Hydrogels by supramolecular crosslinking of terpyridine end group functionalized 8-arm poly ( ethylene glycol )

Metallo supramolecular assemblies of an 8-arm poly(ethylene glycol) partially substituted with terpyridyl end-groups and the transition metal ions Ni, Fe, Co and Zn were studied for their nano-particle formation at dilute conditions and gelation at higher concentrations. The large differences in dissociation rate constants of the metal ligand complexes largely determines the assembly behavior. Thermodynamically stable complexes are generated with Ni, Fe chlorides, which lead to distinct particle sizes of ~200 nm in dilute conditions. The Co and Zn chlorides provide multiple size distributions revealing that mono and bis-complexes are present at equilibrium. Upon complexation, terpyridyl groups move to the outer sphere giving aggregates with a charged surface. At polymer concentrations above 5 wt%, crosslinking upon addition of transition metal ions provides hydrogels. Elastic hydrogels were obtained with Ni, Fe and Co having storage moduli in excess of 20 kPa, whereas Zn gels are relatively viscous. Only Zn gels show a thermoreversible sol to gel transition at a temperature of 25 C independent of polymer concentration. Page 2 of 15 Soft Matter


Introduction
Polymer gels are networks of polymer molecules that are either covalently or physically crosslinked and expanded throughout their volume by a uid. 1 Physically cross-linked polymer gels are based on supramolecular chemistry, classically dened as the chemistry of complex formation through non-covalent interactions. 2These interactions include dipole-dipole interactions, ionic bonding, hydrogen bonding, p-p stacking or combinations of these, such as ion-dipole interactions. 2,3The properties of physically cross-linked gels may vary widely, especially when the interactions are governed by dynamic equilibrium processes.As an example, cation-anion interactions can yield bond strengths up to 350 kJ mol À1 .This type of interaction has been used in several supramolecular polymer gels like calcium alginate gels, 4,5 gellan gum 6 and chitosan polyelectrolyte complexes. 7Although cation-anion interactions can yield mechanically strong gels, the cross-links in these gels can be dynamic due to the relatively high kinetic lability of the interaction. 8][11] Gels formed by supramolecular cross-linking using metal ions as guests and ligands as hosts will also strongly depend on the predominant equilibrium state.A gel is only formed when the host-guest complex is favored at equilibrium.Bond strengths in metal ligand complexes can be as high as those of covalent bonds. 12In certain cases highly stable complexes are formed when multiple Brønsted bases in a molecule donate electrons to a single Lewis acid which is known as the chelate effect. 13,14Importantly, although these interactions can have a high thermodynamic stability, the metal coordination bond can nonetheless possess a relatively high kinetic lability.This combination of relatively high thermodynamic bond stability and kinetic lability makes metal coordination bonds an interesting choice for use as cross-links in polymeric gels.Such systems are currently attracting increasing interest and are termed metallo-supramolecular polymer gels (MSPGs). 157][18][19] Binding with third row transition metal ions ideally proceeds in a two-step process. 20The equilibria involve the stepwise coordination of two terpyridine moieties to a metal ion with different stability constants for the metal-mono(terpyridine) complex (K 1 ) and metal-bis(terpyridine) complex (K 2 ), respectively.The overall equilibrium is represented by the overall stability constant b, the product of K 1 and K 2 .The stability constants are equilibrium constants and are dened by their association and dissociation rate constants. 21he stability of transition metal bis(terpyridine) complexes is affected by several factors, such as the metal ion, temperature, pH and solvent.The metal ion is of paramount importance in this respect as it is known that the overall stability constants (b) of transition metal ion-bis(terpyridine) complexes in water follows the series Ni 2+ > Fe 2+ > Co 2+ > Zn 2+ . 22,23everal extended overviews on terpyridine chemistry and their complexes with a variety of transition metal ions have been published in the past decade. 24,25However, research on MSPGs has mainly focused on complex formation in organic solvents, i.e. on the formation of organogels and only minor research has been directed to hydrogels.Reports on MSPGs formed in water cover either terpyridyl end-group modied thermosensitive block copolymers, a terpyridyl graed amphiphilic block copolymer and end-group modied 3 or 4 armed poly(ethylene glycol). 26Chiper et al. end modied commercially available pluronics with terpyridyl groups and determined their ability to form nano-particles in dilute solution upon complexation with NiCl 2 . 27The aqueous aggregation of these modied pluronics into micelles in the absence of a metal ion took place via hydrophobic interactions of the hydrophobic blocks and terpyridyl groups.Addition of NiCl 2 led to the rearrangement and disruption of the micellar aggregates due to the hydrophilicity of the metal ion-terpyridine complexes generated.Furthermore, intramicellar bis(terpyridine) complexes were formed due to the relatively high effective concentration of these groups at the micelle coronas.At high concentrations of 20 wt%, the pluronic F127 end modied with terpyridyl groups in the presence of half an equivalent Ni 2+ ions showed thermoreversible gelation behaviour with a transition temperature of 34 C.As suggested, gelation depends on the packing of micelles and not on intermicellar Ni 2+ -terpyridine complexes formed.
