Self-Accelerating Click Reaction in Step Polymerization

Article pubs.acs.org/Macromolecules

Self-Accelerating Click Reaction in Step Polymerization Ji-Qiang Chen,† Lue Xiang,†,‡ Xianfeng Liu,†,‡ Xueping Liu,†,‡ and Ke Zhang*,†,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: A self-accelerating double-strain-promoted azide−alkyne cycloaddition reaction (DSPAAC) was used in the step polymerization for the first time. This produced a novel stoichiometric imbalance-promoted homogeneous step polymerization method using sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) and various bis-azide compounds (N3-R-N3) as respective monomers. Because of the self-accelerating property of DSPAAC reaction, the novel step polymerization method could prepare high molecular weight polymers under stoichiometry imbalance condition with excess molar amounts of DIBOD to N3R-N3. Based on the click property of DSPAAC reaction, the novel step polymerization could be efficiently performed in very mild reaction conditions such as in air at room temperature without requiring any metal catalyst or chemical stimulus. Under the present polymerization condition, the preparation of high molecular weight polymers from this novel method was accompanied by the formation of cyclic oligomers with a weight content above 30%. The resultant polymers possessed a unique fluorescence property, which emitted strong fluorescence peaking at 412 nm under UV irradiation at 280 nm.

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INTRODUCTION Based on the concept of equal reactivity of functional groups, the number-average degree of polymerization (X n ) is determined by the well-known equation of (1 + r)/(1 + r − 2rp) for the homogeneous step polymerization with AA and BB type monomers.1,2 In this equation, r is the stoichiometric ratio of monomers and p is the extent of reaction. According to this classical equation, the strict stoichiometry of AA and BB monomers is the essential requirement for the homogeneous step polymerization to produce the high molecular weight polymers. A slight stoichiometry imbalance between AA and BB monomers seriously decreases the molecular weight of the resultant polymers, which significantly deteriorates their mechanical properties and limits their practical applications. In practice, however, the strict stoichiometry is hardly achieved due to the impurity of monomers, side reactions during polymerization, etc. As a result, the novel homogeneous step polymerization methods are highly desired to produce the high molecular weight polymers without requiring the stoichiometry of AA and BB monomers. By eliminating the equal reactivity of functional groups, several stoichiometric imbalance-promoted homogeneous step polymerization methods with AA and BB type monomers have been developed to produce the high molecular weight polymers.3−22 One common feature of these methods lies in the fact that they use the self-accelerating reaction as the condensation reaction. In this approach, the reaction of the first functional group like B in BB monomers with A functional group significantly enhances the reactivity of the remaining B group. The activated B group reacts with A group having a much larger reaction rate constant compared to the original B © XXXX American Chemical Society

group in BB monomers. In this case, the usage of excess molar amount of BB monomers in a certain degree does not deteriorate the molecular weight of the resultant polymers but significantly increases it as a matter of fact.3−22 Because the selfaccelerating reaction property eliminates the effect of excess BB monomers on the second step of condensation reaction between A and activated B groups, the excess BB monomers can be used to enhance the efficiency of the first ratedetermining step of condensation reaction between A group and BB monomers. In addition, the usage of excess BB monomers can assist A group to achieve a quantitative conversion during the polymerization. These altogether facilitate the stoichiometric imbalance-promoted homogeneous step polymerization to produce high molecular weight polymers by avoiding the requirement of stoichiometry between AA and BB monomers.3−22 To date, several self-accelerating reactions have been applied in step polymerization to produce various polymers with high molecular weights under the stoichiometry imbalance condition.4−22 This includes palladium-catalyzed Suzuki−Miyaura polycondensation4−6 and palladium-catalyzed Stille coupling polycondensation7 for conjugated polymers, rhodium-catalyzed oxidative polycoupling of phenylpyrazole and internal diynes for poly(pyrazolylnaphthalene)s,8 rhodium-catalyzed oxidative polycoupling of arylboronic acids and internal diynes for poly(naphthalene)s,9 palladium-catalyzed polycondensation of methyl propargyl carbonate and bisphenols for polyethers,10 Received: June 4, 2017 Revised: July 14, 2017

