2]m[NPPhMe]n, GPC (PDI)b
28 89 200 50 246 20
d 26.5 (1.2) 43.9 (1.2) 8.7 (1.2) d 8.6 (1.1)
18.8 (1.2) 44.4 (1.2) 53.9 (1.2) 2.4 (1.3) 6.3 (1.2) 2.6 (1.3)
DP [NP(OR)2]m[NPPhMe]n, 31P{1H} and 1H NMRc m m m m m m
= = = = = =
28/n = 17 89/n = 18 200/n = 85 50/n = 35 245/n = 56 20/n = 70
a The DP was calculated by relative integration of signal of the end groups (−PCl3+ at 8 ppm and Ph3PN− at 20 ppm) and the signal at −17 ppm ([NPCl2]n, 1[PCl6]). bThe GPC values were obtained using THF with 0.1% w/w of (n-Bu)NBr (see Experimental Section). cDetermined by relative 31P{1H} NMR integration of the signals of the block [NPPhMe] (ca. 3 ppm) and the blocks [NP(OCH2CF3)] (ca. −7 ppm) and [NP(O2C12H8)] (ca. −6 ppm). The 1H NMR signals at ca. 2 ppm (CH3) and ca. 4 ppm (OCH2CF3) were relatively integrated with the aromatic broad signal at ca. 7 ppm (Ph and O2C12H12). dWe did not recover enough sample after the macromolecular substitution to measure the GPC value.
Differential scanning calorimetry (DSC) analysis of the series 15b revealed the transitions characteristic of each block. In all the members of the series 15b it was possible to observe the phase transition T1 corresponding to the block [N P(OCH2CF3)2] in the temperature range from 54 to 58 °C, ca. 10 °C lower than the T1 value of the homopolymer [N P(OCH2CF3)2]n (13, T1 = 67 °C).23c This difference may be explained by the presence of the second block [N P(Ph)(Me)]. In copolymer 15b1 it was also possible to detect the Tg value of the [NP(Ph)(Me)] chains at 35 °C (the reported Tg value for the homopolymer [NP(Ph)(Me)]n is 36 °C).34 In the block copolymers 16b only one Tg value was clearly observed at ca. 122 (16b1), 132 (16b2), and 136 °C (16b3) (see Experimental Section and Supporting Information). As the reported Tg value of the homopolymer [N P(O2C12H8)]n is 165 °C,33 we assigned these values of Tg to the [NP(O2C12H8)] block (a complete study of the segregation of these materials in the solid state and in solvent selective to one of the blocks will be the focus of future publications).
Thus, living samples of mono-end-capped [NPCl2]n (1[PCl6]) with chain lengths varying from 30 to 200 repeat units were prepared (using the initiator 11[PCl6] and Cl3P N−SiMe3 (3); see Experimental Section) and used for the formation of the diblock copolymers. The degree of polymerization (DP) of the corresponding [Ph3PN−(Cl2PN)n− PCl3][PCl6] (mono-end-capped 1[PCl6]) was determined by 31 1 P{ H} NMR spectroscopy by integrating the signal at −17 ppm ([Cl2PN]) and the signals of the end groups (Ph3P N, 21.0 ppm; −PCl3+, 7.8 ppm). As a control experiment, aliquots of all the samples were treated with NaOCH2CF3 to give the polymer [NP(OCH2CF3)2]n (13), which is air and moisture stable and could be analyzed by GPC. Varying amounts of the monomer Cl(Ph)(Me)PN−SiMe3 (14) were added to living samples of mono-end-capped 1[PCl6] in toluene at room temperature (Table 1). The polymerizations were monitored by 31P{1H} NMR spectroscopy, showing in all cases the total consumption of the monomer. The resulting block copolymers [NP(Ph)(Me)]n-b-[N PCl2]m (15a1−15a3 and 16a1−16a3; Scheme 4) were treated with NaOCH2CF3 or a mixture of 1,1′-biphenyl-2,2′-diol (biphenol, C12H10(OH)2) and K2CO333 to afford the desired PP-b-PP diblock copolymers [NP(Ph)(Me)]n-b-[NP(OCH2CF3)2]m (see Scheme 4: 15b1, n = 20, m = 30; 15b2, n = 20, m = 100; 15b3, n = 60, m = 200) and [N P(Ph)(Me)]n-b-[NP(O2C12H8)]m (16b1, n = 35, m = 50; 16b2, n = 60, m = 245; 16b3, n = 70, m = 20), which could be isolated in good yields (Table 1). As expected, the molecular weight distributions were reasonably narrow (Table 2). The low PDIs values obtained (ca. 1.2) even for large values of the DP (degree of polymerization; 15b3, m = 200/n = 85) were a consequence of using mono-end-capped initiators (11[PCl6]) that precluded the possibility of bidirectional chain growth. All the spectroscopic data of the block copolymers 15b1− 15b3 and 16b1−16b3 (see Experimental Section) confirmed the proposed molecular formulas. The 31P{1H} NMR spectroscopy showed no signals corresponding to end groups. As shown by thermogravimetric analysis (TGA) at a rate of 10 °C/min, the block copolymers 15b1−15b3 were thermally stable until ca. 250 °C, losing ca. 98% of their initial mass in a single step centered at 350 °C. The polymers 16b1−16b3 were more stable, losing ca. 65% of the mass in a single step centered at 480 °C. As expected by the presence of the block [N P(O2C12H8)],33 the ceramic yield at 800 °C is higher for the copolymers 16b (ca. 40−50%; see Experimental Section and Supporting Information).