Jochum et al. showed that a terpyridine modied blockcopolymer of hydrophilic poly(triethyleneglycol methylether methacrylate) (PTEGMA) graed with a low percentage of terpyridyl groups and polystyrene readily formed gels upon the addition of NiCl 2 . 28Compared to gels based on only terpyridine modied PTEGMA, the self-assembly of the polystyrene blocks led to additional cross-links resulting in an increased modulus of the gels.
Only a few papers on terpyridyl modied PEG's that can be crosslinked into MSPGs in an aqueous environment have appeared in literature.Schmatloch and Schubert reported on a low molecular weight three-arm PEG terminated with terpyridyl groups. 26When combined with a linear terpyridyl end modied PEO, the addition of Fe 2+ ions it was shown that the branched structure acted as a chain stopper instead of crosslinker.Kimura prepared 4-armed PEG, terminated with terpyridyl groups and studied the complexation behavior of this polymer with Fe 2+ in methanol.The dried polymer formed a stable gel in water at a concentration of 20 wt%.They also showed that varying the pH from 1 to 13 or heating to 90 C did not give visible changes. 29A three armed terpyridyl terminated PEG was recently studied for gelation with cobalt ions.Hydrogels were only formed from a water-soluble polymer having a M n of 8000, but not with lower molecular weight polymers. 30Recently, Yoshida and coworkers showed that terpyridyl end-modied 4 and 8 armed PEG form hydrogels upon complexation with Ru 2+ ions.They used the complexes as a catalyst in the Belousov-Zhabotinsky reaction.The mechanical properties of the gels are almost independent on the number of arms, but gelation kinetics is higher for the 8-armed polymer. 31lthough the above presented examples show that terpyridyl-modied polymers may be used for the design of hydrogels, only minor attention has been given to the assembly process and stability of these systems in an aqueous environment using different transition metal ions.In a recent paper by Rossow and Seiffert supramolecular hydrogels based on 4-arm PEG end-modied with terpyridyl end-groups and Mn 2+ , Zn 2+ and Co 2+ ions were described. 32They showed that network inhomogeneity is higher in Co 2+ complexed gels compared to Zn 2+ complexed gels.However, this nanometer-scale inhomogeneity seems to enforce rather than weaken the gel mechanical properties.In this paper, we describe supramolecular gels formed by the self-assembly of an 8-arm PEG end modied with terpyridyl groups via metal-ligand interaction using Ni 2+ , Fe 2+ , Co 2+ and Zn 2+ ions.The aggregation at dilute aqueous conditions and gelation properties at concentrations above 5 wt% were determined.Importantly, we show that the kinetic stability of the metal-ligand crosslinks in the gels determines the temperature dependent gelation properties of the hydrogels.

Synthesis
The 8-arm poly(ethylene glycol) end-group modied with terpyridyl groups was synthesized by the following procedure.
