
The group transfer polymerization (GTP) of N,N-diethylacrylamide (DEAA) was studied using various combinations of an organic acid of N-(trimethylsilyl)bis-(trifluoromethanesulfonyl)imide (Me 3 SiNTf 2 ), 1-(2,3,4,5,6-pentafluorophenyl)-1,1bis(trifluoromethanesulfonyl)methane (C 6 F 5 CHTf 2 ), and tris(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ) and a silyl ketene acetal (SKA) of 1-methoxy-1-(trimethylsiloxy)-2-methyl-1propene ( Me SKA), 1-methoxy-1-(triethylsiloxy)-2-methyl-1-propene ( Et SKA), 1-methoxy-1-(triisopropylsiloxy)-2-methyl-1-propene ( iPr SKA), and 1-methoxy-1-(triphenylsiloxy)-2methyl-1-propene ( Ph SKA), among which the combination of B(C 6 F 5 ) 3 and Et SKA afforded a relatively better control over the molecular weight distribution. The polymerization behavior of DEAA using B(C 6 F 5 ) 3 and Et SKA was then intensively investigated in terms of the polymerization kinetics, chain extension experiments, and MALDI-TOF MS analyses, by which the polymerization was proven to proceed in a living fashion though an induction period of tens of minutes occurred. The B(C 6 F 5 ) 3 -catalyzed GTP was further extended to various N,Ndisubstituted acrylamides (DAAs), such as N,N-dimethylacrylamide (DMAA), N,N-di-npropylacrylamide (DnPAA), N-acryloylpiperidine (API), N-acryloylmorpholine (NAM), N-(2methoxyethyl)-N-methylacrylamide
Introduction
Polyacrylamides are a sort of important polymer materials for industrial applications, such as flocculants, [1] viscosifiers for oil recovery, and subdermal fillers for aesthetic facial surgery. [2] Although most of the industrial polyacrylamide products have been undoubtedly prepared by conventional radical polymerizations, the precise, efficient, and green synthesis of such polyacrylamides is still an interesting subject from the viewpoint of polymer synthesis. The living polymerization of various acrylamides has been achieved by the living/controlled radical polymerizations (CRPs), such as atom transfer radical polymerization, [3, 4] nitroxide-mediated radical polymerization, [5] and reversible addition-fragmentation chain transfer (RAFT) polymerization, [6] [7] [8] or anionic polymerization. These CRPs are applicable to acrylamide, N-monosubstituted acrylamide, and N,N-disubstituted acrylamide (DAA) except for those bearing radical-sensitive groups, such as N-allyl acrylamide and N-2thioethyl acrylamide. On the other hand, anionic polymerization is only suitable for DAAs without any anionsensitive groups though applicable DAAs are limited to N,Ndimethylacrylamide (DMAA), [9] [10] [11] N,N-diethylacrylamide (DEAA), [10] [11] [12] [13] N,N-di-n-propylacrylamide (DnPAA), [11] Nethylmethylacrylamide (EMA), [11] N-acryloylpyrrolidine (APY), [11] N-acryloylpiperidine (API), [11] N-acryloylmorpholine (NAM), [11] N-acryloyl-2-methylaziridine, [14] Nacryloylazetidine, [15] N-methoxymethyl-N-isopropylacrylamide, [16] N-propyl-N- (3-triisopropoxysilylpropyl) acrylamide, [17] and N-propyl-N-(3-triethoxysilylpropyl)acrylamide. [17] Nevertheless, the anionic polymerization of DAA is still the most reliable method for the precise synthesis of the structurally defect-free polyacrylamides because the CRPs inevitably cause side reactions due to radical transfers. Scheme 1. Syntheses of homo acrylamide polymers, homo acrylamide block copolymers, and hetero block copolymers by organic acid-catalyzed group transfer polymerization method.