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CONCLUSIONS Experimental studies with model compounds on the synthesis of PP-b-PP diblock copolymers by sequential living polymerizations of two different N-silylphosphoranimines indicate that the best synthetic strategy starts with mono-end-capped initiators to avoid bidirectional growth. The previous methodologies using polymers with two active propagating chain ends give materials with a fraction of triblock copolymers. The computational studies performed on model systems have explained these findings. They have demonstrated that the nature of the donor nucleophilic phosphoranimines and the electrophilic cationic end groups of the living polyphosphazenes strongly affect the polymerization reaction, imposing limits to its synthetic potential. Thus, good electron donor phosphoranimines, i.e. XR2PN−SiMe3, react better with electron-poor cationic end groups such as N−PCl3+. Contrary to this, the electron-deficient X3PN−SiMe3 species are almost linear along P−N−Si and do not show an imine-like nucleophilic character, rendering them inferior nucleophiles. Likewise, electron-rich cationic end groups, N−PMe2Cl+, do not offer a sufficiently exposed lobe of the LUMO to ensure efficient attack of nucleophiles. Guided by those considerations, the synthesis of two families of narrow distributions of welldefined and pure crystalline-coil PP-b-PP diblock copolymers of formula [NP(Ph)(Me)]n-b-[NP(OCH2CF3)2]m and [N P(Ph)(Me)]n-b-[NP(O2C12H8)]m has been achieved using 2577
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Table 3. Test Calculations for the Evaluation of Different Computational Methods and the Influence of the Basis Seta
method basis ΔE0 ΔH ΔG a
B3LYP/6-31G(d)
B3LYP/6-11G(d,p)
B3LYP/6-11+G(d)
PBEPBE/6-1G(d)
MPW1MPW91/6-31G(d)
MP2/6-31G(d)
−9.9 −10.1 −10.5
−9.4 −9.6 −9.0
−9.7 −9.9 −8.3
−9.8 −10.0 −8.7
−11.4 −11.5 −11.4
−13.1 −12.2 −14.4
All values are in kcal/mol. corrected) ΔE (at 0 K), enthalpies ΔH (at 298.15 K), and free energies ΔG (at 298.15 K). For all species true minima were obtained, as indicated by the absence of negative frequencies. Natural bond orbital (NBO) charges and orbitals38 were included in the analysis. The effect of the solvent, methylene dichloride, was assessed by employing the polarized continuum model (PCM)39 on the B3LYP/631G(d) optimized structures without further geometrical optimization. The free energies in solution were then obtained by correcting the gas phase reaction free energies with those obtained in solution. However, in order to increase our confidence that this level of theory was adequate for our purposes, test calculations on one of the reactions considered (mechanism 2) were performed