3.3 Results and discussion
3.3.3 Altering core loading
For micellar catalysis numerous parameters can be changed, including the number of micelles, amount of catalyst used and the catalyst loading on the support. To further explore the Michael addition in polymeric nanoreactors, polymers with different catalyst loadings were synthesised (as discussed in Chapter 2, Figure 3.14). The catalyst was incorporated into an amphiphilic block copolymer using post-polymerisation modification where the catalyst was tethered to the hydrophobic
143 block. Once assembled into micelles, the hydrophobic domain (PSt) is protected by the surrounding hydrophilic corona (PDMA).
Figure 3.14: Altering the catalyst loading; red represents the hydrophobic block, blue the hydrophilic block, and the green represents the catalyst, and subsequently assembled to M2.9a-c.
It is possible to calculate the number of micelles in solution and the aggregation number (Nagg) using the particle molecular weight (MW particle). MW particle was
calculated using static light scattering (SLS) at multiple angles (θ) using the Zimm
equation (Equation 3.1 (a)).27 The molecular weight of a particle is determined by
plotting Kc/Rθvsq2; where q2 is the wave vector defined in Equation 3.1 (b), n is the
refractive index of the solvent, and λ is the wavelength of the laser (633 nm). Rθ is
the Rayeigh ratio; this is based on the average scattering intensity (Isample) of the
solution when compared to the intensity of the solvent (Isolvent) and the intensity of a
144
factor calculated using Equation 3.1 (d) while is equivalent to the polymer
refractive index increment, and NA is Avogadro’s constant.
Equation 3.1:The Zimm equation and its derivatives for calculation of Nagg from the Mw of the particle and the polymer (determined by SLS).27, 28
= 3 + 1 + 2 ( ) = sin ( 2) × (4 ) ( ) = − , ( ) =4 ( ) = 1 ( ) = ( )
For each sample, SLS measurements were carried out at a single concentration to produce a partial Zimm plot (Figure 3.15). Extrapolation of the linear fit of the partial Zimm plot to q2 = 0 gives the molecular weight of the particle (MW particle) at a
given concentration (c), as shown in Equation 3.1 (e), hence the first term of the
Zimm equation is equal to zero. By assuming that the second virial coefficient (A2)
is negligible, the third term in Equation 3.1 (a) equals zero, thus allowing for MW
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Figure 3.15: Zimm plot for M2.9a (blue), M2.9b (red) and M2.9c (green) at a concentration of 1 mg mL-1.
Nagg represents the number of polymeric chains per micelle; therefore, a decrease in
the Nagg will result in more nanoparticles forming (per mass of polymer) within the
solutions. Nagg was subsequently calculated using the MW particle and MW polymer,
Equation 3.1 (e). A larger Nagg was observed for M2.9b (Table 3.3), which is
attributed to the shorter PDMA block (DP = 182) in comparison to M2.9a and
M2.9c (DP = 254 and 230 respectively). As a consequence of the shorter hydrophilic
block the corona of these micelles (M2.9b) will have lower curvature allowing for a
higher aggregation number.29 The number of micelles used for the Michael addition
within each solution was calculated using Equation 3.2 (a-b) (Table 3.3).
Equation 3.2: Calculation of the number of micelles using MW particle asdetermined by SLS.
= ( )
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Table 3.3: A summary of the number of nanoparticles in the reaction solution forM2.9a, M2.9b and M2.9c.
Core Loading
% Nagg Number of micelles
M2.9a 1 26 1.9 × 1018
M2.9b 6 115 4.15 × 1017
M2.9c 11 60 7.61 × 1017
As well as a difference in core-catalyst loading these nanoreactors had different Nagg
values, this can potentially affect the catalytic efficiencies of the micelles, as a result
of an increased number of nanoreactors.30
To account for the effects of core loading and the number of nanoreactors present, two methods were adopted for the Michael addition reactions as shown in Figure 3.17. Method A, maintained the number of micelles however the substrate loading was altered to preserve a constant catalyst loading. Method B, decreased the number of micelles present as the core-loading increased, however the same amount of substrate was added and the catalyst loading was maintained at 5 mol%. Both
approaches were carried out with the micellar nanoreactors, M2.9b and M2.9c, this
was used to monitor the effect of both core loading and substrate loading on the micellar nanoreactors.