Ground potassium hydroxide (KOH, 0.68 g, 12.1 mmol) and 8PEG (5.90 g, 0.30 mmol) were dissolved in 240 mL of dimethyl sulfoxide (DMSO) at 60 C in a N 2 atmosphere.Aer 90 min 4 0chloro-2,2 0 :6 0 ,2 00 -terpyridine (0.5 g, 1.87 mmol) was added and the reaction mixture was stirred for another 90 min at 60 C, during which the color of the solution changed from dark red to orange.The reaction mixture was cooled and the products precipitated in diethyl ether.Aer decantation, the sticky, slightly orange product was mixed with 125 mL of brine.The resulting mixture was extracted 4 times with 250 mL of chloroform and the combined organic layers were dried over sodium sulfate.Aer ltration, the chloroform was evaporated under reduced pressure and the remaining liquid was dissolved in dichloromethane and precipitated in cold diethyl ether.The product was dried in vacuum at room temperature (yield: 4.8 g; 77%).Additional purication of the product was performed by dissolving the polymer in water and subsequent dialysis against water for 24 hours.The 8PEG(tpy) 5.4 OH 2.6 was recovered by lyophilization. 1
Dissociation rate constants.Dissociation rate constants of Fe 2+ and Co 2+ -8PEG(tpy) 5.4 OH 2.6 complexes were determined according to a method described by Holyer et al. 22,33 Stock solutions of Fe 2+ and Co 2+ -8PEG(tpy) 5.4 OH 2.6 complexes (metal ion-terpyridyl-groups is 1 : 2 mol mol À1 ) in water were prepared at a concentration of 0.78 mg mL À1 .Subsequently, the resulting solution was added to a fused quartz cuvette and a 100 times molar excess of NiCl 2 $6H 2 O or FeCl 2 in 50 mL of water was added.The change in the absorption at 556 nm (exchange of Fe 2+ by Ni 2+ ) or increase in the absorption at 556 nm (exchange of Co 2+ by Fe 2+ ) were recorded over time at room temperature.
Dynamic light scattering (DLS).Aqueous solutions of 8PEG(tpy) 5.4 OH 2.6 were prepared at a concentration of 3 mg mL À1 .A titration experiment was performed by adding set amounts of metal chloride solution under stirring at room temperature to give M 2+ -terpyridyl molar ratios of 0, 1 : 8, 3 : 8, 1 : 2, 3 : 4 and 1 : 1. Aer addition of an aliquot of the metal ion solution the resulting solution was equilibrated overnight.Size distributions were recorded on a Malvern Instruments Zetasizer Nano ZS at 20 C and a 173 backscatter angle.Solutions containing a M 2+ -terpyridyl molar ratio of 1 : 2 were used to measure the zeta potential.The zeta potentials were recorded (Malvern Instruments Zetasizer Nano ZS) at 20 C, using a 633 nm He-Ne laser and backscattering detection.Polystyrene capillary folded cells were used.
Rheometry.Oscillatory rheometry experiments were performed on an Anton Paar, Physica MCR-201 system, using a PP25 measuring plate (25 mm diameter).Approximately 300 mL of a gel with a concentration of either 5, 10 or 20 wt% were prepared by dissolution of 8PEG(tpy) 5.4 OH 2.6 in water and addition of a calculated amount of metal chloride solution (M 2+ -terpyridyl molar ratio of 1 : 2).The gels were le overnight at room temperature.A gel was placed at the center of the measuring plate at 5 C and the measuring gap was set to 0.3 mm, and excess gel was removed.The system was equipped with an oil based solvent trap and the storage (G 0 ) and loss modulus (G 00 ) were recorded under oscillatory shear.Measurements were performed at a frequency (u) of 1 Hz and strain (g) of 1%.Subsequently, heating and cooling cycles were performed between 5 and 60 C at a rate of 1 C min À1 .Aer the temperature cycles, samples were cooled to 20 C and a frequency and amplitude sweep were performed between u ¼ 0.1-100 Hz (g ¼ 1%) and g ¼ 0.01-100% (u ¼ 1 Hz) to conrm that all samples were within the linear viscoelastic regime (Fig. S4 and S5 †).
1 H-NMR.Proton nuclear magnetic resonance ( 1 H-NMR) spectra were recorded on a Bruker Ascend 400/Avance III 400 MHz NMR spectrometer system using deuterated chloroform as a solvent.Solutions were prepared at a concentration of approximately 10 mg mL À1 and measurements were performed at ambient temperature.