Introduction
Hence, we turned our attention to the group transfer polymerization (GTP), one of the anionic polymerization methods, which is principally suitable for acrylic monomers. [18] [19] [20] [21] The living polymer chain-ends capped by silyl groups in the GTP process are much less reactive than the living anions during the anionic polymerization processes, which is one of the reasons why the GTP proceeds at a moderate temperature and is tolerant to functional moieties, such as vinyl and epoxy groups. [22, 23] In particular, the use of organocatalysts for the GTP of methacrylate and acrylate monomers has significantly improved the livingness of the polymerizations leading to well defined polymers with high molecular weights. [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] In addition, organocatalysts significantly improved the controlled/living characteristics for the GTP of acrylamides compared to previous methods those using conventional catalysts; [18, 19, [35] [36] [37] for example, we reported that organic Brønsted acids, such as bis(trifluoromethanesulfonyl)imide (HNTf 2 ) and 1-(2,3,4,5,6pentafluorophenyl)-1,1-bis(trifluoromethanesulfonyl)methane (C 6 F 5 CHTf 2 ), were used for the controlled GTP of DMAA with a silyl ketene aminal of (Z)-1-dimethylamino-1-trimethylsiloxy-1propene ((Z)-DATP) as the initiator to produce poly(N,Ndimethylacrylamide)s with molecular weights up to 53.9 kg mol −1 and polydispersities of less than 1.06, which was the first report to achieve the living polymerization of DAA in the GTP chemistry. [38, 39] Furthermore, Taton and Gnanou reported that Nheterocyclic carbene was used as an efficient organocatalyst for the GTP of DMAA and the block GTP of DMAA and methyl methacrylate to produce tailored polyacrylamides and diblock copolymers. [40, 41] Thus, of great importance is to elucidate the scope and limit for the GTP of DAA in terms of applicable monomers, available organocatalysts, and initiator design. For example, it is interesting to expand the applicable monomers, such as the DAAs with nucleophilic hetero atoms of oxygen, nitrogen, and sulfur, because these atoms would deactivate the true catalytic species of a silylium cation. In addition, the HNTf 2 -catalyzed GTP of DMAA required the use of the silyl ketene aminal though its synthesis turned out to be more difficult than a silyl ketene acetal (SKA), and the GTP with the silyl ketene aminal as the initiator could not be used to synthesize hetero block copolymers because the silyl ketene aminal group of the growing polymer chain-end could not initiate the GTP of the methacrylate and acrylate monomers. Thus, the comprehensive study for the GTP of DAA coupled with available organocatalysts and initiator design is still one of the challenging and remaining tasks in GTP chemistry.
Introduction
We now report the new efficient combinations of organic acids and SKAs for the GTPs of various DAAs, as shown in Scheme 1. This study describes: (1) the evaluation of the combinations of organic acids and SKAs for the GTP of DEAA, (2) the investigation of the livingness and mechanism of the B(C 6 F 5 ) 3 -catalyzed GTP of DAAs, (3) the monomer expansion, and (4) the synthesis of polyacrylamideb-polyacrylamide homo and polymethacrylate-b-polyacrylamide hetero block copolymers.
Introduction
Materials. Dichloromethane (CH 2 Cl 2 , >99.5%; water content, <0.001%), methanol (MeOH), and deuterated chloroform (CDCl 3 , > 99.8%) were purchased from Kanto Chemicals Co., Inc. N,N-Diethylacrylamide (DEAA), 1-methoxy-1-trimethylsiloxy-2-methyl-1-propene ( Me SKA), N-(trimethylsilyl)bis-(trifluoromethanesulfonyl)imide (Me 3 SiNTf 2 ), 1-(2,3,4,5,6pentafluorophenyl)-1,1-bis(trifluoromethane-sulfonyl)methane (C 6 F 5 CHTf 2 ), trans-3-indoleacrylic acid, N,N-dimethyl acrylamide (DMAA), N-acryloylmorpholine (NAM), and methyl methacrylate (MMA) were purchased from Tokyo Kasei Kogyo Co., Ltd. 1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylidenamino]-2Λ 5 ,4Λ 5catenadi(phosphazene) (t-Bu-P 4 , 1.0 mol L −1 in n-hexane) and sodium trifluoroacetate were purchased from the Sigma-Aldrich Chemicals Co. Tris(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ) was purchased from Wako Pure Chemical Industries, Ltd., and was used after recrystallization from n-hexane at −30 °C. DEAA, DMAA, NAM, MMA, and CH 2 Cl 2 were distilled from CaH 2 , degassed by three freeze-pump-thaw cycles, and stored under an Ar atmosphere prior to use. 1-Methoxy-1-(triethylsiloxy)-2methyl-1-propene ( Et SKA), [42] 1-methoxy-1-(triisopropyl siloxy)-2-methyl-1-propene ( iPr SKA), and 1-methoxy-1-(triphenylsiloxy)-2-methyl-1-propene ( Ph SKA) were synthesized according to previous reports. [43] The Spectra/Por® 6 Membrane (MWCO: 1000) was used for the dialysis. All other chemicals were purchased from available suppliers and used without purification.
