One-Pot Three-Step Polymerization System Using Double Click

Letter pubs.acs.org/macroletters

One-Pot Three-Step Polymerization System Using Double Click Michael Addition and Radical Photopolymerization Matthieu Retailleau,† Ahmad Ibrahim,† Céline Croutxé-Barghorn,† Xavier Allonas,*,† Christian Ley,† and Didier Le Nouen‡ †

Laboratory of Macromolecular Photochemistry and Engineering and ‡Laboratoire de Chimie Organique et Bioorganique, University of Haute Alsace, 3b rue Alfred Werner, 68093 Mulhouse, France S Supporting Information *

ABSTRACT: A click chemistry synthetic strategy based on aza-Michael addition and radical photopolymerization is proposed to generate a polymeric network via three time-controlled steps. The selection of primary diamines and diacrylates allows two consecutive aza-Michael reactions to occur. This reaction sequence affords the unique opportunity to interpose a radical photopolymerization reaction, enhancing the cross-link density. Consequently, the second aza-Michael addition appears as a valuable postconsolidation step of the polymer network.

T

Michael addition can be considered as a two-step reaction: after a first step consisting in the addition of primary amine group on electron poor olefin (referred here as AZ1 reaction), the resulting secondary amine can react with a second olefin (AZ2 reaction).1 AZ1 and AZ2 addition reactions exhibit different kinetic rates.15,16 The present paper aimed to take advantages of this two-step chemistry by introducing an intermediate photopolymerization step. Purposely, a diamine:diacrylate mixture at a molar ratio of 1:4.2 was selected for a better performance of the overall process. The first step corresponding to the addition of the primary diamine on the CC double-bonds will end up with a controlled amount of unreacted diacrylate functions. These latter are then polymerized under a fast photochemical process. The residual unreacted functions that remain after the well-known vitrification19,20 of light cured samples can be further polymerized via AZ2 reaction, achieving highly cross-linked and homogeneous polymer network. The effectiveness of the aza-Michael reaction was first investigated by using liquid state 1H NMR to monitor the addition process of 1,5-diaminopentane (DP) with an ethoxylated bisphenol A diacrylate (EBAD) at a molar ratio of 1:4.2 (DP/EBAD) in bulk. Figure 1 shows the initial compounds and ideal chemical structures of the polymer formed during the two AZ1 and AZ2 steps. It has to be mentioned that dissolving the sample in CDCl3 stops the reaction and allows a precise characterization of the reaction

he Michael addition reaction is recognized as a versatile and effective pathway for the coupling of electron poor olefins with a large panel of nucleophiles.1 The reaction was first reported for carbon nucleophiles but also included various other nucleophiles bearing heteroatom like nitrogen (azaMichael), sulfur (thiol-Michael), and phosphorus to design C− N, C−S, and C−P bonds, respectively.1−3 Owing to its characteristics (high yields, simple to perform, mild conditions, ...), the Michael addition has been identified to fulfill criteria of a “click” reaction.4 Considering these advantages, a dynamically controlled polymerization reaction has been recently designed via thiol-Michael-acrylate addition and photopolymerization.5−7 It aimed to overcome limits related to the chain growth mechanism occurring in photopolymerization (heterogeneity, vitrification, ...)8 by combining two reactions capable of generating two polymers, via chain growth and step growth polymerization. It should be noted that thiols represent a very small fraction of possible nucleophiles, and only few suitable thiols are commercially available. By contrast, amine could be a suitable substitute as aza-Michael addition was used in many areas such as linear step growth polymerization,9−11 hydrogels,12−14 or hyperbranched polymers.15−17 During the course of our research, a paper appears based on this idea, demonstrating the validity of such a concept.18 It was shown that amine can advantageously replace thiol, and the materials obtained using this approach were shown to exhibit quite interesting properties. Nevertheless, intrinsic advantages of using amines were not fully investigated in this work. Indeed, one of the advantages of using amines is that no catalyst is required as they can act as both nucleophiles and bases. Moreover, when using primary amines at a specific ratio, aza© 2015 American Chemical Society

Received: September 16, 2015 Accepted: November 9, 2015 Published: November 13, 2015 1327