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DOI: 10.1021/acs.macromol.7b01173 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules palladium-catalyzed allylation polymerization,11,12 super-acidcatalyzed polycondensations of fluorinated ketones with aromatic monomers,13−17 poly-Radziszewski reaction for imidazolium-based polymers,18 polycondensation of dibromomethane and 4,4′-thiobisbenzenethiol for polythioether,19 polycondensation of bisphenol A and dibromomethane for polyformals,20 and polycondensation of bisphenol A with 2,2dichloro-1,3-benzodioxole for polyorthocarbonates.21 To date, most of the successful stoichiometric imbalance-promoted homogeneous step polymerizations are based on the metalcatalyzed self-accelerating reactions.4−12 This may limit their application in practice since the polycondensation should be performed in oxygen-free conditions at high temperature. For the remaining metal-free ones, the super acid or strong base is required, and the corresponding polycondensations are usually performed at high temperature, still limiting their practical applications.13−22 Recently, a double-strain-promoted azide−alkyne cycloaddition reaction (DSPAAC) has been reported in organic chemistry using sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) and azides as reagents.23−28 In DSPAAC, the cycloaddition of the first alkyne with azide increases the DIBOD ring strain and significantly activates the second alkyne, which reacts with azide much faster than the original DIBOD alkyne groups. It has been demonstrated that the monocycloadduction intermediate is neither isolated nor detected by reacting DIBOD with less amount of azides, strongly indicated the self-accelerating reaction property.23,24 Using a model reaction with DIBOD and benzyl azide as reactants, we have quantified the self-accelerating property of DSPAAC by the parameter of rate constant ratio (k2/k1) between the second (k2) and first (k1) azide−alkyne cycloaddition reactions, in which k2/k1 is calculated as 185 and k1 is calculated as 5.84 × 10−2 M−1 s−1.28 In addition, one more distinct advantage of DSPAAC lies in the fact that it can be efficiently performed without any side reaction in a very mild reaction condition such as in air, at room temperature, and requiring no any catalyst or chemical stimulus. This allows DSPAAC to perfectly fulfill the criterion of click chemistry. On the basis of this, we have developed a novel bimolecular homodifunctional ring-closure method for the formation of pure cyclic polymers using the DIBOD as small linkers to ring-close the homodifunctional linear polymers with azide terminals.28,29 Because of the selfaccelerating property of DSPAAC ring-closing reaction, this novel method eliminates the requirement of equimolar amounts of telechelic polymers and small linkers in traditional bimolecular ring-closure methods for pure cyclic polymers. It facilitates this method to efficiently and conveniently produce varied pure cyclic polymers by employing an excess molar amount of DIBOD small linkers. Herein, we further explore the self-accelerating DSPAAC click reaction in step polymerization for the first time and develop a novel stoichiometric imbalancepromoted homogeneous step polymerization with DIBOD and N3-R-N3 type compounds as monomers (Scheme 1). Since the bis-cycloadducts of DIBOD and monoazido compounds had two regioisometric structures,23−28 this novel step polymerization produced the polymer chains with varied isomer structures derived from the orientation (cis/trans) of the two triazole units within the polymer chain.

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Scheme 1. Stoichiometric Imbalance-Promoted Homogeneous Step Polymerization Based on DSPAAC Click Chemistry, Which Used DIBOD and N3-R-N3 as Monomersa

a

One isomer was used to demonstrate the molecular structures of Poly 1−3.