Results and discussion
The hydroxyl groups of the star shaped 8PEG were partly converted into terpyridyl groups by a nucleophilic aromatic substitution reaction of potassium hydroxide generated alkoxide groups of the 8PEG to 4 0 -chloro-2,2 0 :6 0 ,2 00 -terpyridine as depicted in Fig. 1.The reaction was performed for 1 h and within this time period a color change can be observed from dark red to orange.The nal ratio of hydroxyl to terpyridyl groups was controlled by the molar feed ratio of the reactants, although full conversion of the hydroxyl groups could not be obtained when an excess of the chloro-terpyridine was used.
The reaction was performed in such a way that approximately 5 of the 8 hydroxyl groups were substituted with terpyridyl groups.The 1 H-NMR spectrum conrmed the structure of the product (Fig. 2) and the number of substituted hydroxyl groups was calculated from the ratio of the terpyridine protons (a-e) and the CH 2 -groups of the terminal PEG oxy ethyl unit (f and g).
Of the 8 hydroxyl groups initially present on average 5.4 hydroxyl groups per polymer molecule were substituted by terpyridyl groups affording 8PEG(tpy 5.4 )OH 2.6 .
To determine complexation ratios of the 8PEG(tpy 5.4 )OH 2.6 with Ni(II), Fe(II), Co(II) and Zn(II) ions in water, UV-Vis titration experiments were carried out.Moreover, given the known high kinetic stability of the Ni 2+ bis(terpyridine) complexes, the titration with a NiCl 2 solution was used to determine the average number of terpyridyl-groups per polymer molecule.Upon titration, the evolution of the characteristic adsorption band of the Ni 2+ -complex at 324 nm (Fig. S1A †) was used to determine the molar complex ratio.By denition, the Ni 2+complex absorption will reach a maximum when all terpyridyl groups are complexed.As depicted in Fig. 3A, at a ratio of Ni 2+ to terpyridyl-groups of 1 : 2, the absorption indeed remains constant upon further addition of Ni 2+ ions.The number of terpyridyl groups coupled to the 8PEG can then be calculated from this maximum (i.e., from the intercept of the straight lines).The average number of coupled terpyridyl groups was calculated as 5.4, which is similar to that which was determined by 1 H-NMR.
Titrations with the other transition metal chlorides, Fe 2+ , Co 2+ , and Zn 2+ were performed similarly.From the characteristic Fe 2+ , Co 2+ and Zn 2+ bis-terpyridyl metal ligand charge transfer absorption bands at 556 nm, 307 nm and 322 nm, respectively, the complex ratios were determined (Fig. 3B and S1, S2 †).Titration with Fe 2+ demonstrated a maximum complexation ratio of 1 : 2, whereas the metal ion-terpyridylgroups ratios were somewhat higher for the Co 2+ and Zn 2+ complexes.This is a clear indication that Fe 2+ forms stable bisterpyridyl complexes with 8PEG(tpy 5.4 )OH 2.6 in water, whereas  in the case of Co 2+ or Zn 2+ mono-complexes may also be present. 34Moreover, the decrease of the absorption at higher Co 2+ -terpyridyl ratios is indicative of a lower stability of cobalt complexes compared to iron and nickel complexes.Similarly, adding an excess of Zn 2+ ions results in a decrease of the absorption at 322 nm, indicative of the formation of monocomplexes. 20n the titration experiments, complex formation appears quickly and may even be diffusion controlled.Rate constants for complex formation of transition metal ions and terpyridylgroups have been determined in the past and are generally 10 4 M À1 s À1 . 22The differences observed in the titration experiments with an excess of metal ions are therefore indicative of differences in the dissociation rate constants.Because the dissociation rate constants were expected to have an inuence on the aggregation at dilute conditions and crosslinking into hydrogels at higher concentrations (vide infra), the dissociation rate constants of the transition metal complexes of the polymers in water were rst determined.
Metal exchange experiments in dilute solutions were performed according to a procedure described by Henderson and Hayward. 33Fe 2+ -bis(terpyridyl) complexes were dissociated with a 100-fold excess Ni 2+ and of Co 2+ -bis(terpyridyl) complexes were dissociated with a 100-fold excess Fe 2+ .In both cases, the absorption band of the Fe 2+ -bis(terpyridyl) complexes was followed by UV-Vis spectroscopy.Henderson and Hayward have shown that these exchanges enable the determination of dissociation rate constants of the Fe 2+ and Co 2+ bis(terpyridyl) complexes.
