INVESTIGATION OF AUTOCATALYTIC CROSSLINKING REACTIONS
BASED ON ROOM TEMPERATURE “CLICK” CHEMISTRY AS
VERSATILE TOOL FOR DESIGNING SELF-HEALING POLYMERS
D. Döhler 1, P. Michael 1 and W. H. Binder 1
1 Institute of Chemistry, Chair of Macromolecular Chemistry, Faculty of Natural Sciences II
(Chemistry, Physics and Mathematics), Martin-Luther-University Halle-Wittenberg, von-Danckelmann-Platz 4, Halle (Saale) 06120, Germany – e-mail: diana.doehler@chemie.uni-halle.de; philipp.michael@chemie.uni-diana.doehler@chemie.uni-halle.de; wolfgang.binder@chemie.uni-halle.de
Keywords: autocatalysis, click chemistry, poly(isobutylene), room temperature, self-healing polymer
ABSTRACT
Fast crosslinking and post-crosslinking reactions based on catalytic reactions acting at room temperature are a major focus of materials scientists as highly reliable toolbox for the design of self-healing polymeric materials. While applying liquid and reactive polymeric precursors with different architectures as well as reactions with autocatalytic behavior, the efficiency and thus the self-healing performance of the associated network formation, determined via melt rheology, can be optimized. We therefore investigated the network formation of multivalent polymeric alkynes and azides, namely poly(acrylate)s and poly(isobutylene)s (PIB’s) via the copper(I)-catalyzed alkyne-azide "click" cycloaddition reaction (CuAAC) as well as the corresponding crosslinking kinetics via in situ melt rheology.
Due to the formation of 1,3-triazole rings acting as internal ligands during the catalytic process an acceleration of subsequent “click”-reactions up to a factor of 4.3 (increase of rate constant) was observed. Thus efficient crosslinking within several hours could be enabled at room temperature dependent on the molecular weight while linking the starting viscosity of the polymer mixture as well as molecular mobility to changes in the CuAAC-reactivity.
Based on the deeper understanding of CuAAC-catalysis and the optimization of the reaction rate of CuAAC we subsequently have expanded our crosslinking concept to hyperbranched PIB’s prepared via living carbocationic inimer (initiator-monomer) type polymerization. Accordingly, we created a new crosslinking system based on CuAAC in order to improve the resulting network strand densities and therefore the stiffness of the final crosslinked material.
1. INTRODUCTION
Crosslinking as well as post-crosslinking reactions and their associated network formation are in major focus of materials scientists to create unique self-healing materials with various architectures or to improve existing materials regarding their thermomechanical and physicochemical properties. Accordingly, the quest for such types of reactions proceeding under ambient reaction conditions like low temperature as well as for reactive and liquid reactants generating an efficient amount of network points is very high. We therefore investigated the CuAAC[1-2] as one of the most
prominent “click”-type reactions and valuable as well as versatile tool for the design of self-healing polymers due to its substrate insensitivity and high efficiency even under mild reaction conditions. Thus, we explored the crosslinking reaction of various poly(acrylate)s and PIB’s bearing alkyne- respectively azide-groups and the corresponding reaction kinetics via in situ melt rheology.[3-5]
2. MATERIALS AND METHODS
Rheological measurements were performed on an oscillatory plate rheometer MCR 501/SN 80753612 from Anton Paar (Physica). For all measurements a PP08 measuring system (parallel plated, diameter 8 mm) was used. Measurements were performed at 20 °C. For evaluation of data the RheoPlus/32 software (V 3.40) and OriginPro7 was used. For sample preparation a 1: 1 mixture of an azido-functionalized polymer was used. The frequency sweep of the pure polymer mixturewhich was used as basic measurement was performed with a strain γ of 10.0% and with an angular frequency ω ranging from 100 to 1 rad/s. CuBr(PPh3)3
(0.1 equiv per functional group) was dissolved in CHCl3 (40.0 μL) and was added as
a stock solution to all investigated polymer mixtures. Subsequently, the reaction mixture was mixed with a spatula and was immediately put on the rheometer plate. Measurements were performed with a strain γ of 0.1% or 0.5% and with an angular frequency ω ranging from 100 to 1 rad/s. A frequency sweep was performed every 20 minutes.
