Probing Dense Packed Limits of a Hyperbranched Polymer through

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Probing Dense Packed Limits of a Hyperbranched Polymer through Ligand Binding and Size Selective Catalysis Adam Ellis and Lance J. Twyman* Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K. S Supporting Information *

ABSTRACT: In the area of dendritic chemistry (hyperbranched polymers and dendrimers) it is often generalized that dendrimers are the molecule of choice for smart, selective, or technical applications involving encapsulation or controlled/ selective environments. This is despite the fact that hyperbranched polymers (HBP)s are generally easier and cheaper to synthesize, making them more amenable to large-scale applications. Dendrimers have been successful in these applications by virtue of a dense packed or dense shell limit. This paper describes the synthesis of a series of narrowly dispersed HBPs possessing binding and catalytic cores with a high and uniform loading. Subsequent binding experiments clearly demonstrated the existence of a dense packed limit with respect to polymer molecular weight and ligand size. A series of catalytic experiments were also performed in an attempt to exploit these molecules and their dense packed limit to the area of shape/size selective catalysisan area where dendrimers have previously been used with celebrated success. However, although we were able to show the existence of a dense packed limit, we were initially unable to demonstrate any selectivity based on substrate size or shape. Nevertheless, further studies into core branching motif and multiplicity eventually enabled us to obtain a series of HBPs capable of perturbing the shape/size selectivity of a simple oxidation reaction involving two alkenes. Specifically, we were able to demonstrate a 3.5-fold shift in chemoselectivity toward a smaller alkene of lower reactivity. These results compare favorably with those obtained using dendrimers and allow us to conclude that, with careful thought regarding core design, HBPs are indeed capable of being applied to technical/smart applications involving controlled and selective environments.

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INTRODUCTION Although hyperbranched polymers (HBPs) can be applied to a range of applications,1 it is generally perceived that the most high-end or technical applications are reserved for dendrimers.2 We wanted to challenge this idea and determine whether or not this generalization was valid. We were particularly interested in investigating applications that exploit the use of dense shell/ packing3 properties when applied to selective encapsulation, release, or catalysis. To do this, we need to identify the specific structural and design requirements necessary for HBPs to operate at a similar level of performance when applied to the same high-end or technical applications normally reserved for dendrimers. These structural and design properties are controlled by the synthetic methods used to construct the relevant macromolecule (dendrimer or HBP). Dendrimers are synthesized using stepwise methods.4 Although this is often tedious and time-consuming, it does lead to a series of well-defined structures known as generations. These molecules are monodisperse with regards to molecular weight, structure, and degree of branching (100% by virtue of complete and controlled reactionsi.e., all branching points have reacted). Furthermore, as each dendrimer possesses a unique molecular weight and therefore size, a dendrimer series can be considered quantized with respect to its generations.5 As such, the internal environment, including any packing and © XXXX American Chemical Society

closed shell properties, are switched on or off as we move from one particular generation to the next. This level of precision makes it relatively easy to control local environment (electronic and steric) and results in high selectivity and control in a range of applications.6 A particularly good exemplifier of how this controlled structure can be exploited was described by Suslick and Moore.7 Suslick and Moore showed how porphyrin cored dendrimers could be used to mimic the reactivity of cytochrome P4508 and catalyze the oxidation of various alkenes in a shape/size selective way. The key experiment involved taking a 1:1 mixture of the two substrates and catalyzing the reaction using the porphyrin cored dendrimers. The two substrates used were a large/fat substrate with a high relative reactivity and the second smaller/thinner substrate possessing much lower relative reactivity. When catalyzed by a dendrimer, the results showed that selectivity shifted toward the smaller substrate reactivity, despite it possessing a lower inherent reactivity (compared to the larger substrate). This change in selectivity was by virtue of the dendrimer’s structure, which created a steric barrier around the porphyrin and through which the smaller substrate could pass through more easily. Received: June 27, 2013 Revised: August 1, 2013

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dx.doi.org/10.1021/ma401344d | Macromolecules XXXX, XXX, XXX−XXX

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Conversely, and despite being inherently more reactive, the larger substrate had a more difficult journey to the central porphyrin catalyst. These result demonstrated the ability of a core functionalized dendrimer and, more specifically, how packing around a catalytic core could be exploited to influence selectivity. If HBPs were to act in a same way to the dendrimers described by Suslick and Moore, they would require a similar level of control with respect to their structure. In particular, we would need a polymer that possessed a steric or dense packed limit with respect to molecular weight. However, HBPs are synthesized using a much simpler procedure involving a branching monomer and a single-step random polymerization procedure, generating a mixture of molecules differing in molecular weight, branching architecture, and 3D structure.9 The resulting structures are poorly defined, and any change in internal steric or electronic environment (i.e., dense shell/ packing properties) is gradual and poorly defined with respect to size or molecular weight. It is this lack of control regarding the internal environment that ultimately leads to poor performance in selective encapsulation, release, or catalytic applications. Nevertheless, we proposed to investigate whether or not a well-defined hyperbranched polymer could be applied as enzyme mimetics6,10 capable of replicating the same levels of shape and size selectivity demonstrated by the dendrimers reported by Suslik and Moore. The specific aims of the work described in this paper are threefold. Our initial aim was to obtain a narrowly dispersed, pseudo-generational series of HBPs possessing a catalytic/ binding core in each and every polymer molecule. Having achieved this, we would then determine whether or not (and at what point) an observable and well-def ined steric or dense packed limit existed. The steric properties of the HBP series would be assessed by studying ligand binding to the central core. Finally, catalytic experiments would be carried out either side of any steric limit to test for possible shape/size selectivity. If these aims could be achieved, then we would have demonstrated that, in principle, HBPs can be applied to the more technical and high-end applications usually reserved for dendrimers.