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RESULTS AND DISCUSSION Step Polymerization of DIBOD with M1. Stoichiometric imbalance-promoted homogeneous step polymerization from DSPAAC was first exemplified using M1 and DIBOD as monomer pairs. DIBOD was obtained according to the literature protocol starting from dibenzosuberenone in four steps with overall yield of 6%.28 As shown in Scheme 1, the step polymerization was performed by simply mixing the M1 and DIBOD monomers in CHCl3 at room temperature in air. Because of the click property of DSPAAC reaction, the corresponding Poly 1 could be efficiently produced with varied [M1]0 and S ([DIBOD]0/[M1]0) values. As shown in Figure S1, with S = 1.2 and [M1]0 = 0.05 M, the step polymerization could be finished in 3 h, indicated by the overlapped GPC curves (A) of Poly 1 from 3 h (red) and 7 h (blue) reaction. With S = 1.2 and [M1]0 ≥ 0.1 M, the step polymerization could be finished in only 1 h, confirmed by the overlapped GPC curves (B−D) of Poly 1 from 1 h (red) and 3 h (blue) reaction. The effect of the stoichiometric ratio r on the step polymerization of M1 and DIBOD was systematically investigated by keeping [M1]0 as a constant of 0.2 M. The polymerization was performed with different S values of 0.83, 1.2, 1.5, and 2.0, corresponding to the stoichiometric ratio r values of 0.83, 0.83, 0.67, and 0.5, respectively. Figure 1A shows the FT-IR curves of the raw Poly 1 resulting from the 1 h reaction. The azide absorption peak at 2100 cm−1 completely disappeared using excess DIBOD with S values of 1.2 (red curve), 1.5 (blue curve), and 2.0 (magenta curve), indicating the complete consumption of azide groups in these cases. When using deficient DIBOD with S = 0.83 (black curve), however, the azide absorption peak was clearly observed at 2100 cm−1 as expected, indicating the excess azide groups preserved from this polymerization condition. Figure 1B shows the corresponding UV−vis curves of the raw polymerization solution for Poly 1. The characteristic absorption peak of DIBOD was observed at 273 nm for the cases using excess DIBOD with S values of 1.2 (red curve), 1.5 (blue curve), and 2.0 (magenta curve), which indicated the excess DIBOD survived from the used polymerization condition. The DIBOD absorption peak, however, completely disappeared from the curve (black) with deficient DIBOD (S = 0.83). Resultantly, the combination of FT-IR and UV−vis characterization demon-

EXPERIMENTAL SECTION

All experimental details are in the Supporting Information. B


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Figure 2A shows the corresponding GPC characterization of the resultant Poly 1 with varied S values, whose quantitative analyses are shown in Table 1, runs 1−4. For S = 0.83 corresponding to r = 0.83, the equation of (1 + r)/(1 + r − 2rp) produced a theoretical Xn around 11 for the resultant Poly 1 in the presence of p = 1 (considering the complete consumption of DIBOD in this case). This corresponded to a low theoretical Mn of 3169 for Poly 1, correlating well with the measured Mn value of 2990 from GPC characterization calibrated with PS standards (Figure 2A, black curve; Table 1, run 1). This indicated that the DSPAAC-based step polymerization followed the classical theory in the presence of excess azide groups, in which the usage of excess azides to alkyne groups seriously decreased the molecular weight of the resultant Poly 1. For S = 1.2, the stoichiometric ratio r was calculated as the same value of 0.83. Resultantly, the equation of (1 + r)/(1 + r − 2rp) should produce the same low theoretical Mn of 3169 for the resultant Poly 1 in the presence of p = 1 (considering the complete consumption of azide group in this case). Figure 2A (red curve) shows the corresponding GPC characterization, in which two peak distributions were observed. The major peak at high molecular weight direction with a content of 66.3% had a measured Mn of 130 000 (Table 1, run 2), which was much higher than that (2990) measured for the case with S = 0.83. This proved that the usage of excess DIBOD did not deteriorate the Mn of the resultant Poly 1 but increased it significantly. With keeping increasing S values to 1.5 and 2 corresponding to r values of 0.67 and 0.5, the equation of (1 + r)/(1 + r − 2rp) produced the theoretical Mn as low as 1474 and 883 for the resultant Poly 1 in the presence of p = 1. From the GPC characterization (Figure 2A, blue and magenta curves), however, the major peaks at high molecular weight direction had the measured Mn of 91 300 (Table 1, run 3) and 70 600 (Table 1, run 4) for the resultant Poly 1 with S

Figure 1. FT-IR spectra (A) and UV−vis spectra (B) of raw Poly 1 obtained from [M1]0 = 0.2 M and S = 0.83 (black), 1.2 (red), 1.5 (blue), and 2.0 (magenta).

strated that the deficient reaction groups could be completely consumed and the excess reaction groups could be well preserved from the DSPAAC-based step polymerization because of the click property of DSPAAC and the mild reaction conditions.