Introduction
Measurements. The 1 H (400 MHz) and 13 C NMR (100 MHz) spectra were recorded using a JEOL ECS400. The preparation of the polymerization solution was carried out in an MBRAUN stainless steel glove box equipped with a gas purification system (molecular sieves and copper catalyst) and a dry argon atmosphere (H 2 O, O 2 <1 ppm). The moisture and oxygen contents in the glove box were monitored by an MB-MO-SE 1 and MB-OX-SE 1, respectively. Size exclusion chromatography (SEC) in DMF containing lithium chloride (0.01 mol L −1 ) was performed at 40 °C using a Jasco high performance liquid chromatography (HPLC) system (PU-980 Intelligent HPLC pump, CO-965 column oven, RI-930 Intelligent RI detector, and Shodex DEGAS KT-16) equipped with a Shodex Asahipak GF-310 HQ column (linear, 7.6 mm × 300 mm; pore size, 20 nm; bead size, 5 µm; exclusion limit, 4 × 10 4 ) and a Shodex Asahipak GF-7M HQ column (linear, 7.6 mm × 300 mm; pore size, 20 nm; bead size, 9 µm; exclusion limit, 4 × 10 7 ) at the flow rate of 0.6 mL min −1 . The M n,SEC and M w /M n of the obtained polymers were determined by the RI based on poly(methyl methacrylate) (PMMA) with the M w (M w /M n )s of 1.25 × 10 6 g mol −1 (1.07), 6.59 × 10 5 g mol −1 (1.02), 3.003 × 10 5 g mol −1 (1.02), 1.385 × 10 5 g mol −1 (1.05), 6.015 × 10 4 g mol −1 (1.03), 3.053 × 10 4 g mol −1 (1.02), and 1.155 × 10 4 g mol −1 (1.04), 4.90 × 10 3 g mol −1 (1.10), 2.87 × 10 3 g mol −1 (1.06), and 1.43 × 10 3 g mol −1 (1.15), respectively. The matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) measurements were performed using an Applied Biosystems Voyager-DE STR-H mass spectrometer with a 25 kV acceleration voltage. The positive ions were detected in the reflector mode (25 kV). A nitrogen laser (337 nm, 3 ns pulse width, 106-107 W cm −2 ) operating at 3 Hz was used to produce the laser desorption, and the 200 shots were summed. The spectra were externally calibrated using a sample prepared from narrow-dispersed polystyrene (Chemco Scientific Co., Ltd., M n = 3.6 kg mol −1 , M w /M n = 1.08, 30 µL, 10 mg mL −1 in THF), the matrix (1,8-dihydroxy-9-(10H)-anthracenone, 30 mg mL −1 , 100 µL), and the cationizing agent (silver trifluoroacetate, 10 mg mL −1 , 15 µL) with a linear calibration. Samples for the MALDI-TOF MS were prepared by mixing the polymer (1.5 mg mL −1 , 10 µL), the matrix (trans-3-indoleacrylic acid, 10 mg mL −1 , 90 µL), and the cationizing agent (sodium trifluoroacetate, 10 mg mL −1 , 10 µL) in THF.
Polymerization of acrylamide monomers.
A typical procedure for the polymerization is as follows. A stock solution (40.0 µL, 2.00 µmol) of B(C 6 F 5 ) 3 in CH 2 Cl 2 (0.05 mol L −1 ) was added to a solution of DEAA (127 mg, 1.00 mmol) and Et SKA (8.65 mg, 40.0 µmol) in CH 2 Cl 2 (1.81 mL) at room temperature (25 °C) . After 70 min of the polymerization, MeOH was added to the solution to quench the polymerization. The crude product was purified by dialysis against MeOH followed by lyophilization of the resulting polymers from their aqueous solutions. Yield: 80.0 mg (63%); SEC (RI): M n,SEC = 3.36 kg mol −1 , M w /M n = 1.14. In the post polymerization experiments and block copolymerizations, the additional monomer was added to the reaction mixture just after a small portion of the reaction mixture was taken for the determination of the monomer conversion and the M n,SEC and M w /M n of the product in the first polymerization.
Polymerization of acrylamide monomers.