effects observed with dendrimers. As a consequence of the controlled synthetic procedure used to construct dendrimers, it is trivial to place a catalytic group at the core.12 Specifically, the core can be used to start or end the synthetic sequence, ensuring total incorporation (after purification). This is not the case for hyperbranched polymers, where the core is often distributed unevenly across the full molecular weight range or is concentrated in a particular molecular weight fraction.13 For any study involving core incorporation and isolation, it is essential that the core distribution is controlled. Although it is desirable for each and every polymer molecule to possess a core, it is not essential. However, it is paramount that the core is distributed evenly across the polymers molecular weight distribution, as this allows us to compare a series of HBPs that differ in molecular weight (pseudo-generations).14 In a conventional one-pot hyperbranched polymer synthesis carried out under kinetic conditions, an even distribution of core molecule cannot be guaranteed, with core units being distributed randomly across the molecular weight range (or not incorporated at all). Despite this difficulty there are a number of methods that have been reported for core incorporation.15 In our case we have used reversible reaction conditions to control core incorporation with respect to distribution and final molecular weight. A reversible reaction is dominated by thermodynamics and will result in a statistical distribution of core units across all molecular weights. If the conditions are such that the equilibrium is on the side of the products, then in principle a core molecule will be incorporated in each and every polymer molecule. We approached this problem by applying a reversible/equilibrium procedure using a transesterification procedure. We elected to use 3,5-diacetoxybenzoic acid 116 as the AB2 monomer as it generates the same repeat unit used by Suslick and Moore when synthesizing their shape/size selective catalytic dendrimer.7 As such, the electronic environment created within our HBP will be very similar to that generated in Suslick’s dendrimer. The AB2 monomer 1 was synthesized from 3,5-dihydroxybenzoic acid by reaction with acetic anhydride (Scheme 1).15,16 Tetraacetoxyphenylporphyrin 3 was selected

RESULTS AND DISCUSSION Synthesis of Tetra(acetoxyphenyl)porphyrin Cored Hyperbranched Polymers. Because of the lack of control regarding the synthesis of HBPs, dense packing will be dependent on a number of different and variable properties. These include molecular weight, which is effectively a measure of size, which in turn controls the packing density. However, this is not necessarily going to affect dense packing in the same way observed for dendrimers and is due to the reduced degree of branching that occurs with HBPs.11 This reduction in branching results in a more flexible and open structure with less geometric constraints. As such, any dense packing is likely to occur at a higher molecular weight for HBPs in comparison to dendrimers. Another important parameter to be considered is polydispersity. In a polydisperse solution there will be a mix of polymers with a range of molecular weights and sizes; inevitably, some of these will be below the dense packing limit. This is not an issue for dendrimers, whose controlled stepwise synthesis ensures a monodisperse structure. A final problem relates to the level of core or catalyst incorporation. This is probably the most significant consideration when comparing the properties of a hyperbranched polymer and a dendrimer, particularly when trying to mimic the generational

Scheme 1. Synthesis of 3,5-Diacetoxybenzoic Acid

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as the central porphyrin unit as it contains four acetoxy groups, each of which can react with the carboxylic acid group on the AB2 repeat units (via a reversible transesterification process). The porphyrin 3 was synthesized using 4-acetoxybenzaldehyde 2, which was obtained after acetylation of 4-hydroxybenzaldehyde using acetyl chloride and triethylamine (Scheme 2).17 Scheme 2. Synthesis of 4-Acetoxybenzaldehyde

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HBP that possessed a p-nitrophenol core with a 100% level of incorporation.3 Specifically, 3,5-diacetoxybenzoic acid 1 was polymerized with porphyrin 3 in a 20:1 molar ratio using diphenyl ether as solvent (Scheme 4). The mixture was placed in a round-bottom flask and heated to 225 °C for 45 min. The temperature was then lowered to 180 °C and the pressure reduced to 5 mmHg for 4 h. Placing the flask under reduced pressure allows the removal of the acetic acid byproduct, driving the equilibrium in favor of the product. Once the reaction time was complete the crude mixture was dissolved in refluxing tetrahydrofuran and precipitated into a large excess of cold methanol to give the porphyrin cored polymer, TAPPHBP 6 in 65% yield by mass. The presence of porphyrin in the polymer sample was immediately apparent from the reddishbrown color of the product and was substantiated by 1H NMR results, which showed small coincident sharp and broad resonances at chemical shifts corresponding to the porphyrins aromatic protons at 8.89 ppm. This suggested a mixture of “f ree” and incorporated porphyrins with similar evidence provided by GPC. In addition to a broad peak for the polymer, a second peak was seen at the low molecular weight end of the chromatogram. This peak was small accounting for 6% of the total peak area. GPC analysis using a fixed wavelength UV detector at 418 nm indicated that this peak was indeed from free porphyrin and at the same time demonstrated the presence