Figure 2. (A) GPC curves of raw Poly 1 obtained from [M1]0 = 0.2 M and S = 0.83 (black), 1.2 (red), 1.5 (blue), and 2.0 (magenta). (B) GPC curves of raw Poly 1 obtained from S = 1.2 and [M1]0 = 0.05 M (black), 0.1 M (red), 0.2 M (blue), and 0.3 M (magenta). GPC curves of raw Poly 2 (C) and Poly 3 (D) obtained from [M1]0 = 0.2 M and S = 0.83 (black), 1.2 (red). THF (for A and B) or DMF (for C and D) was used as the eluent, and PS standards were used for calibration. C


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Macromolecules Table 1. Synthesis and Characterization of Poly 1, Poly 2, and Poly 3 runa

[M]0b (mol/L)

Sc

theoretical Mnd

Mne (g/mol)

Mwe (g/mol)

Mw/Mne

1 2 3 4 5 6 7 8 9 10 11

0.2 0.2 0.2 0.2 0.05 0.1 0.3 0.2 0.2 0.2 0.2

0.83 1.2 1.5 2.0 1.2 1.2 1.2 0.83 1.2 0.83 1.2

3169 3169 1474 883 3169 3169 3169 2285 2285 2759 2759

2990 130000 91300 70600 52000 122700 159100 3130 144200 6540 84500

4296 297500 215200 180800 87300 253500 407300 5680 197200 11900 132900

1.44 2.29 2.36 2.56 1.68 2.07 2.56 1.81 1.37 1.82 1.57

polymere (%)

oligomere (%)

66.3 67.3 66.3 31.8 51.9 68.8

33.7 32.7 33.7 68.2 48.1 31.2

32.5

67.5

46.0

54.0

a

The polymerization was performed at room temperature in air using CHCl3 as solvent. bInitial molar concentration of bis-azide monomers. cInitial molar ratio of DIBOD and bis-azide monomers ([DIBOD]0/[M]0). dCalculated from the equation of (1 + r)/(1 + r − 2rp) with p = 1. eCalculated from GPC characterization.

each other. It became less favorable with increasing the polymer molecular weight.2 This caused the two distributions in the GPC curves of the resultant Poly 1 in the presence of excess DIBOD, as shown in Figure 2A red, blue, and magenta curves. Based on above theory, the lower molecular weight Poly 1 oligomers should have cyclic molecular topology. To confirm this, MALDI-TOF MS was used to characterize the molecular structure of Poly 1 oligomers. As shown in Figure 4A, the full spectra (top) shows that the absolute molecular weights of Poly 1 oligomers were detected expanding from 2000 to 7000. From the expanded spectra (bottom), three peak distributions were observed, which could be accurately assigned to cyclic Poly 1 ionized with H+, Na+, and K+. The linear contaminants were not observed in this case. In addition, a regular m/z interval of ca. 588 was observed between the neighboring peaks in the main distribution, which corresponded to the molar mass of the repeating unit of Poly 1. This strongly indicated that the Poly 1 oligomers indeed had the cyclic molecular topology. Figure 4B shows the MALDI-TOF mass spectra of the high molecular weight Poly 1. Since the high molecular weight polymers were discriminated in MALDI-TOF MS measurement, only a small fraction of Poly 1 with relatively lower molecular weights expending from 4000 to 11 000 were detected with a low signal-to-noise ratio (top). From the expanded spectra (bottom), two peak distributions were accurately assigned to cyclic Poly 1 ionized with Na+ and K+. The linear contaminants were still not observed in the measured molecular weight range. A regular m/z interval of ca. 588 also existed between the neighboring peaks of the main distribution, corresponding to the molar mass of the repeating unit of Poly 1. Based on the above MALDI-TOF characterization, cyclic topology dominated the molecular structure of Poly 1 with molecular weight up to 11 000 at least. In the kinetically controlled step polymerization, the cyclization in the early stage of polymerization consumed monomers to form the inert and stable cyclic oligomers, which theoretically resulted in a decrease of molecular weight for the final polymers.2,30−34 As a resultant, if the cyclization could be suppressed in the early stage of polymerization, the molecular weight could be theoretically increased for the final polymers. It is known that the monomer concentration plays a key role to affect the cyclization during the polymerization. The linear polymerization favors a high monomer concentration while the cyclization prefers a low monomer concentration. Resultantly, the monomer concentration could be used to conveniently