Results and discussion GTP of N,N-diethylacrylamide using various organic acids and silyl ketene acetals. The GTP of N,N-diethylacrylamide (DEAA) was carried out using various organocatalysts and silyl ketene acetals (SKAs), such as Me SKA, Et SKA, iPr SKA, and Ph SKA, in CH 2 Cl 2 at room temperature (~25 °C) to find a suitable combination of a catalyst and an initiator for the controlled/living GTP of DEAA. We employed three types of organocatalysts, such as the Lewis acid of tris(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ) and N-(trimethylsilyl)triflylimide (Me 3 SiNTf 2 ), the Brønsted acid of pentafluorophenylbis(triflyl)methane (C 6 F 5 CHTf 2 ), and the Lewis base of t-Bu-P 4 , which have been previously reported as efficient catalysts for the GTP of (meth)acrylates. [24] The polymerizations of DEAA were carried out under the conditions of [DEAA] 0 = 0.5 mol L −1 and [DEAA] 0 /[Initiator] 0 = 50, and the polymerization results are summarized in Table 1 . The acidic organic catalysts were obviously more effective than the basic catalyst of t-Bu-P 4 , since the molecular weights obtained from the acid-catalyzed GTP were better controlled. Given that the t-Bu-P 4 -catalyzed GTP proceeding through the dissociative mechanism, in which the active species was an enolate anion conjugating with a carbanion, was rather similar to the conventional anionic polymerization, the propagating ends of the enolate anions were so reactive that they readily caused side reactions or termination reactions which led to broad molecular weight distributions (MWDs) or no polymerization. We reported that the GTPs of acrylates, such as methyl acrylate and n-butyl acrylate, using the acidic catalysts smoothly proceeded in a quantitative monomer conversion by optimizing the bulkiness of the silyl group of the employed SKA, in which iPr SKA was found to be the most effective initiator for preparing the corresponding well-defined polymers. [34] In this study, all the acid-catalyzed GTPs of DEAA were completed with quantitative monomer consumption within the set polymerization times except for the B(C 6 F 5 ) 3 -catalyzed polymerization using iPr SKA. The acidcatalyzed polymerizations using Me SKA and Ph SKA (runs 5, 8, 9, 12, 13 and 16) afforded broad MWDs as M w /M n s >1.22 and those using iPr SKA (runs 7 and 11) produced poly(N,Ndiethylacrylamide)s (PDEAAs) with MWDs of 1.19. In contrast, the polymerizations using Et SKA with a moderately bulky silyl group produced the most narrowly-distributed PDEAAs, among which the B(C 6 F 5 ) 3 -catalyzed polymerization afforded the narrowest MWD. The number-averaged molecular weights (M n,SEC s) of these resulting PDEAAs were estimated using size exclusion chromatography based on PMMA standards due to the lack of appropriate PDEAA standards, meaning that the M n,SEC values should be slightly deviated from the exact values. In this study, we mainly used the MWD as the evaluating criterion and thus selected B(C 6 F 5 ) 3 as the catalyst for the following polymerizations due to its operating convenience, unless otherwise instructed.
Polymerization of acrylamide monomers.
Living nature of the B(C 6 F 5 ) 3 -catalyzed GTP of DEAA. The living nature of the B(C 6 F 5 ) 3 -catalyzed GTP of DEAA was assessed by the kinetic studies, the MALDI-TOF MS measurements, and the post-polymerization experiments. The polymerization using Et SKA for the kinetic study was implemented in CH 2 Cl 2 at 25 °C, [DEAA] 0 /[ Et SKA] 0 /[B(C 6 F 5 ) 3 ] 0 = 25/1/0.05, and [DEAA] 0 = 0.5 mol L −1 . Figure 1 shows the zero-and first-order kinetic plots and the M n,SEC and M w /M n dependence on the monomer conversion (Conv.). Surprisingly, the kinetic plots displayed a distinct induction period of ca. 25 min. The further discussion in this regard is described in greater detail in the following section about polymerization mechanism. After the induction period, the polymerization normally proceeded as a propagation reaction till the complete consumption of the monomer in 20 min, during which no retardation of the polymerization was observed, indicating that no termination reaction occurred during the entire propagation process. The polymerization turned out to be zero-order, rather than a first-order reaction. The M n,SEC and M w /M n dependence on the monomer conversion (Conv.) shown in Figure 1 (b) indicated that the M n,SEC of the obtained PDEAA linearly increased from 1.65 to 4.00 kg mol -1 with the increasing monomer conversion, while their M w /M n s were as narrow as <1.17, which indicated the homogeneous growth of the PDEAA chain in the propagation stage. Excluding the induction period, the above polymerization possessed true living characteristics. A MALDI-TOF MS spectrum of the obtained PDEAA in Figure 2 provided a more detailed insight into the chemical structure of the resulting