The porphyrin was then synthesized as shown in Scheme 3, using the methods developed by Rothemund18 and Adler and Longo.19 Scheme 3. Synthesis of TAPP 3, Zn-TAPP 4, and Fe-TAPP 5

Having obtained the building blocks, the next step was to synthesize the porphyrin cored hyperbranched polymer. As previously mentioned, a reversible strategy was used to ensure the porphyrin core was evenly distributed across all molecular weights. We have previously used this strategy to synthesize a

Scheme 4. Synthesis of Porphyrin Cored Hyperbranched TAPP-HBP 6 and Metalated Derivatives

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of porphyrin groups in the polymer itself. Unfortunately, due to insolubility of the porphyrin in methanol, it was not possible to separate “free” porphyrin from the polymer through purification of the polymer using the established trituration procedure. Thus, chromatography using an alumina column was used to separate the free porphyrin from the rest of the polymer mixture. Subsequent analysis confirmed removal of free porphyrin. GPC now showed a single broad trace covering molecular weights greater than that of the free porphyrin, and the sharp peaks corresponding to unincorporated porphyrin were no longer present in the 1H NMR. As well as the porphyrin resonances in the aromatic region, a strongly deshielded peak at minus 2.03 ppm was also seen for the NH protons. Along with a visible Soret band in the UV spectra,20 this is a diagnostic characteristic associated with all porphyrins.21 The resulting polymer sample possessed a molecular weight of 11 700 and polydispersity of 1.90. Mass spectrometry was carried out on the bulk polymer sample, although problems with disproportionate ionization meant that peaks were not observed above ∼2000 mass units. The resulting spectrum did however demonstrate full incorporation of the porphyrin core for the lower molecular weight fractions. Peaks were observed at 178 mass unit intervals, with each peak corresponding to polymers consisting of n monomer residues and a single porphyrin core (674 + n178). Peaks from cyclized and “coreless” polymers were not observed. Although mass spectrometry provided direct evidence for 100% core incorporation, it was only true for the low molecular weight fractions (mass spectrometry on the higher molecular weight fractions was inconclusivesee above). To prove 100% incorporation across all molecular weights, the polymer needed to be fractionated into a series of different sizes and the level of core incorporation determined for each (using a non-mass spectrometric method). Fractionation was also important with respect to achieving the main aims of this work, specifically ensuring a series of narrowly dispersed “pseudo” dendrimers where potential dense packing properties could be observed. The polymer was therefore fractionated using preparative GPC in the form of a column packed with SX-1 Biobeads.22 The extent of core incorporation was quantified by comparing the polymer’s molecular weight determined using the polymer’s bulk property (i.e., GPC) with the molecular weight obtained from a core group analysis (i.e., UV of the porphyrin core). Because of the way these values are calculated, it is unlikely that these two values will be identical. For example, it is well-known23 that GPC calibrated using linear standards underestimates molecular weight of a globular hyperbranched polymer. As such, the molecular weight cannot be lower than that obtained using GPC and can be referred to as Mnmin.15 On the other hand, it may be possible that molecular weights determined using a core unit analysis may be overestimated. This is due to the assumption that ALL polymer molecules will possess a core unit. If this is not the case, then polymers without core units will effectively contaminate the mixture and increase the relative proportion of repeat units relative to core units. Therefore, in the case of core analysis the molecular weights obtained represent a maximum possible value and are referred to as Mnmax.15 The data obtained for all polymer fractions are shown in Table 1. The apparent level of incorporation for each fraction can be obtained by dividing Mnmin by Mnmax; the results are also shown in Table 1. The ratio of the two molecular weights indicates a level of incorporation around 60% and is consistent across all fractions, demonstrating

Table 1. Minimum Core Incorporation Calculated from GPC and UV Analysisa fraction

Mnmin (GPC)

Mnmax (UV)

minimum core incorporation (%)

bulk 1 2 3 4

8500 3000 5500 11000 15100

14250 5000 9500 18500 24500

59 60 58 61 62

a

Mnmin by GPC is a function of the minimum number of polymer molecules, and Mnmax by UV is a function of the minimum number porphyrin units. Thus, dividing Mnmin by Mnmax gives the minimum level of core incorporation. These are expressed as percentages.

the desired even distribution for the core molecule. Furthermore, having already proven 100% incorporation for the low molecular weight fractions (via mass spectrometry), it follows that the bulk polymer sample must also possess a core incorporation approaching 100%. The two protons meta to the carboxyl function exist in a number of different environments and resonate as a series of signals between 8.03 and 7.83 ppm in the 1H NMR spectrum. There are three well-defined peaks at 7.50, 7.35, and 7.20 ppm corresponding to the para protons, which are present in three distinctive environments. These are the dendritic, linear, and terminal repeat units; integrating these peaks and applying Fréchet’s equation returned a value of 50% for the degree of branching. After polymerization the porphyrin’s pyrrolic resonance could be seen as a singlet at 8.89 ppm. Although only slightly shifted from the position of the same proton peak in the starting material, a split or unsymmetrical peak pattern was not observed. A similar situation was also observed for the porphyrin’s aromatic peak at 8.24 ppm. As such, there is no clear evidence for unreacted acetoxy groups (on the porphyrin) and confirms that all four of the porphyrin’s acetoxy groups have reacted with monomer. However, the degree of polymerization from each site will vary dramatically, and a variety of structures will exist. If we consider two extremes, they are: (a) A polymer with four large and roughly equal HBP sections (one from each of the pophyrin’s acetoxy groups). This structure is similar to that of a core functionalized dendrimer. (b) A polymer with one very large and dominant HBP section (with a degree of polymerization equivalent to 4 times that observed for each HBP section in (a) above). This structure is more like that observed for dendrons (Figure 1).