The dissociation and association of the metal ion bis-(terpyridyl) complexes in the presence of an excess of stronger complexing metal ions can be described by a series of equilibrium reactions as depicted in Fig. 4.During metal ion exchange the initial bis-complex dissociates (equilibrium 1) and the resulting free terpyridyl group rapidly forms a mono-complex with the stronger complexing metal ion (equilibrium 2).The mono-complexes subsequently disproportionate and biscomplexes are nally formed (equilibrium 3).
[Fe 2+ (tpy) 2 ] t ¼ [Fe 2+ (tpy) 2 ] 0 e ÀkÀ2,Fet (1) The exchange of Co 2+ by Fe 2+ ions from the Co 2+ -bis-(terpyridyl) complex proceeds relatively quickly and could be accurately t over the rst 20 minutes of the metal ion exchange.Within this time span, metal ion exchange is dominated by Co 2+ -bis(terpyridyl) dissociation.Thereaer, the data deviate from the t due to Co 2+ -mono(terpyridyl) dissociation.The t yields a dissociation rate constant k À2,Co of 1.4 Â 10 À3 s À1 , similar to the value presented in literature for the dissociation rate constant of a Co 2+ complex of a linear terpyridyl endgroup modied PEG (1.5 Â 10 À3 s À1 ). 33The exchange of Fe 2+ by Ni 2+ ions from the Fe 2+ -bis(terpyridyl) complex proceeds rather slowly and the data could be t according to eqn (1) over at least 6 hours, yielding a dissociation rate constant of 8.1 Â 10 À7 s À1 .This dissociation rate constant is lower than that determined for an Fe 2+ complex of terpyridyl end-group modied linear PEG (5.0 Â 10 À6 s À1 ) but higher than that of an unsubstituted terpyridine (1.6 Â 10 À7 ). 22,33The differences observed in the dissociation rate constants could be a result of more effective shielding of the bis(terpyridyl) complexes by the 8-arm polymer.This may limit the accessibility of the complexes by metal ions in solution.The dissociation rate constants of bis(terpyridyl) complexes with Ni 2+ and Zn 2+ could not be determined.For the Ni 2+ complexes this is due to their high stability.The Zn 2+ complexes have too low a stability and mainly free ligands are present (equilibrium 1 in Fig. 4 is to the right).
In dilute aqueous solutions (3 mg mL À1 ), 8PEG-TEP 5.4 OH 2.6 forms nano-particles with an average diameter of $10 nm.Please note that intensity plots do not provide ratios of particle size distributions, as larger particles scatter more than small particles.However, such plots give insight into changes in aggregation phenomena that can be caused by structural changes.The particle size of the 8PEG-TEP 5.4 OH 2.8 as measured with DLS appeared similar to that of the 8 armed PEG(OH) 8 revealing that the polymer does not aggregate in water at dilute conditions.Titration of 8PEG(tpy 5.4 )OH 2.6 with the transition metal ions Ni 2+ and Fe 2+ resulted in a distinct shi of the average particle size (Fig. 6).Upon titration, a new distribution of particles with an average size of $200 nm appears, whereas the distribution of the small particles of $10 nm shis to lower values with decreased intensities.A high resolution SEM picture is presented in the ESI (Fig. S6).† The distinct change in size distribution indicates the formation of Ni 2+ -terpyridyl complexes disrupting the hydrophobic interactions between terpyridyl groups and causing rearrangement to minimize the surface free energy.At a Ni 2+ -terpyridyl-groups ratio of 1 : 2 groups the smaller particles are no longer present and addition of an excess metal ions does not result in changes of the particle size distribution.The decrease in the average diameter of the small particles ($10 nm) upon addition of Ni 2+ ions is similar as observed by Chiper et al. on terpyridyl end-group modied pluronics. 27They explained this decreasing average particle size by rearrangement into intramicellar complexes, generating ower type micelles.Here, the distinct change to particles with an average diameter of $200 nm upon titration also reveals a fast rearrangement.Likely the Ni 2+ -terpyridyl complexes formed rearrange to the outer surface of the particles.Both the charge distribution and shielding of charges by PEG may contribute to the stability of the particles (vide supra).