at higher levels of theory for this functional, 6-311G(d,p) and 6-311+G(d) (Table 3). The results indicated that the difference in the zero-point corrected potential energy and the reaction enthalpy at 25 °C was no more than 0.5 kcal/mol. For the reaction free energy, the difference was maximally 2.2 kcal/mol, which was sufficiently low to conclude that the 6-31G(d) level of theory is appropriate for this study. Two other DFT functionals, PBE040 and MPW1PW91,41 and also the ab initio MP2 functional42 at the 6-31G(d) level were tested with the same reaction in order to ensure that the results were method independent (Table 3). This proved to be the case. The largest difference to B3LYP was, as expected, found using the ab initio MP2 functional, but even here, the reaction free energy was predicted to be only 3.9 kcal lower than the B3LYP result. These results showed that B3LYP at a level of theory of 6-31G(d) is a suitable method to analyze the systems of interest in this paper. 1. Synthesis of Cl(Me)(R)PN−SiMe3 (7, R = Me; 14, R = Ph). First, Me2P−N(SiMe3)2 (17) was prepared as previously described.36 To a solution of 6.40 g (29.0 mmol) of highly pure 17 in CH2Cl2 (50 mL) at −78 °C was added 6.90 g (29.0 mmol) of C2Cl6. The mixture was stirred at −78 °C for 2 and 2.5 h, respectively, at room temperature (31P{1H} NMR spectroscopy showed only the presence of an unique signal at 22.7 ppm corresponding to the desired product 7). After all volatiles had been removed under vacuum, the product was purified by distillation at reduced pressure (5 mbar) and room temperature, to yield 7 as a clear colorless oil (75%). Data for 7 are as follows. 31P{1H} NMR (CDCl3; δ, ppm): 23.7. 1H NMR (CDCl3; δ, ppm): 0.09 (s, 9H, N−Si((CH3)3); 1.95 (d, 2JH−P = 13.82 Hz, 6H, P−CH3). 13C{1H} NMR (CDCl3; δ, ppm): 2.81 (d, 3 JC−P = 5.82 Hz); 26.51 (d, 1JC−P = 84.63 Hz). The phosphoranimine Cl(Me)(Ph)PN−SiMe3 (14) was prepared by the same procedure followed for 7 and was isolated in moderate yield (52%) by distillation at reduced pressure (2 mbar) at 50 °C. The starting material Me(Ph)P−N(SiMe3)2 (18) was prepared as previously described.34 Data for 14 are as follows. 31P{1H} NMR (CDCl3; δ, ppm): 15.8. 1 H NMR (CDCl3; δ, ppm): 0.11 (s, 9H, N−Si((CH3)3); 2.06 (d, 2JH−P = 13.82 Hz, 6H, P−CH3); 7.39−7.87 (m, 5H, P−C6H5). 13C{1H} NMR (CDCl3; δ, ppm): 3.0 (d, 3JC−P = 5.27 Hz); 26.69 (d, 1JC−P = 84.66 Hz); 128.44 (d, 2JC−P = 14.24 Hz); 130.31 (d, 3JC−P = 11.71 Hz); 131.98 (d, 4JC−P = 2.41 Hz); 136.45 (d, 1JC−P = 122.30 Hz).
the initiator [Ph3PNPCl3][PCl6]. These materials are excellent candidates to micellation studies, as shown by the preliminary results of the first solid-state self-assembly studies now in progress.