Figure 1. For a single molecular weight there are a number of possible structures with respect to polymerization. Two extreme structures are shown above: (left) a tetrafunctionalized polymer possessing four roughly equivalent HBP sections; (right) a monofunctionalized polymers with a large HBP section (di- and trisubstituted structures as well as a number of other nonsymmetrical structures are also possible). D


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as the first was not completely reliable or reproducible (extremely dependent on vacuum and temperature); fractionation would also reduce the polydispersity. Therefore, the bulk hyperbranched polymer was resynthesized as described above using a 40:1 molar ratio of AB2 monomer 1 and the porphyrin unit 3. Zinc was inserted into the porphryin core using excess zinc acetate at room temperature in DCM. Purification was achieved by filtration to remove unreacted zinc acetate followed by reducing the volume of solvent by rotary evaporation before trituration into excess methanol to yield the zinc inserted hyperbranched polymer Zn-TAPP-HBP 7 in an overall yield of 62% by mass (Scheme 4). Confirmation of successful insertion was provided by 1H NMR, in which the highly shielded peak at minus 2.03 ppm was absent; this corresponds to the inner protons that are removed upon metal insertion. Further confirmation of successful functionalization was provided by UV/vis spectrometry in which the four Q-bands of the starting material at 515, 548, 592, and 648 nm were replaced by two resonances at 548 and 589 nm. For control experiments zinc was also inserted into the free base porphyrin 3 yielding the zinc porphyrin Zn-TAPP 4, using a similar procedure (Scheme 3). Prior to performing the UV/vis titrations, it was necessary to fractionate and purify the zinc functionalized polymer. Preparative size exclusion chromatography was employed in the form of a glass column (3 cm diameter) using SX-1 Biobeads and DCM as the eluent. This process allowed (500 mg of) the polydisperse bulk hyperbranched polymer to be separated into more discrete molecular weights with lower polydispersities, and the data are shown in Table 2.

Nevertheless, due to the same back folding properties observed for dendrons,24 the porphyrin units will remain encapsulated by the polymer matrix, and a very similar environment will exist for all possible structures (of similar molecular weight). Although it was not possible to control the molecular weight precisely via the core/monomer ratio, it was possible to influence the molecular weight by altering this ratio in favor of whatever product was desired. As described above, when using a 20:1 ratio of AB2 monomer relative to the porphyrin core a polymer with an Mn of approximately 8500 was obtained. If the ratio was doubled (to 40:1), then a polymer with roughly double the molecular weight was obtained (Mn of around 18 500). While this ratio could be used to favor a certain approximate molecular weight, the temperature, reaction time, and the quality of vacuum were paramount. For example, if the vacuum was poor, a relatively small polymer would be produced regardless of the monomer/core ratio. This was exemplified when a polymer with a very low Mn value of 2000 (and a PD of around 4.0) was obtained when using a 20:1 ratio. This was much smaller than polymers obtained from similar experiments (using the same 20:1 ratio) and was probably due to a poor vacuum resulting in inefficient removal of the acetic acid byproduct. However, the product was returned to a reaction flask containing solvent (diphenyl ether), and the mixture was heated back up to 180 °C and a vacuum of 5 mmHg applied. When the product was analyzed, an Mn value similar to that recorded from previous experiments was obtained (around 9000). These experiments confirm the reversible nature of the reaction and how this can be used to generate hyperbranched polymers that possess specific and functional units with very high levels of incorporation. As described above, this is a key requirement when comparing the properties of hyperbranched polymers with those of dendrimers. Probing Dense Packing of the Zn-TAPP-HBP through Ligand Binding Studies. After confirming the reversibility and high/even levels of core incorporation of the hyperbranched polymer system, investigations were conducted to determine any site isolation or dense packing properties of the resulting polymers. Specifically we wanted to determine whether there was a sudden break in binding properties, similar to that observed for dendrimers at their dense packed or dense shell limit,3 or a linear or nonspecific relationship between binding and molecular weight, as is the case for simple linear polymers.25 The presence of a porphyrin at the center allows for direct monitoring of access to the core through UV binding experiments. This can be achieved using zinc metalloporphyrin cored polymers and studying the binding of various nitrogen ligands, enabling any trends to be identified relative to ligand size. In addition, analysis of binding affinity to a particular polymer fraction with respect to the different sized ligands would indicate whether the hyperbranched system was capable of excluding larger ligands while allowing access to smaller substratesa requirement for future size selective catalysis experiments. However, the first requirement was to obtain a pseudo-generational series of hyperbranched polymers possessing a zinc metalloporphyrin core. One approach would be to carry out a number of polymerizations using an increasing monomer to core ratio. Alternatively, a bulk metalloporphyrin polymer could be synthesized and then fractionated using the chromatographic method described above and separate the polymer into the required pseudo-generational series of hyperbranched polymers. We opted for this second method