polymer and further into the polymerization reaction. The spectrum showed only one population of molecular ion peaks and the distance between any two neighboring molecular ion peaks was ca. 126.9 Da, which well corresponded to the exact molecular weight of DEAA as the repeating unit. In addition, the m/z value of each molecular ion peak definitely suggested the sodium-cationized polymer composition of [MeO 2 CMe 2 C-DEAA n -H + Na + ] (molecular formula: C 7n+5 H 13n+10 N n O n+2 Na), e.g., the m/z value of 3302.33 Da for a specified peak corresponded to a sodiumcationized 25-mer polymer structure of [MeO 2 CMe 2 C-DEAA 25 -H + Na + ] with the theoretical monoisotopic value of 3302.55 (molecular formula: C 180 H 335 O 27 N 25 Na). This result strongly supported the fact that the polymerization proceeded without any side-reactions. However, the molecular ion peaks had a broad distribution ranging from 1500 to 9000 Da, which is considered to be caused by the lower initiation rate than that of the propagation. The initiating end was also confirmed by the 1 H NMR measurements, i.e., the methoxy protons of the Et SKA residue at the α-terminal of PDEAA was clearly christinahopkins@example.net ppm (peak b) along with those due to the main-chain protons of christinahopkins@example.net (peaks c and f), 2.20-2.86 (peak d), and 2.90-3.62 (peak e) ppm ( Figure S2(a) ). Mechanism of the B(C 6 F 5 ) 3 -catalyzed GTP of DEAA. Given the unique polymerization behavior of the B(C 6 F 5 ) 3 -catalyzed GTP of DEAA using Et SKA, we proposed a plausible mechanism, as shown in Scheme 2. The B(C 6 F 5 ) 3 -catalyzed GTP of DEAA has three elementary reactions, including (1) monomer activation by B(C 6 F 5 ) 3 , (2) initiation reaction, and (3) propagation reaction. For (1), it was reported that an aromatic amide showed an extremely high coordination affinity toward the highly electrophilic B(C 6 F 5 ) 3 and the equilibria lay far toward the formation of coordination adducts (a high equilibrium constant). [44] This trend is also adaptable to the α-unsaturated amide of DEAA in our case. As shown in Figure 4(a) , the proton signals of the equimolar mixture of DEAA and B(C 6 F 5 ) 3 for [DEAA] 0 = [B(C 6 F 5 ) 3 ] 0 = 0.167 mol L −1 showed very clear chemical shifts, which suggests the formation of the coordinating products of DEAA*. In addition, each peak showed an almost complete shift, indicating the equilibrium constant (K eq ) for (1) is rather high though we did not calculate the exact K eq value. The high K eq implies that B(C 6 F 5 ) 3 in the solution mixture almost exists in the DEAA*. It should be noted that the signals due to the vinyl protons of DEAA showed an upfield shift after coordinating with B(C 6 F 5 ) 3 , while those due to the ethyl group showed a downfield shift. This result might indicate that B(C 6 F 5 ) 3 is located in an anti fashion toward the diethylamino group of DEAA because the upfield shift is considered to be caused by the ring current effect of the pentafluorophenyl ring of B(C 6 F 5 ) 3 . The monomer activation was also verified by 13 C NMR measurements, as shown in Figure 4 (b). For (2), the initiation reaction occurs between the Et SKA and DEAA* and produces a silyl ketene aminal from the silyl ketene acetal of Et SKA. The initiation reaction turns out to slowly proceed and shows a distinct induction period. This phenomenon can be firmly attributed to the mismatch of the chemical structures between Et SKA and DEAA because the B(C 6 F 5 ) 3 -catalyzed GTP of DEAA using a silyl ketene aminal does not have such an induction period, as determined in our unpublished study. When comparing the structures of Et SKA and the generated silyl ketene aminal, the Et SKA has an additional methyl group bonded to the ene group, which hinders the initiation reaction; on the other hand, the acetal group in Et SKA has a lower electron-donating ability than that of the aminal group in the silyl ketene aminal, which indicates that the ene group of Et SKA is less nucleophilic toward DEAA* in comparison to that of the silyl ketene aminal. Therefore, these two structural factors of Et SKA result in the induction period and relatively slow initiation reaction. This fact, in turn, suggests that it is of great significance to design the initiator possessing a similar structure with the monomer. For (3), the propagation reaction shows the living characteristics commonly appearing in a living polymerization. It should be emphasized that the propagation reaction of this polymerization system is strongly suggested to be a zero-order reaction, as shown in Figure 1(a) . This can be well explained by the following description. As a premise, the propagation rate (k p ) in each addition reaction is assumed to be a constant. The total monomer consumption rate can then be expressed as
Polymerization of acrylamide monomers.