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EXPERIMENTAL SECTION
Materials. All solvents used in this work (THF, CH2Cl2, toluene, n-hexane, diethyl ether; Merck) were dried using an appropriate drying agent prior to use. C2Cl6 (Aldrich) and the PCl5 (Merck) were purified by sublimation under reduced pressure and storage under a nitrogen atmosphere. PCl3 (Aldrich), SO2Cl2 (Aldrich), and PhPCl2 (Aldrich) were distilled prior to use. MeLi (1.6 M in diethyl ether), nBuLi (1.6 M in n-hexanes), LiN(SiMe3)2, Br2, Ph3PN−SiMe3 (5), NaH (60% in mineral oil), and CF3CH2OH (all from Aldrich) were used without further purification. The N-silylphosphoranimines Cl3PN−SiMe3 (3)35 and Br(Me2)PN−SiMe3 (6)36 and the initiators [Ph3PNPCl3][X] (X = Cl, PCl6)37 were prepared according to the literature procedures. General Methods. All reactions were carried out under an atmosphere of nitrogen using common Schlenk techniques in an inertatmosphere glovebox (M. Braun). The sequential precipitations of the final block copolymers from concentrated THF solutions to water, alcohol, and n-hexanes were performed in the air. All IR spectra were recorded with a Perkin−Elmer Paragon 1000 spectrometer. Wavenumbers are in cm−1. NMR spectra were recorded at room temperature on Bruker NAV-400, DPX-300, AV-400, and AV-600 instruments. 1H and 13C{1H} NMR spectra are given relative to Si(CH3)4. 31P{1H} NMR spectra are given in relative to external 85% aqueous H3PO4. 19F NMR resonances are given relative to an external reference of CF3COOH. The C, H, N analyses were performed with an Elemental Vario Macro. GPC traces were measured on Perkin− Elmer equipment with a Model LC 250 pump, a Model LC 290 UV, and a Model LC 30 refractive index detector. The samples were eluted with a 0.1% by weight solution of tetra-n-butylammonium bromide in THF through Perkin−Elmer PLGel (Guard, 105, 104, and 103 Å) at 30 °C. Approximate molecular weight calibration was obtained using narrow molecular weight distribution polystyrene standards. Tg values were measured with a Mettler DSC Toledo 822 differential scanning calorimeter equipped with a TA 1100 computer. Thermogravimetric analyses were performed on a Mettler Toledo TG 50 TA 4000 instrument. The polymer samples were heated at a rate of 10 °C/min from ambient temperature to 800 °C under a constant flow of nitrogen. Calculations. Unless otherwise noted, the calculations were carried out at the B3LYP28 level of theory with standard split-valence polarized 6-31G(d) basis sets on all elements. In order to calculate the thermochemical parameters for the reactions discussed, full geometry optimizations were followed by vibrational frequency calculations, which allowed us to obtain the corrections for zero-point energy, internal energy, and free energy (at 298.15 K) in the rigid-rotor harmonic oscillator limit. All energies are presented in kilocalories per mole. We report the potential energy differences (zero-point energy 2578
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Organometallics
Article