Table 2. Pseudo-Generational Series of the Zn-TAPP-HBP 7a theoretical values for equivalent dendrimer

MW

repeat units

generation

1450 2874 5722 11418 23522

4 12 28 60 128

1.0 2.0 3.0 4.0 5.0

experimentally determined values for fractionated Zn-TAPP-HBP-5

fraction

MW (GPCMn)

repeat units

pseudogeneration

1 2 3 4 5 6 7

1650 2150 5900 8750 11500 13900 16800

5 8 29 45 62 75 91

1.0 1.5 3.0 3.5 4.0 4.5 5.0

a

Half-generations were assigned to hyperbranched polymers when the number of terminal groups and the molecular weights fell between those for the full generations. Values for the equivalent dendrimer are also shown for comparison.

It is clear from the table that early generations are very close in molecular weight; this placed a limit on the fractionation procedure. The narrow molecular weight divergence at low generations is irrelevant for quantized dendrimers but does create a difficulty for nonquantized poly dispersed hyperbranched polymers. By taking the molecular weight and estimating the number of repeat units for each fraction, we were able to obtain a series of pseudo-generations. These values, along with those corresponding to the equivalent dendrimers, are shown in Table 2. Inevitably, overlap exists for some of the lower molecular weight fractions due to the similar molecular weight values moving from one generation to another (this is less of a problem for higher generations as E


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the divergence of molecular weights becomes more marked between each generation). Consequently, a judgment was made as to which pseudo-generation each fraction belonged. This dictated that some fractions are not included in the data set, and fractions with ambiguous molecular weights were not used; only fractions with molecular weights corresponding closely to their pseudo-generation are included. Half-generations were assigned to hyperbranched polymers when the number of repeat units and the molecular weights were between the theoretical full generations (G1.5, G3.5, and G4.5). Once the zinc inserted polymer had been fractionated and characterized, the next step was to probe whether or not this polymeric system was capable of dense packing, core isolation, and size selectivity. Therefore, binding experiments were carried out in the form of UV/vis titrations using pyridine and other pyridine derivatives.26 Binding experiment would be carried out using the fractionated hyperbranched polymers and a series of ligands with different molecular volumes or size. If dense packing was observed at different molecular weights for this series of ligands, this could indicate a selective capability of these hyperbranched polymers with respect to size. The specific pyridyl ligands used must differ in size and must be free of groups in the ortho positions which may block the interaction. Based on these criteria, the following ligands were chosen: pyridine, 3,5-lutidine, and 3-phenylpyridine. These are shown along with their space-filling models in Figure 2. The titrations

Figure 3. Plots showing how pyridine binding affinity changes with respect to polymer molecular weight for the Zn-TAPP-HBP 7. All data normalized for binding to the core porphyrin (Zn-TAPP 4) and also normalized relative to each ligand. (For reference, the Ka values were 8600, 11 400, and 4950 M−1 for pyridine, 3,5-lutidine, and 3phenylpyridine, respectively.)

a superior polar environment, with the additional support of favorable π−π stacking interactions,27 creating a partition effect. A similar effect on microenvironment has been reported for polyether dendrimers.28 The pattern is the same for all ligands; at an Mn value of around 8500 the binding constants become smaller and continue to decrease in a linear fashion as the polymer’s molecular weight increases. This is clear and obvious evidence of dense packing. The molecular weight at which dense packing occurs can be determined by placing a best fit line through the two obvious linear regions of each plot. The molecular weight at which they cross is the dense packing limit. For all ligands the dense packed limit occurs at an Mn value of around 7200 (±500) or between G3.0 and G3.5. Interestingly, this is the same point at which dense packing often occurs for dendrimers.29 However, due to the polydispersity of hyperbranched polymers, any dense packed limit is likely to occur over a much broader range than that observed for the quantized dendrimers. For hyperbranched polymers it is probably more appropriate to describe dense packing limits with respect to a molecular weight range. Therefore, in the case of binding to our hyperbranched polymer ZnTAPP-HBP 7, dense packing occurs between molecular weights (Mn) of 6000 and 8000 (for the ligands studied). It is interesting to note that despite the increase in size for the largest ligand (3-phenylpyridine), the dense packed limit is the same. This initially seemed to suggest that shape or size selectivity does not take place. However, if the binding is studied in more detail, it is noticeable that Ka does not plummet to zero and reasonable binding was possible for all ligands across the molecular weight range studied. However, it is also noticeable that the rate of change in Ka with respect to molecular weight is greater for the larger 3phenylpyridine ligand. Specifically, when comparing Ka for the porphyrin core with that obtained for the pseudo-fourthgeneration polymer studied, we observe a 11−12% reduction in binding for the pyridine and 3,5-lutidine ligands. This contrasts to a much larger 29% reduction in binding strength for the 3phenylpyridine ligand, which is significant and does suggest an

Figure 2. Pyridyl ligands used to probe steric, electronic, and dense packing properties of the porphyrin HBPs. These ligands difference in size, electronics, and hydrophobicity.