It is assumed that no side reactions take place during the polymerization process. Thus,
Polymerization of acrylamide monomers.
Considering that the real-time [DEAA] is much higher than [B(C 6 F 5 ) 3 ] and K eq is significantly high, it is thus rational to assume that the [DEAA*] remains constant, the same as [B(C 6 F 5 ) 3 ] 0 , except for the very late time of the entire propagation period. Thus,
Polymerization of acrylamide monomers.
Equation (iv) is derived from equations (i), (ii), and (iii) as Obviously, the propagation is a zero-order reaction. During the propagation period, the monomer consumption is linearly dependent on the polymerization time once the initial conditions are fixed, except for that in the late stage of the propagation period due to the changing [DEAA*] after consumption of most of the monomer, and independent of the monomer concentration, which has also been fully reflected in the following zero-order kinetic plots of the B(C 6 F 5 ) 3 -catalyzed GTPs of other acrylamide monomers ( Figure 5 ).
Scheme 2.
A plausible mechanism for the B(C 6 F 5 ) 3 -catalyzed GTP Table 2 . All the polymerizations homogeneously proceeded and gave quantitative monomer conversions within the set polymerization time. The M n,SEC s estimated by the SEC measurements ( Figure S1 ) of the obtained PDMAA, PDnPAA, PAPI, PNAM, PMMEAA, PBMEAA, PDAlAA, and PMPAA were 6. 35, 14.7, 8.65, 13.5, 8.40, 16.2, 13.5, 8 .44 kg mol −1 when their corresponding [monomer] 0 /[ Et SKA] 0 ratios were 100, 100, 100, 100, 50, 100, 100, 50, respectively. It should be emphasized that it is meaningless to do the comparison between the M n,SEC s and the theoretical molecular weights (M n,calcd. s) because of the inconsistency between the obtained polymers and the PMMA standards. Interestingly, the resulting PDMAA, PDnPAA, and PMMEAA had broad MWDs of 1.22, 1.23, and 1.28, respectively, while PAPI, PNAM, PBMEAA, PDAlAA, and PMPAA had narrow ones of 1.11, 1.15, 1.14, 1.09, and 1.10, respectively. The M w /M n value significantly depended on the used monomer. This dependence should be significantly assigned to the k i /k p difference in the polymerizations of the above monomers, i.e., the polymerizations of API, NAM, BMEAA, DAlAA, and MPAA seem to have good balance between k i and k p , while those of DMAA, DnPAA, and MMEAA have a poorer balance. It is rather understandable that both k i and k p vary when the used monomer is changed, which results in the k i /k p variation. Since it is seemingly impossible to accurately measure the k i s due to the induction period, we did not obtain the detailed k i /k p values and do not undertake a further discussion in this regard. The chemical structures of the obtained polymers were also confirmed by 1 H NMR and MALDI-TOF MS measurements like that for PDEAA, as shown in Figures S2 and S3 .
Scheme 2.
Apart from DEAA, the polymerization kinetics of DMAA, API, BMEAA, and DAlAA were also carried out in CH 2 Cl 2 at room temperature and an argon atmosphere for the same Figure 5 and all the t i s and k p.obs. s are summarized in Table 3 . It is very clear that all the propagation reactions of the polymerizations of DMAA, DEAA, API, BMEAA, and DAlAA are zero-order , the k p s for DMAA, DEAA, API, BMEAA, and DAlAA were calculated to be 640, 1308, 100, 173, 248 L mol −1 min −1 , respectively. API had the longest induction period, DEAA, BMEAA, and DAlAA in the middle, and DMAA the shortest, which well corresponded to the bulkiness order of their substituents at the nitrogen atom. On the other hand, the propagation rates were in the monomer order of DEAA > DMAA > DAlAA > BMEAA > API, indicating that the hindrance of the functional and bulky substituents of the N-allyl, N-2-methoxyethyl, and N-piperidyl groups significantly decreased the propagation rate. Synthesis of acrylamide homo diblock copolymer by the B(C 6 F 5 ) 3 -catalyzed GTP. The living nature of the B(C 6 F 5 ) 3catalyzed GTP of DAA was also used to prepare acrylamide diblock copolymers. DMAA and DAlAA were used for the block copolymerization with DEAA by a sequential monomer addition method. The polymerization results are shown in Table 4 . Twenty-five equivalents of the first monomer were quantitatively polymerized to produce the first-stage polymer product, after which 50 equivalents of the second monomer were added