2. Preparation of the Initiator [Ph 3PNPCl3][PCl 6] (11[PCl6]). Typical Procedure. To a solution of 1.0 g (4.8 mmol) of PCl5 in 10 mL of CH2Cl2 was added a solution of 0.84 g (2.40 mmol) of Ph3PN−SiMe3 (10) in 10 mL of CH2Cl2 with vigorous stirring. The reaction was monitored by 31P{1H} NMR spectroscopy, and after 2.5 h all volatiles were removed, giving a white solid (31P{1H} NMR δ 28.32, 1.00, and −296.80) isolated in good yield (91%). During the isolation process some hydrolysis always occurred (ca. 5% by integration; 31P{1H} NMR δ 13.80, d, 14.7 Hz and −7.35 d, 14.7 Hz). The product cannot be stored for long periods because it is easily hydrolyzed (even in a glovebox). In Situ Procedure. To a 0.03 g (0.14 mmol) solution of PCl5 in 1 mL of CH2Cl2 was added 0.025 g (0.07 mmol) of a Ph3PN−SiMe3 (10) solution in 1 mL of CH2Cl2 with strong stirring. The reaction was monitored by 31P{1H} NMR spectroscopy, giving quantitative formation of 11[PCl6] after 40 min. At this point the phosphoranimine 3 was added to give mono-end-capped living samples of [N PCl2]n (1[PCl6]). This procedure avoids the formation of the hydrolysis product and was chosen in most of the preparations described in this work. 3. Synthesis of the Block Copolymer [NP(Me)2]4-b-[N P(OCH2CF3)2]102 (12b). In a J. Young tube, to a well-stirred solution of 11[PCl6] (50 mg, 0.08 mmol) in 2 mL of CH2Cl2 (prepared in situ, as was explained before) was added a solution of 3 (1.80 g, 7.50 mmol) in CH2Cl2 (1 mL). The reaction mixture was maintained for 5 h at room temperature with a considerable increase of the viscosity. 31 1 P{ H} NMR spectroscopy revealed the total consumption of 3 (−54 ppm) and the formation of mono-end-capped 1[PCl6] (−17 ppm). At this point, an aliquot of the reaction mixture was treated with NaOCH2CF3 to obtain the polymer [NP(OCH2CF3)2]n (13) for a GPC analysis. All volatiles were removed, and the solution of the mono-end-capped 1[PCl6] was treated with a solution of 0.35 g (2.28 mmol) of 9 in 2 mL of CH2Cl2. The mixture was stirred at room temperature for 72 h. 31P{1H} NMR spectroscopy showed the total consumption of monomer 9 and signals of the formation of side products ([NP(Me)2]3, 34.2 ppm; [NP(Me)2]4, 27.1 ppm; [N P(Me)2]5, 22.3 ppm, overall ca. 10%). The reaction mixture was then treated overnight at room temperature with NaOCH2CF3 to yield the block copolymer 12b, which was purified by sequential precipitations from concentrated solutions of THF into water (×2) and n-hexanes (×2). The polymer 12b was dried for 1 day under reduced pressure at 40 °C (yield 40%). Data for 12b are as follows. 31P{1H} NMR (DMSO-d6; δ, ppm): −7.61 (m, br, NP(OCH2CF3)2; 9.59 (m, br, NPMe2). 1H NMR (DMSO-d6; δ, ppm): 1.41 (m, br, NP(CH3)2); 4.50 (m, br, N P(OCH2CF3)2. 13C{1H} NMR (DMSO-d6; δ, ppm): 29.08 (s, br, P(CH3)2); 63.14 (2JC−F = 84.66 Hz, NP(OCH2CF3)2); 123.1 (3JC−P = 5.52 Hz; 1JC−F = 217.48 Hz, NP(OCH2CF3)2). 19F NMR DMSOd6; δ, ppm): −78.37 (s, br). FT-IR (KBr pellets, cm−1): 2968.2 (νC−H(sp3)); 2362.1; 1423.9.0; 1287.3 (νPO−C); 1170.4 (νPN); 1082.0(νP−OC); 963.5; 888.0; 843.8; 661.8; 564.6; 521.7. GPC: Mn = 12.8 × 104, PDI = 1.3 (12.4 × 104 first block, PDI = 1.13). TGA (10 °C/min under N2): the polymer was stable until 240 °C, lost 98% of its initial mass in a single step centered at 400 °C. DSC (10 °C/min under N2): T1[NP(OCH2CF3)2] = 58.3 °C. Anal. Calcd (found) for C3.92H4.08F5.76NO1.92P (236.32 g/mol); C, 22.03 (19.92); H, 1.75 (1.74); N, 5.97 (5.93). 