were performed using a dichloromethane solution of Zn-TAPPHBP 7 and titrating in 10−20 μL aliquots of a pyridyl ligand solution and recording a UV/vis spectrum after each addition. The changes in absorbance for the bound peak were plotted against the concentration of ligand added, and the binding constant, Ka, was determined by fitting the data points to a 1:1 binding isotherm.26 A number of repeats were carried out for each Mn (pseudo-generation), and a value for Ka was determined for each. These were then averaged with respect to molecular weight. Plotting the results with respect to generation would involve the use of a nonlinear scale in the xaxis, with equal distances between each generation. However, when moving across a generational series the molecular weight differences get larger as the generations get higher and a generational plot would not necessarily provide an accurate picture of events. Therefore, our binding results are plotted with respect to Mn (Figure 3). This plot shows a small increase in the binding constant from the free porphyrin core (effectively G0) to the G3 polymer with molecular weight 5900. Although this could be due to error, it is also possible that the polymer is starting to generate a good environment for the pyridine ligands, presumably by providing F


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Table 3. Binding Data of Pyridyl Ligands to Zn-TAPP-HBP 7 ligand

Ka porphyrin core (M−1)

Ka of HBP G4 (Mn ∼ 11 500) ( M−1)

onset of dense packing (Mn)

reduction in bindinga (%)

pyridine 3,5-lutidine 3-phenylpyridine

8600 (±400) 11400 (±850) 4950 (±350)

4700 (±200) 6900 (±500) 1000 (±75)

7200 (±500) 7100 (±500) 7200 (±500)

11 (±5) 12 (±7.5) 29 (±3)

a

Calculated by comparing the Ka values obtained from the porphyrin core unit (Zn-TAPP 4) and the pseudo-fourth-generation Zn-TAPP-HBP 7 (Mn 11 500).

substrate 9 to occur equally well with a catalytically functionalized polymer above or below its dense packed limit. Conversely, we might predict that the larger cyclic alkene substrate 11 would struggle to reach the catalytic core of a hyperbranched polymer whose molecular weight was above the dense packed limit, resulting in a slower reaction and a lower yield. We proposed to study these reactions using the same method reported by Suslick and Moore,7 specifically using iodosylbenzene as the oxygen source and a metalloporphyrin as the catalyst. Although zinc porphyrins were used to study and probe binding, they are not suitable as oxidation catalysts. There are a number of suitable metals that can be inserted into a porphyrin;30 in this case iron (Fe(III)) was used as the products are robust and make good oxidation catalysts.31 We therefore set about synthesizing Fe(III) metalloporphyrin32 cored hyperbranched polymers above and below the dense packed limit as well as aiming for a polymer with a molecular weight close to the dense packed limit. For control and reference experiments we would also required the simple Fe(III) metalloporphyrin 5, which was synthesized as shown in Scheme 3. Insertion of iron into free base porphyrin 3 was achieved by reaction with a large excess of iron(II) chloride in the presence of 2,6-lutidine.33 After refluxing for 4 h under nitrogen, the mixture was exposed to air as it cooled, oxidizing the Fe(II) to Fe(III). After work-up and column chromatography, the mass spectra showed a peak corresponding to the target molecule, but also a series of peaks above the molecular ion. This reaction was repeated a number of times, but on every occasion an identical mass spectrum was obtained. It was assumed the extra peaks were due to an unknown species strongly ligated to the metal center. Therefore, the porphyrin was dissolved in dichloromethane and washed with 1 M hydrochloric acid. The organic phase was collected, the solvent removed and the product reanalyzed. On this occasion the EI mass spectrum contained a single dominant peak of the molecular ion and no peaks of a higher value. Further authentication for the product Fe-TAPP 5 was provided by UV/vis analysis, which showed a reduction in the number of Q-bands from four to two, a diagnostic change for a metal inserted porphyrin. Because of the paramagnetic iron, substantial broadening of the peaks occurred in the 1H NMR spectrum.34 With the porphyrin itself metalated, it was also necessary to insert iron into the porphyrin cored hyperbranched polymer. The polymerization was repeated using the unmetalated porphyrin and metal insertion performed after purification (Scheme 4). The synthesis was performed in a similar manner to that described above for the free porphyrin, using iron(II) chloride and 2,6lutidine; however, after refluxing TAPP-HBP 6 for 4 h the solution was filtered through a short plug of silica to remove unreacted iron chloride, washed with dilute HCl, and precipitated into excess methanol to give Fe-TAPP-HBP 8 in good yield. GPC analysis carried out after the acid wash returned an Mn value of 12 100, which was almost identical to

element of shape and/or size selectivity with respect to ligand size and the polymer’s molecular weight; the data are summarized in Table 3. Probing Shape and Size Selectivity of the Fe-TAPPHBP via Catalysis. The binding data presented above confirmed the porphyrin cored hyperbranched polymers possess a dense packed limit at a molecular weight around 8000. The data also suggest the polymers were capable of size selective binding. That is, at a molecular weight below 8000 a relatively open structure persists, and this allows unhindered access to the core, whereas at higher molecular weights (above 8000) the polymer possesses a tighter more compact structure and access to the core is restricted. This effect is more pronounced for the bigger polymers and larger ligands. Therefore, assuming all other things are equal, it should be possible for smaller substrates to access the core in preference to larger substrates. We now wish to exploit this effect and study the possibility of size selective catalysis. Suslick and Moore demonstrated the selective capability of similar dendrimers using a mixture of linear and cyclic octenes.19 In an attempt to make a valid comparison between dendrimers and hyperbranched polymers, we decided to study the same catalytic reaction (Scheme 5), that is, the catalytic epoxidation of a linear alkene 9 and a cyclic alkene 11, in isolation and together in equal ratios. Scheme 5. Iron Porphyrin Catalysed Oxidation of Small/ Thin Alkene 9 and a Larger/Fat Alkene 11