to continue the polymerization, leading to producing the acrylamide diblock copolymers. For all the polymerizations, the monomers were fully consumed in the fixed time no matter during which polymerization stage. The polymerizations of DEAA with DMAA and DAlAA produced the corresponding diblock copolymers of PDEAA-b-PDMAA and PDEAA-b-PDAlAA, respectively. In addition, the same diblock copolymers could also be obtained by changing the addition order of the acrylamide monomers. The resulting PDEAA-b-PDMAAs had relatively broad MWDs, while the PDEAA-b-PDAlAA s had MWDs narrower than 1.17. As already mentioned, PDEAA, PDMAA, and PDAlAA are thermoresponsive, water-soluble, and reactive acrylamide polymers. Therefore, these diblock copolymers are expected to have unique properties. Synthesis of hetero diblock copolymer by the Lewis acidcatalyzed GTP. Given the fact that a SKA initiator has been applicable for the Lewis acid-catalyzed GTPs of both methacrylate and acrylamide monomers, we then attempted the synthesis of hetero-block copolymers composed of methacrylate and acrylamide blocks by the sequential GTP method. Twenty-five equivalents of methyl methacrylate (MMA) were first completely polymerized; afterwards, 50 equivalents of a DAA monomer (DEAA, DAlAA, or NAM) were added to continue the second-stage polymerization. The block copolymerization under the reverse addition of MMA and a DAA monomer did not work because the first polymerization of an acrylamide monomer produces a PDAA bearing a silyl ketene aminal (SKAm) as the living end, which has been known not to initiate the polymerization of a methacrylate monomer. The block copolymerization of MMA and DEAA (run 29, Table 5 ) was first undergone using B(C 6 F 5 ) 3 as the catalyst. However, the second-stage polymerization of DEAA did not proceed at all though the first-stage polymerization of MMA was fairly successful, probably because the large amount of B(C 6 F 5 ) 3 required by GTP of MMA stopped the polymerization of DEAA. To solve this problem, C 6 F 5 CHTf 2 and Me 3 SiNTf 2 with a much higher acidity than B(C 6 F 5 ) 3 were then used. The block copolymerizations (runs 30-35, Table 5 ) afforded quantitative monomer conversions for both the MMA and DAA monomers and perfect molecular weight control of the PMMAs were obtained in the first-stage polymerizations. The initiation efficiencies of the first-stage polymerizations of MMA estimated from the M n,calcd. /M n.SEC s were extremely close to 100%. However, the sequential polymerization of a DAA monomer afforded mixed polymer products as confirmed by the SEC measurements in Figure 6 , of which one trace points to the hetero-block copolymer of PMMA-b-PDAA and the other to the unreacted PMMA.For example, the C 6 F 5 CHTf 2catalyzed block copolymerization of MMA and DEAA using Et SKA (run 30) afforded the mixed polymer products of PMMA-b-PDEAA and PMMA. The M n.SEC of PMMA-b-PDEAA was 47.2 kg mol −1 , which was significantly higher than the M n.calcd. of 8.96 kg mol −1 calculated based on the premise that all the SKA ends of PMMA obtained in the firststage polymerization initiated the second-stage polymerization of DEAA. The mixed polymer products were isolated to obtain the pure PMMA-b-PDEAA and PMMA by preparative SEC in order to estimate the initiation efficiency in the second-stage polymerization and confirm the chemical structure of the unreacted PMMA. Because of the perfect Figure S4 indicated that this unreacted PMMA product was due to the desilylated PMMA obtained after quenching with MeOH. This result strongly suggested that no side reactions occurred during the entire polymerization course of MMA, and supported the fact that a part of the living SKA ends of PMMA did not initiate the second-stage polymerization of DEAA, leading to the eventual low initiation efficiency. This phenomenon can be well explained by the fact that the polymerization rate of DEAA from a SKAm end is significantly greater than the initiation rate, i.e., the reaction rate of the Mukaiyama-Micheal reaction of the SKA end of PMMA with DEAA*. [38] This leads to the fact that the polymerization of DEAA was completed upon the initial SKAm formation from the Mukaiyama-Micheal reaction of only a part of the SKA ends of PMMA with Scheme 3. A schematic diagram for the diblock GTP of MMA and DAA.
Scheme 2.