4. Block Copolymers [NP(Ph)(Me)]n-b-[NP(OCH2CF3)2]m (15b1, n = 20, m = 30; 15b2, n = 20, m = 100; 15b3, n = 60, m = 200). The procedure described corresponds to the synthesis of 15b2. The polymers 15b1 and 15b3 were prepared in a similar manner. In a J. Young tube, to a quickly stirred solution of 11[PCl6] (40 mg, 0.06 mmol) in 2 mL of CH2Cl2 (prepared in situ, see explanation above) was added a solution of 3 (1.44 g, 6.10 mmol) in CH2Cl2 (1 mL). After 30 h at room temperature, 31P{1H} NMR spectroscopy revealed the quantitative formation of mono-end-capped 1[PCl6] (−17 ppm). As a control, an aliquot of the reaction mixture was treated with NaOCH2CF3 to give polymer 13 for a GPC analysis. All the volatiles were removed, and the resulting dry mono-end-capped 1[PCl6] was treated with a solution of 0.30 g (1.22 mmol) of the N-
silylphosphoranimine 14 in 2 mL of toluene. The reaction was allowed to proceed at room temperature until 31P{1H} NMR spectroscopy showed the total consumption of the monomer 14 (96 h). The reaction mixture was treated overnight at room temperature with NaOCH2CF3 to yield the block copolymer 15b2, which was purified by sequential precipitations from concentrated solutions of THF into water (×2) and n-hexanes (×2) and dried for 1 day under reduced pressure at 40 °C. Yield: 48%. Data for 15b1 are as follows. Yield: 50%. 31P{1H} NMR (DMSOd6; δ, ppm): −7.68 (m, br, 28 P, NP(OCH2CF3)2; 4.5 (m, br, 16.8 P, NPMePh); 19.51 (m, br, 1P, end group). 1H NMR (DMSO-d6; δ, ppm): 1.42 (m, br, 3H, NP(CH3)Ph); 4.40 (m, br, 7.1H, N P(OCH2CF3)2; 7.51 (m, br, 5H, NP−C6H5). 13C{1H} NMR (DMSO-d6; δ, ppm): 22.75 (s, br, P(CH3)2); 63.46 (2JC−F = 37.37 Hz, NP(OCH2CF3)2); 118.03; 121.63; 125.35; 130.48 (NP-C6H5); 129.79 (dd, J = 5.52 Hz; J = 216.79 Hz, NP(OCH2CF3)2). 19F NMR (DMSO-d6; δ, ppm): −75.65 (s, br). FT-IR (KBr pellets, cm−1): 3075.3 (νC−H(aromat)); 2968.1 (νC(sp3)−H); 2356.7; 2331.9; 1548.1; 1418.3 (νCC(aromat)); 1287.7 (νPO−C); 1172.4 (νPN); 1089.6 (νP−OC); 963.9; 872.4; 844.3; 806.2; 743.8 (δPNP); 696.8 (δC−H); 661.6; 564.4; 516.4. GPC: Mn = 19.0 × 104, PDI = 1.4. TGA (10 °C/min under N2): the polymer was stable until 280 °C, lost 97% of its initial mass in a single step centered at 400 °C. DSC (10 °C/min under N2): T1[N P(OCH2CF3)2] = 54.3 °C, Tg[NPMePh] = 35 °C. Anal. Calcd (found) for C5.08H5.44F3.84NO1.28P (204.91 g/mol); C, 30.06 (29.78); H, 2.76 (2.68); N, 6.92 (6.84). Data for 15b2 are as follows. Yield: 48%. 31P{1H} NMR (DMSOd6; δ, ppm): −7.66 (m, br, 89 P, NP(OCH2CF3)2; 3.16 (m, br, 18 P, NPMePh); 19.13 (m, br, 1P, end group). 1H NMR (DMSO-d6; δ, ppm): 1.55 (m, br, 3H, NP(CH3)Ph); 4.38 (m, br, 18H, N P(OCH2CF3)2; 7.51 (m, br, 5H, NP−C6H5). 13C{1H} NMR (DMSOd6; δ, ppm): 22.75 (s, br, P(CH3)2); 63.46 (2JC−F = 37.85 Hz, N P(OCH2CF3)2); 128.60; 131.40 (NP−C6H5); 129.59 (dd, J = 9.92 Hz; J = 284.79 Hz, NP(OCH2CF3)2). 19F NMR (DMSO-d6; δ, ppm): −76.18 (s, br). FT-IR (KBr pellets, cm−1): 2971.7 (νC(sp3)−H); 1457.5; 1421.7 (νC=C(aromat)); 1288.9 (νPO−C); 1177.8 (νPN); 1080.6 (νP−OC); 963.3; 872.4; 879.6; 845.2; 744.5 (δPNP); 694.4 (δC−H); 661.2; 560.9; 515.7. GPC: Mn = 44.5 × 104, PDI = 1.2 (26.5 × 104 first block, PDI = 1.2). TGA (10 °C/min under N2): the polymer was stable until 280 °C, lost 98% of its initial mass in a single step centered at 400 °C. DSC (10 °C/min under N2): T1[NP(OCH2CF3)2] = 55.4 °C. Anal. Calcd (found) for C4.54H4.72F4.92NO1.64P (223.97 g/ mol). C, 27.18 (24.34); H, 1.67 (2.12); N, 6.14 (6.25). Data for 15b3 are as follows. Yield: 60%. 