DOI: 10.1021/acsmacrolett.5b00675 ACS Macro Lett. 2015, 4, 1327−1331

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ACS Macro Letters

Figure 1. (a) 1H NMR (A−C) and HR-MAS (D) spectra recorded for the polymerization of DP+ EBAD with a 1:4.2 molar ratio for (A) 1 min; (B) 80 min; (C) 6 days; (D) 20 days and (b) chemical structures of starting reactants (EDAB and DP) and ideal structures of adducts (EBAD-DP and EBAD-DP-EBAD).

kinetics. Protons were assigned according to the NMR spectra displayed in Figure 1. To ensure proper identification of protons that are interacting with the nitrogen, 2D COSY NMR was carried out after 5 h (see Figure S1). More details are reported in the Supporting Information. Samples were also investigated after 20 days by HR-MAS. For sake of clarity, only protons involved in the aza-Michael reaction will be described in details. Signals between 7.06 and 7.21 ppm and between 5.59 and 6.56 ppm correspond to the resonance of benzylic (j) and vinylic (d1, e1, and f1) protons in EBAD, respectively. The signal at 2.81−2.97 ppm was attributed to protons linked to the α-carbons (e2) in the EBAD groups of EBAD-DP. The chemical shift at 2.34− 2.51 ppm refers to hydrogens attached to the α-carbons in the DP group (a3) and β-carbons (f3) in EBAD groups of EBADDP-EBAD (see Figure S2). Michael addition was followed via the decrease of EBAD and the formation of EBAD-DP and EBAD-DP-EBAD as estimated on the basis of 1H NMR chemical shift data. Ratios of the different species consumed or generated during the two-step reaction are reported in Figure 2. Primary amines react with double-bonds (AZ1) increasing the EBAD-DP content to 19.1% after 80 min (see Table S1). At

Figure 2. Experimental data and fitted conversion kinetics of the azaMichael addition for EBAD (black square and black line), EBAD-DP (green circle and green line), and EBAD-DP-EBAD (blue triangle and blue line) with molar ratio of 1:4.2 for DP and EBAD, respectively.

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DOI: 10.1021/acsmacrolett.5b00675 ACS Macro Lett. 2015, 4, 1327−1331

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ACS Macro Letters that time, a slight consumption of EBAD-DP is simultaneously observed, indicating that a second reaction proceeds. The latter corresponds to AZ2 reactions between unreacted acrylates and generated secondary amines. From Figure 2, it can be seen that AZ2 reaction starts after almost full completion of AZ1. This step acts as a dark cross-linking process and ends up after 20 days. To determine the reaction rate constants, a modeling of both AZ1 and AZ2 was performed. Michael addition polymerization of DP with EBAD can be described by eqs 1 and 2 corresponding to the reactivity sequence of the primary and secondary amines.21,22 The reaction tends to follow secondorder kinetics based on the concentration of the acrylate and the amine.1 k1

EBAD + DP → EBAD − DP k2

EBAD − DP + EBAD → EBAD − DP − EBAD

Scheme 1. Schematic Representation of the Three-Step Process (with 1:4.2 Molar Ratio)

(1) (2)

k1 and k2 are the rate constants of the AZ1 and AZ2 reactions, respectively. Assuming this mechanism, it is then possible to express the disappearance and appearance rate of reagents and products (eqs 3, 4, 5, and 6): d[EBAD] = −k1[EBAD][DP] − k 2[EBAD − DP] dt × [EBAD] d[DP] = −k1[EBAD][DP] dt

(3)

(4)

d[EBAD − DP] = k1[EBAD][DP] − k 2[EBAD − DP] dt × [EBAD]

(5)

d[EBAD − DP − EBAD] = k 2[EBAD − DP][EBAD] dt (6)

By numerically solving this system of coupled nonlinear differential equations, time−conversion curves can be fitted to the experimental data extracted from NMR analysis (Figure 2), leading to the determination of k1 and k2: k1 = (4.2·10−4 ± 0.6·10−4)M−1·s−1 k 2 = (1.0·10−6 ± 0.2·10−6)M−1·s−1