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Figure 3.16: Preparation of catalytic solutions with 5mol% catalyst concentration with respect to the reaction mixture using micelles of different core loadings. (A) Addition of different concentrations of substrates ensuring the same reaction volume and number of micelles. (B) Changing the number of micelles added yet maintaining a constant volume and concentration of substrate. These schematics are not to scale.
Comparison of M2.9b and M2.9c prepared via method A, therefore resulting in a
similar number of micelles in the reaction, indicated that core loading increased the efficiency of the catalysis. Initial catalysis showed higher conversions with the
M2.9c, as a consequence of the increased number of active sites within the core of the micelle. Similar conversions were found after 24 hours with 5% core loading reaching 85% conversion over 24 hours compared to 83% conversion for the 10% core loading (Table 3.4). These initial micellar solutions were then diluted, decreasing the number of micelles in solution but ensuring the same substrate concentration (method B), and the catalysis activity was then investigated.
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Figure 3.17: The percentage conversion for the Michael addition against time, with (A) M2.9b and (B) M2.9c in triplicate, before dilution (method A, red) and after dilution (method B, blue), in triplicate: error bars represent standard deviation from averaged multiple measurements.
Notably, diluting the system (method B) leads to higher initial conversions (Figure 3.17, blue diamonds). This was attributed to the lower concentration of the
substrates, trans-chalcone and dibenzyl malonate, used in method B (as shown in
Table 3.5). At this lower concentration the substrates can readily disperse into the core of the micelles allowing for catalysis to take place. In contrast higher substrate concentrations (method A) decrease the availability of the TBD catalyst within the core, leading to longer reaction times.
Table 3.4: A summary of the number of nanoparticles in the reaction solution for the diluted and non- diluted micelles (M2.9b and M2.9c) in triplicate.
Micelles Method dibenzyl malonate (mM) Concentration of Number of micelles %Conversion 24 hours
M2.9b A 2.2 × 10-2 4.2 × 1017 85 ± 1.0
M2.9b B 5.7 × 10-3 1.0 × 1017 91 ± 0.6
M2.9c A 4.9 × 10-2 7.6 × 1017 83 ± 1.6
M2.9c B 5.7 × 10-3 7.2 × 1016 99 ± 0.6
149 The core loading of the micelles was subsequently compared. After dilution of the
micelle solutions (method B) containing M2.9b and M2.9c, the molar concentration
of TBD within the Michael addition reaction was the same as for M2.9a (0.0375
mM). Through altering the core loading of TBD within the micellar nanoreactors, increased conversions were observed, Figure 3.18.
Figure 3.18: The percentage conversion for the Michael addition reaction with different catalyst core loadings, with M2.9a (blue diamonds, 1% core loading) M2.9b (red squares, 6% core loading) and M2.9c (green triangles, 11% core loading) against time with 5 mol% catalyst loading, in triplicate: error bars represent standard deviation from averaged triplicate measurements.
It has been hypothesised in the literature that the number of nanoreactors present
within a solution is directly proportional to the rate of catalysis.30 Indeed, we have
shown that dilution of a micellar sample resulted in an increase in the conversion
after 24 hours. Through dilution of the nanoparticles M2.9c, the number of micelles
has decreased by an order of magnitude, yet the conversion increased from 83% after 24 hours to 99%. Hence decreasing the number of micelles has given rise to higher conversions, which is contradictory to previous reports.30 We attribute this to the
150 greater accessibility of the catalyst to the substrate. Moreover, increasing the core loading has led to a prominent increase in activity of the micellar nanoreactors; when changing the core loading from 1% to 11% the conversion after 24 hours has increased from 46% conversion to 99%.
We can summarise that, for this system, increasing catalyst loading within the core improved accessibility of TBD for catalysis. Based on the information garnered about these micellar nanoreactors up to this point, we propose this covalently bound catalysis is now limited by two factors: diffusion of substrates into the core and the accessibility of the catalyst.