The specific substrates selected were cyclooctene, which is a relatively large substrate, and 1-octene, which is a small (thin) linear alkene. The linear substrate has a terminal alkene, which is relatively electron poor and therefore much less reactive than the cyclic alkene, which is more electrophilic. This increase is due to the higher level of substitution and increased sigma donation, leading to a more electron-rich alkene compared to the equivalent linear system. This is an ideal scenario when trying to study the effect of size on reactivity, specifically when trying to encourage the reaction between a catalyst and a small but unreactive substrate, in the presence of a more reactive but larger substrate. To test this, reactions were carried out on both substrates using catalytically functionalized hyperbranched polymers below and above their dense packed limit. If a size/ shape selective reaction is possible, we would expect any reaction using the smaller (but unreactive) linear alkene G


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the Mn of TAPP-HBP 6 (11 700), confirming that the structure remained intact during metalation. As before, UV/vis analysis showed that the number of Q-bands had been reduced from four to two, confirming iron insertion. Because of the size of the polymer, which effectively shields the central iron, line broadening in the 1H NMR was significantly reduced, and some structural information remainedthe most important of which was the disappearance of the free base porphyrin NH peak at −2.03 ppm, confirming metal insertion. The bulk Fe-TAPP-HBP 8 was fractionated using a Biobead column, and the molecular weights of each fraction were analyzed by GPC. Fractions were required with Mn values above and below the dense packed limit, as well as one with an Mn reasonably close to the dense packed limit (previously shown to be somewhere between the G3.0 and G3.5 pseudo generation). Table 4 below shows the fractions selected for the

Now in a position to study the catalytic properties of the hyperbranched polymers, the individual oxidation of each alkene with each polymer fraction (or porphyrin core) was studied. The reaction for the linear alkene 9 is shown in Scheme 7. It is important to ensure that all catalyst/porphyrin concentrations are equivalent when studying a series of polymers. Therefore, the concentration of all solutions with respect to the amount of porphyrin was checked by UV spectroscopy and adjusted if necessary, to ensure equivalent porphyrin concentrations for all polymer fractions (see Experimental section in the Supporting Information). As well as the dominant epoxide products, a number of other oxidation products can also form, either directly or after initial formation of the epoxide. Therefore, the reactions were monitored by following the reduction product, that is, the conversion of iodosyl benzene to iodobenzene (which is a measure of all oxidation reactions catalyzed by the porphyrin).36 A second series of experiments would study the product yields when each polymer (and core) was reacted with an equimolar mixture of both alkenes. This set of experiments would indicate whether or not the hyperbranched polymers could influence a shape/size selective reaction. Gas chromatography was used to quantify the outcome of the catalyzed oxidation reactions by monitoring the epoxide and the iodobenzene; however, it was first necessary to calibrate the instrument using the alkene starting materials, the epoxide products, the iodobenzene side product, and an internal standard (n-dodecane). An initial blank experiment indicated that iodosylbenzene reacted with the porphyrin in the absence of alkene (producing iodobenzene). However, it has been shown that if a large excess of alkene is used, then this side reaction is suppressed (effectively saturating the rate of alkene oxidation).37 A second blank experiment was carried out to ascertain if the alkenes could react with the oxygen source in the absence of porphyrin catalyst. The GC results showed that although oxidation took place, the yields were extremely low and not at a level that would effect the results from the catalytic experiments. With the instrument calibrated and the blank experiments performed, investigation into the activity of the porphyrin catalyst could begin. The yield of oxidation products when using just the core porphyrin unit (i.e., pseudo-generation G = 0) were 74% and 96% for the linear and cyclic alkenes, respectively.38 This indicates that the cyclic alkene is more reactive, which is consistent with previous results and the inherent differences in reactivity between a mono- and disubstituted alkene.39 To make comparisons easier, the yields of the linear and cyclic species are normalized relative to the value obtained using the reference porphyrin (Fe-TAPP 5, i.e., the G = 0 pseudo-generation) and plotted against pseudogeneration (Figure 4).40 The results for the linear alkene show that the activity of G2, G3, and G4 remained similar to that of the free porphyrin G0 (within the 10% error). However, for the cyclic alkene, there appears to be a steady decrease in yield and therefore activity.