Contrary to the distinct complete change in size distribution upon titration with Fe 2+ or Ni 2+ chlorides, the titration with Co 2+ or Zn 2+ chlorides yields nano-particle suspensions with multiple size distributions (Fig. 7).Similarly as described above, upon the addition of Co 2+ the average size of the small particles is shied to lower values.However, the formation of larger particles is less pronounced.Intermediate sized particles (average diameter of 20-100 nm) are observed during titration at Co 2+ -terpyridyl-groups ratios of 1 : 4 and 3 : 8. Furthermore, small micellar type nano-particles with an average diameter of 5 nm are still present at a Co 2+ -terpyridyl ratio of 1 : 2.Even adding an excess of Co 2+ relative to the amount of terpyridylgroups results in a narrowing of the particle size distribution at 200 nm diameter while the small particle distribution at 5 nm still present (data not shown).The larger variation of the nanoparticle size distribution is likely caused by the relatively low stability of the Co 2+ -terpyridyl complexes compared to the Ni 2+ -terpyridyl and Fe 2+ -terpyridyl complexes in water.Possibly, exchange of Co 2+ ions between terpyridyl groups occurs, allowing the formation of both mono-complexes and bis-complexes into intermediate sized particles.Upon addition of distinct volumes of a Zn 2+ chloride solution, similar changes were observed.The multimodal size distributions observed indicate that both mono and bis-complexes are present at equilibrium.At higher metal ion to terpyridyl-group ratios only small changes were observed in the size distribution of the Zn 2+ and Co 2+ complexes, the distribution at 200 nm becoming more narrow, whereas the distribution of the Fe 2+ and Ni 2+ complexes did not change.The changes in size distribution by complex formation of the terpyridyl-groups with transition metal ions indicate the reorganization of the initially present micelles.The zeta potentials were determined at a metal ion-terpyridyl ratio of 1 : 2 and revealed signicant differences between the particle or particle distributions formed with Fe 2+ or Ni 2+ and those formed with Zn 2+ or Co 2+ .For the latter the determined zeta potentials gave an average positive surface charge of 5 and 8 mV, respectively.The Ni 2+ and Fe 2+ -terpyridyl complexes form only larger particles at this ratio providing a larger positive charge of 17-18 mV.Although particle suspensions prepared by Ni 2+ and Fe 2+ -terpyridine complexation are more stable than those comprising Zn 2+ or Co 2+ complexes, they will still tend to occulate over time as their zeta potential is lower than 30 mV.
Addition of transition metal ions to aqueous solutions of 8PEG(tpy 5.4 )OH 2.6 at concentrations higher than 3 wt% produced hydrogels instead of discrete particles.Gelation took place instantaneously.Previous research on three and four armed poly(ethylene glycol)s having terpyridyl end-groups indicated that stable gels could be obtained with either Co 3+ or Fe 2+ . 26,30but the mechanical properties of the gels were not studied.Considering the large differences in aggregation behavior, we studied the temperature dependent mechanical properties of the hydrogels by oscillatory rheology (see Table 1).The effect of temperature on the gels was assessed by measuring changes in the storage (G 0 ) and loss (G 00 ) moduli between 5 and 60 C applying cyclic temperature measurements.Such cyclic measurements can show that the gels are in equilibrium and may show temperature dependent gelation.The mechanical properties presented in Table 1 are from the last cooling cycle.
The damping factor of the Ni 2+ and Fe 2+ gels varies from approximately 1 Â 10 À3 to 5 Â 10 À3 , which demonstrates that elastic hydrogels were obtained.The much higher damping factor of the Co 2+ gels at higher temperatures and Zn 2+ gels reveals a much more viscous behavior.
As depicted in Fig. 8 the storage moduli of the Ni 2+ and Fe 2+ gels increase with an increase in temperature according the theory of rubber elasticity (eqn (3)).In this equation G 0 is the storage modulus in Pa, r is the cross-link density in moles of elastically effective network chains per m 3 , R is the gas constant (8.314J K À1 mol À1 ) and T is the temperature in K.