values of 1.5 and 2, which were much higher than the theoretical ones. These again indicated that the higher molecular weight Poly 1 could be obtained in the presence of much molar excess of DIBOD. In addition, the slight decrease of measured Mn for Poly 1 with increasing S value above 1 correlated well with the previously published experimental results and theoretical calculations.7,19,21 Resultantly, the DSPAAC -based step polymerization indeed behaved the stoichiometric imbalance-promoted property, in which the high molecular weight polymers could be practically produced in the presence of excess DIBOD. In addition to the major peak with high Mn, a minor peak was also observed at low molecular weight direction of the GPC curves (Figure 2A) for the cases using excess DIBOD with S values of 1.2 (red), 1.5 (blue), and 2.0 (magenta). The precipitation process could be used to efficiently separate the two GPC peak distributions corresponding to the high molecular weight Poly 1 and low molecular weight Poly 1 oligomers. Using the case with S = 1.2 and [M1]0 = 0.2 M as an example, the GPC curves (Figure S2A) of the separated high molecular weight Poly 1 (red) and low molecular weight Poly 1 oligomers (blue) from precipitation overlapped well with the high and low molecular weight distributions of the raw Poly 1 GPC curve (black). In addition, GPC with a multiangle laser light scattering detector was used to calculate the absolute molecular weight of the purified high molecular weight Poly 1. Using DMF with 0.01 M LiBr as the eluent, the dn/dc, absolute Mw, and Mw/Mn were calculated as 0.1404 mL/g, 325 100, and 1.82, respectively. Figure 3A shows the 1H NMR characterization of the purified high molecular weight Poly 1, in which the peak assignments and integrations clearly indicated the formation of Poly 1. Figure S3B shows the corresponding 1H NMR spectrum of the pure Poly 1 oligomers, where the spectrum was same as that (Figure 3A) of high molecular weight Poly 1. Because of the absence of equilibration reactions, the DSPAAC-based step polymerization could be assigned to kinetically controlled polymerization.2,30−34 In this case, cyclization competed with linear polymerization during whole polymerization process and produced the stable cyclic polymers continuously as a result.2,30−34 In addition, the cyclization effects were more obvious in the early stage of polymerization when the polymers had relatively lower molecular weight.2 This was because the kinetic feasibility for the cyclization reaction depended on the probability of two end groups encountering D


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Figure 4. MALDI-TOF MS of low molecular weight Poly 1 oligomers (A) and high molecular weight Poly 1 (B), which was synthesized with [M1]0 = 0.2 M and S = 1.2. One isomer was used to demonstrate the molecular structures of the resultant Poly 1.

Figure 3. 1H NMR spectra (in CDCl3) of the high molecular weight Poly 1 (A), Poly 2 (B), and Poly 3 (C) purified from precipitation, which was synthesized with [M]0 = 0.2 M and S = 1.2. One isomer was used to demonstrate the molecular structures of the resultant Poly 1− 3.

manipulate the molecular weight of resultant Poly 1 and the content ratio of the high and low molecular weight parts of Poly 1 as well. To demonstrate this, the polymerization was performed at a constant S value of 1.2 with varied [M1]0 of 0.05, 0.1, 0.2, and 0.3 M. Figure 5A shows the FT-IR curves of the raw Poly 1 resulted from 3 h (for 0.05 M) and 1 h (for 0.1, 0.2, and 0.3 M) reaction. The azide absorption peak completely disappeared at 2100 cm−1 for all cases, indicating the complete

Figure 5. FT-IR spectra (A) and UV−vis spectra (B) of raw Poly 1 obtained from S = 1.2 and [M1]0 = 0.05 M (black), 0.1 M (red), 0.2 M (blue), and 0.3 M (magenta).