31P{1H} NMR (DMSOd6; δ, ppm): −7.61 (m, br, 200 P, NP(OCH2CF3)2; 2.16 (m, br, 85 P, NPMePh); 19.43 (m, br, 1P, end group). 1H NMR (DMSO-d6; δ, ppm): 1.46 (m, br, 3H, NP(CH3)Ph); 4.37 (m, br, 5H, N P(OCH2CF3)2; 7.55 (m, br, 4.9H, NP−C6H5). 13C{1H} NMR (DMSO-d6; δ, ppm): there was not enough resolution to see the [N P(Me)Ph] carbons; 63.46 (2JC−F = 37.85 Hz, NP(OCH2CF3)2); 129.59 (d, J = 2.06 Hz, NP(OCH2CF3)2). 19F NMR (DMSO-d6; δ, ppm): −76.18 (s, br). FT-IR (KBr pellets, cm−1 ): 3056.0 (νC−H(aromat)); 2979.8 (νC(sp3−H)); 1452.8; 1422.0 (νCC(aromat)); 1293.6; 1171.1 (νPN); 1080.2 (νP−OC); 963.1; 876.7; 844.4; 744.1 (δPNP); 694.2 (δC−H); 661.5; 565.4; 512.3. GPC: Mn = 54.0 × 104, PDI = 1.2 (44.0 × 104 first block, PDI = 1.2). TGA (10 °C/min under N2): the polymer was stable until 280 °C, lost 98.5% of its initial mass in a single step centered at 400 °C. DSC (10 °C/min under N2): T1[N P(OCH2CF3)2] = 58.6 °C. Anal. Calcd (found) for C4.9H5.2F4.2NO1.4P (211.27 g/mol); C, 30.06 (27.86); H, 1.83 (2.48); N, 6.69 (6.63). 5. Block Copolymers [NP(Ph)(Me)]n-b-[NP(O2C12H8)2]m (16b1, n = 35, m = 50; 16b2, n = 60, m = 245; 16b3, n = 70, m = 20). The polymers 16b1−16b3 were prepared using the same methodology explained for 15b1−15b3 by changing the conditions employed to the macromolecular substitution of the hydrolytically unstable polymers 16a1−16a3. We describe the macromolecular substitution of 16b2. The block copolymer 16a2 [NP(Ph)(Me)]60-[NPCl2]245 was treated with 1.2 equiv of the 1,1′-biphenyl-2,2′-diol (C12H10O2) per [NPCl2] and 4 equiv of K2CO3 per biphenol in refluxing THF 2579
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Organometallics
Article
overnight. 31P{1H} NMR spectroscopy showed total substitution of the chlorine atoms (the signal at −17 ppm for [NPCl 2 ] n disappeared and a new signal at −6.1 ppm for [NP(C12H10O2)]n appeared). The reaction mixture was then precipitated into water. A white solid was obtained, which was purified by precipitation from concentrated solutions of THF into water, isopropyl alcohol, and nhexanes. The polymer 16b2 was dried for 3 days at 40 °C under reduced pressure. Yield: 52%. Data for 16b1 are as follows. Yield: 43%. 31P{1H} NMR (CDCl3; δ, ppm): −5.68 (m, br, 1.5P, [NP(O2C12H8)]; 3.1 (m, br, 1P, N PMePh); 26.12 (s, 0.09P, [NP(O2C12H8)3]). 1H NMR (CDCl3; δ, ppm): 1.42 (m, br, 1H, NP(CH3)Ph); 7.51 (m, br, 5.6H, NP− C6H5 and NP(O2C12H8)]). 13C{1H} NMR (CDCl3; δ, ppm): 21.71 (s, br, P(CH3)2); 122.85 (s, br); 125.06 (s, br); 127.61 (s, br); 129.25 (m, br); 130.94 (s, br) and 149.60 (m, br) (NP−C6H5 and [N P(O2C12H8)]). FT-IR (KBr pellets, cm−1): 3056.3 (νC−H(aromat)); 2957.4 and 2922.8 (νC−H(sp3)); 1603.1, 1582.7, 1501.0, and 1437.1 (νCC(aromat)); 1246.0 (νC−OP); 1192.3 (νPN); 1095.1 (νP−OC); 1044.8, 1037.5, and 1013.1 (δC−H); 941.2 (δPO−C); 785.4 (δPNP); 750.9 and 716.7 (δC−H); 693.4; 609.2; 590.7; 534.0; 438.9. GPC: Mn = 2.44 × 104, PDI = 1.3 (87.3 × 104 first block, PDI = 1.2). TGA (10 °C/min under N2): the polymer was stable until 350 °C, lost 56.5% of its initial mass in a single step centered at 500 °C. The final ceramic residue was 38.5% of the initial mass. DSC (10 °C/min under N2): a single Tg was observed at 121.6 °C. Anal. Calcd (found) for C9.99H8.00N1.00O1.20P1.00 (192.23 g/mol); C, 62.42 (63.10); H, 4.18 (4.16); N, 7.28 (7.11). Data for 16b2 are as follows. Yield: 52%. 