The model is consistent with the experimental results with a margin of error of about 5% (Figure 2). According to the literature,21,22 primary amine reactivity is higher to that of the formed secondary amine, a fact that is in line with the relative values of k1 and k2. The k2/k1 ratio clearly demonstrates a twostep reaction process where the first click reaction forms a branched poly(amino ester) polymer which is cross-linked via a second click reaction. This difference of reactivity opens the way to interpose a photochemical step between AZ1 and AZ2 processes (Scheme 1). The overall reaction mechanism is therefore the following: 1/AZ1, formation of a branched prepolymer; 2/hν, photochemically cross-linked polymer network in a fast time scale; and 3/AZ2, slow postconsolidation leading to highly crosslinked polymer network. This original three-step approach (Scheme 1) was investigated by real-time FTIR, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DP and

EBAD at a molar ratio of 1:4.2 (promoting the AZ1 reaction) as well as a photoinitiator (Irgacure 819, 2.5 wt %) were mixed together. AZ1 reaction immediately proceeds and 1H NMR results (Figure 2) show that EBAD content decreases to 74.4% after 80 min (i.e., an EBAD conversion of 25.6%). The resulting Tg of −31 °C (Table 1) is consequently low and gives rise to a branched prepolymer that can be easily handled. The resulting prepolymer was subsequently photopolymerized under LED at Table 1. EBAD Conversion and Tg Obtained by DSC after Each of the Three Steps (AZ1, hν, and AZ2) step 1

DP/EBAD molar ratio 1:4.2 1329

step 2

step 3

conv (%)

Tg (°C)

conv (%)

Tg (°C)

conv (%)

Tg (°C)

19

−31

78

16

91

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DOI: 10.1021/acsmacrolett.5b00675 ACS Macro Lett. 2015, 4, 1327−1331

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ACS Macro Letters 395 nm during 2 min, with an irradiance of 70 mW·cm−2. The corresponding conversion kinetic is shown in Figure 3.

Figure 4. Storage modulus vs temperature at different reaction times: t = 0 d corresponds to AZ1 + hν only; t = 25 d corresponds to additional completion of AZ2. Figure 3. EBAD conversion kinetics of a three-step process (molar ratio DP:EDAB 1:4.2).

Aside from increasing thermomechanical properties, AZ2 affords an elegant route to produce materials with more homogeneous network architecture and, hence, is highly responsive to thermal changes. This opens up new fields of applications such as thermal responsive smart materials produced through a one-pot, three-step process.

Through this photochemical process, EBAD conversion quickly evolved from 25% to 78%. As a consequence, the Tg increases up to 16 °C. The limited EBAD conversion could result from the high amine concentration since it is known to inhibit or delay free radical polymerization.23 However, a photoinitiator increase up to 4 wt % does not lead to further conversion during step 2 (see Figure S3). Thus, polymer vitrification19,20 mainly accounts for the limited conversion (which results from restricted mobility of reactive functions) and low Tg. Interestingly, when the light is switched off, the conversion of double-bonds gradually increased with time, a fact which is ascribed to the AZ2 dark reaction. After 10 days, the residual double-bonds are almost fully converted and the polymer exhibits a Tg of 39 °C. Through its click nature, AZ2 exhibits a large thermodynamic driving force which enables the reaction to occur in a vitrified media and lead to further crosslinking. This outstanding feature effectively vanishes the vitrification limitation occurring in conventional photopolymerization. In absence of a photochemical step between AZ1 and AZ2, the final Tg is only −11 °C after 25 days (see Table S2). Thermomechanical properties are obviously enhanced but in a limited extend compared to the three-step approach. Finally, it has to be mentioned that the overall process can be speed up by changing the starting reagents: compounds with higher nucleophilic and electrophilic power would reduce AZ1 reaction time.1 Regarding AZ2, heating the sample appears to be an effective way to consequently shorten the reaction time (see Table S2). To further point out the positive impact of AZ2 on final properties, mechanical phase transitions were measured by DMA along the three-step process. Results are plotted in Figure 4. As expected, the mechanical behavior of the material significantly evolved during AZ2 postconsolidation. Indeed, the mechanical transition sharpened: its width (i.e., the difference between the onset and the endset) was drastically reduced from +31 °C (t = 0 d) to 9 °C (t = 25 d). According to the literature,24,25 a well-defined mechanical transition confirms the formation of a homogeneous network. Thus, by clicking dangling ends of the network obtained from the uncontrolled radical chain growth mechanism, AZ2 enables a reorganization of the network.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00675. Materials and methods used for this study. Detailed 1H NMR and 2D COSY NMR spectra and results. Influence of photoinitiator content and temperature (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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