consumption of azide group in with the presence of excess DIBOD. Figure 5B shows the corresponding UV−vis curves of the raw polymerization solution, where the absorption peak of excess DIBOD was clearly observed at 273 nm for all cases. E


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in Figures S2B and S2C. By virtue of GPC with a multiangle laser light scattering detector and DMF with 0.01 M LiBr as eluent, the dn/dc, absolute Mw, and Mw/Mn were calculated as 0.1379 mL/g, 115 800, and 1.68 for the purified high molecular weight Poly 2 and 0.1179 mL/g, 74 700, and 1.79 for the purified high molecular weight Poly 3, respectively. Figures 3B and 3C show the 1H NMR characterization of the purified high molecular weight Poly 2 (B) and Poly 3 (C), in which the peak assignments and integrations confirmed the successful formation of Poly 2 and Poly 3. Fluorescence Property. More interestingly, the stoichiometric imbalance-promoted homogeneous step polymerization from DSPAAC click reaction produced the polymers bearing fluorescence property. Figure 6A shows the UV−vis curves for

Figure 2B shows the GPC curves of the resultant Poly 1, whose quantitative analysis is shown in Table 1, runs 2 and 5−7. With increasing [M1]0 from 0.05 M (black), to 0.1 M (red), to 0.2 M (blue), and again to 0.3 M (magenta), the GPC peak distribution (Figure 2B) of high molecular weight Poly 1 shifted to higher molecular weight direction continuously. This corresponded to the measured Mn (Table 1) increased from 52 000 (run 5), to 122 700 (run 6), to 130 000 (run 2), and again to 159 100 (run 7). In addition, the GPC peak distribution (Figure 2B) of Poly 1 oligomers kept at the same position, but the peak content (Table 1) decreased from 68.2% (run 5), to 48.1% (run 6), to 33.7% (run 2), and again to 31.2% (run 7). The same GPC peak position of Poly 1 oligomers from different [M1]0 indicated the molecular weight of Poly 1 oligomers was not affected by varying the initial monomer concentration in this step polymerization. This again confirmed the inert and stable cyclic molecular topology for the resultant Poly 1 oligomers formed in the early stage of polymerization. As a result, in the stoichiometric imbalancepromoted homogeneous step polymerization based on DSPAAC click reaction, the usage of higher monomer concentration could efficiently suppress the formation of cyclic oligomers in the early stage of polymerization. This led to the decrease of cyclic oligomer content and the simultaneous increase of molecular weight for the final polymers Step Polymerization of DIBOD with M2 and M3. To demonstrate the universality of this novel stoichiometric imbalance-promoted homogeneous step polymerization, bisazide monomers of M2 with pure alkyl bonds and M3 containing ester bonds were randomly selected to polymerize with DIBOD and form the corresponding Poly 2 and Poly 3 (Scheme 1). The polymerization was performed with a constant initial monomer concentration of 0.2 M and different S values of 0.83 and 1.2. Figures S4A and S4B show the FT-IR characterization of the raw Poly 2 (A) and Poly 3 (B) resulting from 1 h reaction. Azide absorption peaks were observed at 2100 cm−1 for both curves (black) of Poly 2 (A) and Poly 3 (B) using deficient DIBOD (S = 0.83), which completely disappeared in the presence of excess DIBOD (S = 1.2) for both cases (red curves). Figures S4C and S4D show the corresponding UV−vis curves of the raw polymerization solution for Poly 2 (C) and Poly 3 (D). Comparatively, DIBOD absorption peaks completely disappeared at 273 nm for the cases with deficient DIBOD (S = 0.83) (black curves), while they were observed for the cases with excess DIBOD (S = 1.2) (red curves). This clearly indicated that the deficient reaction groups were consumed and the excess reaction groups were preserved during the DSPAAC-based step polymerization. Figures 2C and 2D show the GPC characterization of the resultant Poly 2 (C) and Poly 3 (D). The corresponding quantitative analyses are included in Table 1 as runs 8−11. With deficient DIBOD (S = 0.83), the GPC curves (black) showed that only low molecular weight Poly 2 (Mn = 3130, run 8) and Poly 3 (Mn = 6540, run 10) were obtained. With excess DIBOD (S = 1.2), however, two peak distributions with high and low molecular weights were observed in the GPC curves (red) of both cases. As shown in Table 1, a Mn of 118 100 was obtained for the high molecular weight part of Poly 2 with a content of 32.5% (run 9) and a Mn of 84 500 was calculated for the high molecular part of Poly 3 with a content of 46.0% (run 11). The low molecular weight Poly 2 and Poly 3 oligomers could also be conveniently removed by the precipitation process, which was demonstrated by the GPC characterization