31P{1H} NMR (CDCl3; δ, ppm): −6.70 (m, br, 4.4P, [NP(O2C12H8)]; 1.4 (m, br, 1P, N PMePh); 26.12 (s, 1.8, [NP(O2C12H8)3]). 1H NMR (CDCl3; δ, ppm): 1.42 (m, br, 1H, NP(CH3)Ph); 7.51 (m, br, 13.5H, NP− C6H5 and NP(O2C12H8)]). 13C{1H} NMR (CDCl3; δ, ppm): 21.65 (s, br, P(CH3)2); 122.83 (s, br); 124.97 (s, br); 127.40 (s, br); 128.85 (s, br); 129.22 (s, br); 129.70 (d); 130.70 (s, br) and 148.64 (m, br) (NP−C6H5 and [NP(O2C12H8)]). FT-IR (KBr pellets, cm−1): 3061.1 (νC−H(aromat)); 2955.4 and 2921.2 (νC−H(sp3)); 1603.9, 1582.8, 1500.8, and 1477.6 (νCC(aromat)); 1245.9 (νC−OP); 1192.4 (νPN); 1095.1(νP−OC); 1044.8, 1037.5, and 1013.1 (δC−H); 941.2 (δPO−C); 785.4 (δPNP); 749.9 and 716.2 (δC−H); 693.8; 609.4; 589.7; 536.0; 439.0. GPC: Mn = 6.34 × 104, PDI = 1.2. TGA (10 °C/min under N2): the polymer was stable until 350 °C, lost 58% of its initial mass in a single step centered at 500 °C. The final ceramic residue was 37% of the initial mass. DSC (10 °C/min under N2): a single Tg was observed at 132.1 °C. Anal. Calcd for C11.07H6.52N1.00O1.63P1.00 (210.56 g/mol); C, 63.15 (64.41); H, 3.12 (3.07); N, 6.65 (6.46). Data for 16b3 are as follows. Yield: 40%. 31P{1H} NMR (CDCl3; δ, ppm): −5.68 (m, br, 1P, [NP(O2C12H8)]; 1.01 (m, br, 3.5, N PMePh). 1H NMR (CDCl3; δ, ppm): 1.45 (m, br, 1H, N P(CH3)Ph); 7.51 (m, br, 2.43H, NP−C6H5 and NP(O2C12H8)]). 13 C{1H} NMR(CDCl3; δ, ppm): 21.36 (s, br, P(CH3)2); 122.13 (s, br); 125.67 (s, br); 127.81 (s, br); 129.02 (s, br); 129.07 (s, br); 129.26 (s, br); 129.55 (s, br); 129.97 (s, br), 130.54 (s, br) and 148.97 (m, br) (NP−C6H5 and [NP(O2C12H8)]). FT-IR (KBr pellets, cm−1): 3054.2 (νC−H(aromat)); 2955.4 and 2921.2 (νC−H(sp3)); 1591.7, 1501.3, and 1476.9 (νCC(aromat)); 1297.5 (νC−OP); 1191.1 (νPN); 1095.8 (νP−OC); 1070.4, 1038.2, and 1013.5 (δC−H); 944.32 (δPO−C); 874.7; 786.6 (δPNP); 749.1 and 717.0 (δC−H); 694.1; 608.7; 590.3; 522.4. GPC: Mn = 2.6 × 104, PDI = 1.3. TGA (10 °C/min under N2): the polymer was stable until 300 °C, lost 48.5% of its initial mass in a single step centered at 500 °C. The final ceramic residue was 51.5% of the initial mass. DSC (10 °C/min under N2): a single Tg was observed at 136.3 °C. Anal. Calcd (found) for C8.92H8NPO0.46 (167.40 g/mol); C, 63.94 (64.01); H, 4.78 (4.57); N, 8.36 (8.15).
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copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (G.A.C.);
[email protected] (A.P.S.);
[email protected] (A.S.).
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ACKNOWLEDGMENTS We are grateful to the MICINN for a Juan de la Cierva program (A.P.S.) and Project (CTQ2010-18330) and the European Commission (Marie Curie-ERG program, project UE-10-APGAC-256431). A.S. thanks the Fonds der Chemischen Industrie (FCI) for a Young Researcher’s Fellowship.
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DEDICATION Dedicated to the memory of Prof. F. Gordon A. Stone. REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and tables giving a theoretical discussion and details, TGA and DSC of the block copolymers 15b1−15b3 and 16b1−16b3, and representative GPC traces of the block 2580
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Article
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