INVESTIGATION OF NOVEL CROSSLINKING REACTIONS AS
VERSATILE TOOL FOR DESIGNING SELF-HEALING POLYMERS
P. Michael1, D. Döhler1, W. H. Binder1
1
Institute of Macromolecular and Technical Chemistry, Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany - Email: philipp.michael@chemie.uni-halle.de
Keywords: self-healing polymers, mechanochemistry, copper(I)-alkyne-azide “click” reaction
ABSTRACT
In recent times the development of self-healing polymeric materials is focused on investigations of fast and efficient crosslinking processes. These should either be able to form new highly branched networks in order to restore thus the initial material properties or to enhance existing materials in view of their thermomechanical and physicochemical properties.
We therefore investigated the copper(I)-catalyzed alkyne-azide "click" cycloaddition reaction (CuAAC) of multivalent polymeric alkynes and azides with respect to crosslinking kinetics, crosslinking efficiencies as well as autocatalytic effects [1-3]. For this purpose a library of several multivalent poly(acrylate)s and poly(isobutylene)s (PIB’s) bearing alkyne respectively azide functionalities were synthesized. An efficient network formation could be observed via in situ rheology measurements within several hours even at room temperature. Moreover a dependency of the network formation rate on the molecular weight and the functional group densities could be proofed.
Based on these results we expanded the concept of CuAAC to hyperbranched PIB’s prepared via living carbocationic inimer (initiator-monomer) type polymerization[4] to study the kinetics and autocatalysis of CuAAC in dependency of branching frequency and to improve the resulting network strand densities and therefore the stiffness of the final crosslinked material.
Beside this we pursue another intrinsic self-healing approach investigating various mechnochemical concepts based on the autonomous activation of a latent catalyst by external force [5]. On this occasion we prepare a polymer based metal-complexes which are able to trigger crosslinking reactions under applied force.
1. INTRODUCTION
Self-healing materials are of great interest for a wide range of applications in material science. Especially the increasing lifetime of polymer compounds lead to the necessity of repairing polymer materials intrinsically, without additional stimulus. Therefore, a fast crosslinking processes based on catalytic reactions acting at room temperature as well as reactive and liquid precursors are required for the development of efficient self-healing concepts.
2. CONCEPT
Thus, the copper(I)-catalyzed alkyne-azide "click" cycloaddition reaction (CuAAC) was chosen and the network formation of multivalent low-molecular weight alkynes as well as of multivalent telechelic or orthogonal functionalized polymeric alkynes and azides were investigated [1-3]. These liquid precursors were encapsulated into melamine-formaldehyde resins and were embedded into a copper(I)-catalyst containing, high molecular weight poly(isobutylene) (PIB) matrix (see Figure 1). The network formations as well as the restoration of mechanical strength were investigated via melt-rheology respectively dynamic mechanical analysis (DMA).
Figure 1: Concept for a shear sensitive self-healing material based on the encapsulation of azide- and alkyne-functionalized compounds embedded in a
high-molecular weight PIB matrix containing finely dispersed Cu(I)Br(PPh3)3 as catalyst.
Rupture of the capsules and thus release of reactive compounds is induced by shear
force resulting in network formation via CuAAC (figure reprinted with permission from
[3] Copyright (2011) WILEY-VCH).
Resulting on these investigations, an efficient network formation depending on the molecular weight and the functional group density could be observed at room temperature via in situ rheology measurements within 90 min up to several hours. Polymers with low molecular weight and simultaneously high functional group density show the fastest crosslinking behavior [1].
In order to get a deeper understanding of the crosslinking kinetics and the autocatalytic effects of this reaction, additional melt-rheology, NMR- and IR-spectroscopy as well as differential scanning calorimetry (DSC) measurements were applied in solution and in the bulk state. After screening various reaction conditions, including copper(I) catalysts and different types of polymers for the “click” reaction, like star-shaped azido- and alkyne-telechelic PIB’s (Mn = 5500 - 30000 g/mol) synthesized via living carbocationic polymerization (LCCP) or random copolymers of propargyl acrylate and n-butyl acrylate (Mn = 7000 - 23400 g/mol, alkyne content ranging from 2.7 - 14.3 mol% per chain) prepared via nitroxide mediated polymerization (NMP), an auto-acceleration ranging from a factor of 1.6 to 4.3 could be observed. The autocatalytic effect based on the formation of 1,3-triazole rings during the reaction which act as internal ligands within the catalytic process Moreover, the close special relationship between the neighboured alkyne groups effected the acceleration of the second alkyne addition step according to the proposed mechanism of Fokin et al. [6] (see Scheme 1).
N3 N3 N3 n n n 18.53 μm 1 2 3 ICSHM2013_________________________________________________________________________________ 334
O O O O O O O O O O O O O O O O O O O O O O N3 O O CuBr(PPh 3)3 ligand II I II I O N3 O N3 O O O O O N1 N2 N3 Cu O N3 O O O N1 N2 N3 Cu O O O Cu O O N1 N2 N3 O N3 O O O Cu N1 N2 N3 PIB PIB PIB PIB II I O Cu O O N1 N2 N3 Cu O N3 N1 N2 N3 PIB PIB * * O O O O R N1 N2 N3 Cu PIB R1 Cu R = nBA, CH2-C CH ran R1= alkyne polymer II
Scheme 1: Proposed mechanism of autocatalysis for crosslinking multivalent poly(acrylate)s and PIB’s functionalized with alkyne- respectively azide-groups applying Cu(I)Br(PPh3)3 and TBTA as catalytic system at 20 °C ((scheme reprinted
with permission from [1] Copyright (2012) American Chemical Society)).
Beside this we pursue another intrinsic self-healing approach basing on the mechanochemical activation of latent metal catalysts by external force. On this occasion we prepare a polymer based metal-complexes which are able to trigger crosslinking reactions under applied force (see Scheme 2).
Scheme 2: Schematic concept of a shear force induced mechanochemical scission of a metal complex with polymeric ligands resulting in the activation of a catalyst and
triggering of a crosslinking reactions.
REFERENCES
[1] Döhler, D.; Michael, P.; Binder, W. H.; Autocatalysis in the Room Temperature Copper(I)-Catalyzed Alkyne–Azide “Click” Cycloaddition of Multivalent Poly(acrylate)s and Poly(isobutylene)s; Macromolecules; 2012, 45 (8), 3335–3345.
[2] Schunack, M.; Gragert, M.; Döhler, D.; Michael, P.; Binder, W. H.; Low-Temperature Cu(I)-Catalyzed “Click” Reactions for Self-Healing Polymers; Macromol. Chem. Phys.; 2012, 213 (2), 205-214.
[3] Gragert, M.; Schunack, M.; Binder, W. H.; Azide/Alkyne-“Click”-Reactions of Encapsulated Reagents: Toward Self-Healing Materials; Macromol. Rapid Commun.; 2011, 32 (5), 419-425.
[4] Paulo, C.; Puskas, J. E.; Synthesis of Hyperbranched Polyisobutylenes by Inimer-Type Living Polymerization. 1. Investigation of the Effect of Reaction Conditions; Macromolecules; 2001, 34 (4), 734-739.
[5] Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P.; Activating catalysts with mechanical force; Nature Chem.; 2009, 1 (2), 133-137.
[6] Rodionov, V. O.; Fokin, V. V.; Finn, M. G.; Mechanism of the Ligand-Free CuI-Catalyzed Azide–Alkyne Cycloaddition Reaction; Angew. Chem. Int. Ed.; 2005, 117 (15), 2250-2255.