DEAA*, and that the unreacted living PMMAs were left in the polymer products, as depicted in Scheme 3. The slower initiation rate than the polymerization rate is also the reason why the resulting block copolymers showed relatively broad polydispersities. In contrast, the C 6 F 5 CHTf 2 -catalyzed block copolymerization of MMA and DEAA using the more reactive Me SKA (run 31) showed an improved initiation efficiency of the second-stage polymerization of DEAA (36%) and produced a PMMA-b-PDEAA with a M n.SEC (44.8 kg mol −1 ) lower than that of PMMA-b-PDEAA obtained in run 30 due to the enhanced initiation rate between Me SKA and DEAA*. These polymerization behaviors were also observed for the Me 3 SiNTf 2 -catalyzed block copolymerizations of MMA and DEAA using Et SKA and Me SKA (runs [32] [33] . It is rather clear that in order to obtain a higher initiation efficiency and better polymerization control of the secondstage polymerization is to have the appropriate balance between the initiation (from SKA end of PMMA) and polymerization rates. Considering this, the block copolymerizations using the more reactive Me SKA and less reactive DAA monomers of DAlAA or NAM were further investigated by the Me 3 SiNTf 2 -catalyzed GTP, as shown in runs 34-35. Me SKA was used to enhance the initiation rate, while DAlAA and NAM were used to reduce the polymerization rate. Obviously, the polymerization control over the molecular weight and polydispersity of the final block copolymers was remarkably improved in these two block copolymerizations as the M n.SEC s of 17.0 (PMMA-b-PDAlAA, run 34) and 11.4 (PMMA-b-PNAM, run 35) kg mol −1 were much closer to their corresponding M n.calcd. s of 10.2 and 9.66 kg mol −1 , respectively. Especially when NAM was used as the second monomer, the block copolymerization turned out to be almost perfect ( Figure 6(f) ). In short summary, these results strongly suggest that the perfect block copolymerization of different acrylic monomers can be achieved by carefully selecting the reactivity-matched comonomers. This has been the first finding in this regard in the GTP field.
Conclusions
Various combinations of acidic organocatalysts (Me 3 SiNTf 2 , C 6 F 5 CHTf 2 , and B(C 6 F 5 ) 3 ) and SKAs ( Me SKA, Et SKA, iPr SKA, and Ph SKA) were proven to be significantly efficient for the GTPs of DEAA to produce well-defined PDEAAs, among which the polymerization using B(C 6 F 5 ) 3 and Et SKA showed a relatively better control over the molecular weight distribution. The B(C 6 F 5 ) 3 -catalyzed GTP using Et SKA was applicable to various N,N-disubstituted acrylamides (DAAs) including the water-soluble, thermoresponsive, and reactive ones. All the B(C 6 F 5 ) 3 -catalyzed polymerizations showed an obvious induction period of tens of minutes, after which it proceeded as a zero-order reaction in a living fashion. The occurrence of the induction period was clearly attributed to the structural mismatch between the SKA and DAA. In addition, the initiation and propagation rates of a DAA were significantly dependent on its N-disubstituents and the balance between them was the key factor for well controlling the polymerization, i.e., the GTPs of API, NAM, BMEAA, and DAlAA with relatively bulky substituents were wellcontrolled due to the good balance of the initiation and propagation rates, while those of DMAA, DEAA, DnPAA, and MMEAA with less bulky substituents had worse polymerization control. Acrylamide polymers with an average DP of no more than 200 could be synthesized by the B(C 6 F 5 ) 3catalyzed GTP method. The livingness of the polymerization system was suitable for synthesizing homo block copolymers composed of different acrylamide blocks and hetero block copolymers of PMMA-b-PDAA by the sequential GTP method. Importantly, it has been shown for the first time that a perfect hetero-block copolymer of PMMA-b-PNAM could be synthesized by selecting the reactivity-matched comonomers of MMA and NAM using the combination of the much stronger Lewis acid catalyst, Me 3 SiNTf 2 , and Me SKA. This information is crucially helpful for designing the well-defined hetero-block copolymers, which are currently under investigation. As an expectation, these resulting homo acrylamide polymers, homo block acrylamide copolymers, and hetero block copolymers in this study are applicable for various purposes such as polymer electrolytes, smart polymer materials, and flocculating agents. This study has provided significant progress in the organocatalyzed GTP of DAAs and is expected to help polymer chemists in designing new acrylamide polymers.

{'paper_id': '22781105', 'title': 'Organic acids as efficient catalysts for group transfer polymerization of N,N-disubstituted acrylamide with silyl ketene acetal: polymerization mechanism and synthesis of diblock copolymers', 's2_url': 'https://api.semanticscholar.org/CorpusID:22781105', 'source': 'S2ORC'}