Figure 6. UV−vis spectra (A) and fluorescence spectra (B) of the high molecular weight Poly 1 (black), Poly 2 (red), and Poly 3 (blue) purified from precipitation, which was synthesized with [M]0 = 0.2 M and S = 1.2. Chloroform was used as measurement solvent. Fluorescence spectra were recorded with a UV irradiation of 280 nm.

the purified high molecular weight Poly 1, Poly 2, and Poly 3, in which the curves of Poly 2 (red) and Poly 3 (blue) were overlapped, having a weak absorption peak at 280 nm, and the curve (black) of Poly 1 showed a stronger absorption peak at 300 nm. Figure 6B shows the corresponding fluorescence spectra, in which a strong fluorescence peak at the same position of 412 nm was observed for all three cases with a UV irradiation of 280 nm. The overlapped fluorescence spectra for Poly 1, Poly 2, and Poly 3 with different R segments (Scheme 1) indicated that the fluorescence irradiated from reacted DIBOD segments inside polymer chains. To further confirm the origination of the fluorescence, the model compounds 1 and 2 were synthesized by reacting DIBOD with benzyl azide and n-butyl azide, respectively. The corresponding chemical structures and synthesis details are shown in the Supporting Information. Figure S5 shows the UV−vis and fluorescence spectra of the model compounds 1 (black curve) and 2 (red curve). As shown in Figure S5B, the overlapped fluorescence F


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spectra were observed for model compounds 1 and 2 and the purified Poly 1, Poly 2, and Poly 3. This clearly indicated that the polymer fluorescence originated from the individually reacted DIBOD segments inside polymer chains but not from the collective effect of repeating units in a polymeric structure.

CONCLUSIONS A novel stoichiometric imbalance-promoted homogeneous step polymerization method was developed based on the selfaccelerating DSPAAC click reaction. By eliminating the concept of equal reactivity of functional groups in the traditional step polymerization, this novel method successfully prepared high molecular weight polymers under stoichiometry imbalance condition using excess molar amounts of DIBOD to N3-R-N3. In this approach, the self-accelerating property of DSPAAC eliminated the effect of excess DIBOD on the second cyclization of activated alkyne moiety and azide group during the polymerization. By virtue of this, the excess DIBOD was used to enhance the reaction rate of the first rate-determining cyclization of DIBOD and azide group and also to achieve a quantitative conversion of deficient azide group during the polymerization. This assisted the DSPAAC-based step polymerization to efficiently produce high molecular weight polymer without requiring the stoichiometry between DIBOD and N3R-N3 monomers. In addition, by using a higher initial molar concentration of N3-R-N3 monomer to suppress the cyclization in the early stage of the step polymerization, the molecular weight of the resultant polymers could be further enhanced and the content of the cyclic oligomers from the early stage of polymerization could be simutaneously decreased. Considering the mild reaction condition of DSPAAC click reaction and the wide variety of N3-R-N3 monomers, this novel stoichiometric imbalance-promoted homogeneous step polymerization method is expected to play an important role in the formation of high molecular weight polymers with varied molecular structures and functionalities. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01173. Experimental section and Figures S1−S5 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.Z.). ORCID

Ke Zhang: 0000-0001-5972-5127 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support was primarily provided by National Science Foundation of China (21622406 and 21604089) and Ministry of Science and Technology of China (2014CB932200). K.Z. thanks the Bairen project from The Chinese Academy of Sciences for support. G


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Self-Accelerating Click Reaction in Step Polymerization

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