Depending on concentration, the Co 2+ gels show a different change in storage and loss modulus with temperature.The damping factors of the Co 2+ gels are approximately twenty vefold higher at 60 C than at 10 C. As illustrated in Fig. 9, the storage modulus of the 10 wt% gel increases with temperature similarly as observed for the Ni 2+ and Fe 2+ gels.Contrary, 5 wt% Co 2+ gels show an initial increase in the storage modulus between 5 and 25 C followed by a decrease upon further heating.This is a clear indication that temperature affects the number of effective crosslinks in the gel.
This becomes even more pronounced for the Zn 2+ gels.Increasing the temperature results in a gel to sol transition.The transition from a gel to sol occurs at approximately at 25 C and is reversible upon cooling.Within the temperature range investigated, the storage modulus decreases with increasing temperatures, whereas the loss modulus increases up to the transition temperature and decreases upon further heating.This trend is witnessed for all Zn 2+ gels at different concentrations.These results suggest that the increasing elastic modulus  upon a temperature increase is counteracted by a decrease in elastic contribution due to dissociation of complexes.Thus, although temperature increases the modulus, the equilibrium cross-link density decreases, resulting in an overall decreasing modulus.Moreover, the high modulus of the Zn-complexes is likely the result of efficient crosslinking.This can be due to the lability of Zn-complexes formed, which allows easy reorganization.This is also reected in the rather high loss modulus and thus less elastic networks formed.At higher concentrations the mechanical properties of the Zn gels are increasing due to a higher crosslink density.
Interestingly, the gel to sol transition temperature of the Zn 2+ gels does not change signicantly with polymer content even though the modulus does increase at higher concentrations.The increasing cross-link density with polymer content resulting in an increased modulus is in accordance with the rubber elasticity theory.The temperature independent and reversible sol to gel transition suggests that more cross-links per unit volume are disrupted upon increasing temperatures in the higher polymer content gels compared to gels with a lower polymer content.At this moment we only can give a hypothesis to this phenomenon.The mechanical stability of hydrogels depends on the number of crosslinks formed and the inherent stability of the supramolecular crosslinks.Assuming the association rate of complex formation is very high, hydrogel properties likely depend on the dissociation rate constant of the complexes which has an Arrhenius type temperature dependency.This is reasonable as the formation of terpyridine complexes is an equilibrium process.Thus, at a certain temperature, the ratio between mono-complex and bis-complex (i.e.cross-links) per unit volume will be independent of the polymer content and free polymer chain ends will be formed more rapidly in respect to temperature in the higher polymer content gels compared to those with a lower polymer content.Therefore, polymer chain mobility will increase more rapidly with temperature, yielding a similar transition temperature for a gel with higher polymer content compared to a gel with lower polymer content.

Conclusions
The differences in dissociation rate constants of a supramolecularly crosslinked terpyridine end-group modied 8-arm  PEG (8PEG-terpyridyl) with transition metal ions is reected in nanoparticle formation at low concentrations and hydrogel properties at concentrations above 5 wt%.Whereas Ni 2+ or Fe 2+ complexes at low concentrations afforded nanoparticles, Co 2+ or Zn 2+ complexes were mainly present as micellar type aggregates.Hydrogels formed by 8PEG-terpyridyl with Ni 2+ and Fe 2+ were elastic and showed minor changes with temperature between 5 and 60 C. Due to the low kinetic stability of the Co 2+ and Zn 2+ complexes, the loss modulus of the hydrogels is largely inuenced by temperature and led for the Zn 2+ complexes to a reversible sol gel transition at 25 C. Importantly, the sol-gel transition temperature of the Zn 2+ complexes was independent of polymer content.

Fig. 5
Fig. 5 Absorption at 556 nm as a function of time of the Fe 2+ to Ni 2+ exchange (squares) and of the Co 2+ to Fe 2+ exchange (circles).Lines represent fits of the measured data.

Fig. 4
Fig. 4 Equilibria involved in the metal ion exchange of bis(terpyridyl) complexes.Rate constants are indicated by k m,n , in which m refers to a mono (1) or bis-complex (2) and n refers to the metal ion as depicted in the figure.