3. RESULTS AND DISCUSSION
Three-arm-star azido-telechelic PIB’s were synthesized with molecular weights from 5500 g/mol to 30000 g/mol and were crosslinked with three-arm-star alkyne-telechelic PIB with a molecular weight of 6300 g/mol via CuAAC at 20 °C in order to study the influence of the correlated decreasing concentration of the reactive functional groups from 0.238 to 0.076 M on the crosslinking kinetics. Accordingly, the gelation time, determined via in situ melt rheology, increased from 290 to 855 minutes with increasing molecular weight respectively decreasing functional group density. This observation was in line with the starting viscosity of the polymer mixtures which increased as well with increasing molecular weight respectively decreasing functional group density while linking reactivity and molecular mobility of the polymer chains. Thus, an autocatalytic effect of the CuAAC of multivalent PIB’s could be observed as illustrated in figure 1 showing the correlation of the rate constants k vs time t for crosslinking PIB’s with azide- respectively alkyne-groups and applying bromotris(triphenylphosphine)copper(I) as catalyst.[3]
0 100 200 300 400 500 600 700 800 900 0 400 800 1200 1600 2000 k [M -3 ·mi n -1 ] t [min]
Figure 1: Correlation of the rate constants k vs time t at 20 °C for crosslinking PIB’s with azide- respectively alkyne-groups applying
bromotris(triphenylphosphine)copper(I) as catalyst.
In accordance with the development of the gelation times the strongest autocatalytic effect could be observed for crosslinking the polymer mixture with the lowest molecular weights and thus with the highest concentration of functional groups. Consequently the increase of the rate constants k became less prominent for crosslinking polymer mixtures with higher molecular weights.
The observed autocatalytic effect could be explained due to different functionalities of the formed triazole rings during CuAAC which can act as internal ligand while forming the corresponding copper(I)-acetylide/ligand complex or which can preorientate the functional groups next to the catalytically active copper(I) center and thus, promoting further click reactions. [3]
Due to the autocatalysis of the CuAAC of polymeric azides and alkynes enabling efficient crosslinking reactions at room temperature we subsequently have expanded the crosslinking concept to hyperbranched PIB’s. For the synthesis a living carbocationic inimer (initiator-monomer) type polymerization was used according to Puskas et al.[6] as illustrated in figure 2.
TiCl4, DtBP
- 80 °C +
Figure 2: Synthesis of hyperbranched PIB’s via living carbocationic inimer (initiator-monomer) type polymerization.
Due to the architecture of the hyperbranched PIB’s and the higher amount of functional groups per molecule even shorter gelation times could be observed in comparison to star-shaped PIB’s while crosslinking azide- respectively alkyne-functionalized polymers at room temperature and using CuBr(PPh3)3 as catalyst.
4. CONCLUSION
The crosslinking of various reactive polymeric alkynes and azides via CuAAC was investigated at room temperature while studying the autocatalytic behavior of the reaction via in situ melt rheology. Due to the triazole rings acting as internal ligands during the crosslinking process an acceleration of the rate constant up to a factor of 4.3 dependent on the reactivity of functional groups could be observed enabling the design of polymeric networks for self-healing applications without the need of any external stimuli like elevated temperature. Thus, a highly efficient crosslinking system suitable for the processing of self-healing materials based on polymeric alkynes and azides could be highlighted due to strong and designable autocatalytic effects.
ACKNOWLEDGEMENTS
We are grateful for the grant DFG BI 1337/8-1 (within the SPP 1568 “Design and Generic Principles of Self-Healing Materials”) and the EU-project IASS for financial support.
REFERENCES
[1] W. H. Binder, R. Sachsenhofer, ‘Click’ Chemistry in Polymer and Materials Science, Macromol. Rapid Commun. (2007) 15-54.
[2] W. H. Binder, R. Sachsenhofer, ‘Click’ Chemistry in Polymer and Material Science: An Update, Macromol. Rapid Commun. (2008) 952-981.
[3] D. Döhler, P. Michael, W. H. Binder, Autocatalysis in the Room Temperature Copper(I)-Catalyzed Alkyne-Azide "Click" Cycloaddition of Multivalent Poly(acrylate)s and Poly(isobutylene)s, Macromolecules (2012) 3335-3345.
[4] M. Schunack, M. Gragert, D. Döhler, P. Michael, W. H. Binder, Low-Temperature Cu(I)-Catalyzed “Click” Reactions for Self-Healing Polymers, Macromol. Chem. Phys. (2012) 205-214.
[5] M. Gragert, M. Schunack, W. H. Binder, Azide/Alkyne-“Click”-Reactions of
Encapsulated Reagents: Toward Self-Healing Materials, Macromol. Rapid Commun. (2011) 419-425.
[6] C. Paulo, J. E. Puskas, Synthesis of Hyperbranched Polyisobutylenes by Inimer-Type Living Polymerization. 1. Investigation of the Effect of Reaction Conditions, Macromolecules (2001) 734-739.