Table 4. Theoretical Values for Pseudo-Generations vs the Values for Obtained Fractions of Fe-TAPP-HBP 8 repeat unitsa

molecular weight

a

pseudo-generation

theoretical

obtained

theoretical

obtained

0 (Fe-TAPP) 2 3 4

901 3038 5888 11588

901 3200 5400 12100

0 12 28 60

0 13 25 63

Rounded to the nearest integer.

catalytic experiments along with data regarding the number of repeat units and the corresponding pseudo generation. The core unit would be used as reference and control catalyst during the future epoxidation and catalytic experiments and is also included in the table. Iodosylbenzene 13, was used as the oxygen source for the catalytic reactions and was synthesized as shown in Scheme 6. The synthesis is trivial and iodosylbenzene Scheme 6. Synthesis of Iodosylbenzene 13

was prepared by the stirring iodobenzene diacetate in 3 M sodium hydroxide for 45 min.35 The solid obtained was collected and thoroughly washed with water followed by chloroform, if left to stand for a few days the product can decompose (and is particularly sensitive to prolonged exposure to light). However, as the product was easy to synthesis and characterize, a fresh batch was prepared each time a series of catalytic and epoxidation reactions were performed (a control reaction was always carried out to test the activity of the oxygen source).

Scheme 7. Oxidation of Alkene 9 to the Epoxide 10 and Other Oxidation Products

H


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no shape or size selectivity, whereas values higher than unity indicate a decreased ratio and therefore higher selectivity toward the less reactive linear alkene. It is clear from the graph that no selectivity was observed, and the hyperbranched polymer system displayed selectivity similar to that of the free porphyrin for all molecular weights. The free porphyrin, G0, displayed a preference for cyclooctene; this corresponds directly with the relative reactivity of the two alkenes. This demonstrates that although able to catalyze the reaction, the product ratio remains in line with the relative reactivity of the two alkenes. This suggests that the catalytic porphyrin core is not sterically crowded enough to prevent access of the larger substrate and slow down the reaction. This is despite the fact that binding studies clearly indicated size selectivity. Therefore, when considering the catalytic and binding studies together, we can conclude that although a steric barrier is generated, it does not prevent the substrate reaching the catalytic core and reacting over the time scale of the experiment. To slow down substrate access, an increased steric barrier is required. Clearly the problem is not completely related to the polymers molecular weight, and simply making the polymer bigger will not necessarily constitute a solution. An alternative way to increase sterics around the porphyrin core is to modify the core branching architecture. In this case the polymer is initiated from the 4-position of the porphyrin’s phenyl ring, and polymerization occurs away from the core. This initiation motif unavoidably leaves a volume of space surrounding the porphyrin core, despite the occurrence of dense packing. Therefore, sterics may not be the only issue but also the free volume surrounding the active site. As such, it may be more effective to consider steric crowding around the core as well as steric effects related to polymer size. To do this would require the initial position of polymeric growth to be less remote from the porphyrin core but still be symmetrically arranged as for TAPP 3. This is shown schematically in Figure 6, where it can be seen how the polymer grows away from the porphyrin when initiated from the single 4-position. However, an increased steric environment is possible if the initiation or growth point is moved one substitution position closer to the porphyrin. This could be achieved symmetrically if the polymer backbone started from the 3- and 5-positions on the phenyl

Figure 4. Relative yields of all oxidation products obtained from the oxidation of 1-octene (9) and cyclooctene (11) using catalytic experiments using Fe-TAPP-HBP 8 as the catalyst. Yields quoted relative to those obtained using the G = 0 pseudo-dendrimer (which were 74% and 96%, respectively).

For the larger G4 pseudo-generation there is an approximate 24% reduction in yield. As such, it suggests that there may be some element of selectivity when these oxidations are catalyzed by this polymer system. To conclusively determine whether or not selectivity was possible, a mixed experiment using both alkenes in a 1:1 ratio was carried out. This experiment was performed identically to the activity experiments described above, except for the simultaneous addition of 1 mmol of each alkene. Because iodobenzene is produced in both reactions (i.e., oxidation of linear and cyclic alkenes), the reactions were followed by measuring the yield of the corresponding linear and cyclic epoxides, 10 and 12, respectively (Scheme 5). The results are shown in Figure 5 and the data normalized with respect to

Figure 5. Selectivity of epoxide products from a mixed experiment using the Fe-TAPP-HBP system 8. Yields normalized with respect to the inherent reactivity difference between the linear (terminal) alkene 9 and the more reactive cyclic alkene 11. Therefore, values higher than 1.00 indicate a shift in reactivity toward the less reactive linear alkene.

the free porphyrin as before. In addition, a further normalization was carried out to take into account the inherent reactivity differences of the cyclic and linear alkenes. As such, values obtained for the linear system were normalized to take into account the increased reactivity of the cyclic alkene. Therefore, a value of unity represents a 14:1 ratio and indicates

Figure 6. Schematic showing how the point of branching can affect the steric environment around the polymers central catalytic porphyrin. Although two HBP phenyl substituents are shown, the polymers are likely to be a mix of mono-, di-, tri-, and tetrafunctionalized systems (each possessing one or two HBP arms). I


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Scheme 8. Synthesis of Tetra(3,5-diacetoxyphenyl)porphyrin 16 and Its Metalated Derivatives 17 and 18

this was achieved using a standard demethylation procedure.39 Specifically, porphyrin 14 was reacted with an excess of boron tribromide, followed by water. The product obtained after purification showed that the methoxy peak at 55.6 ppm in 13C NMR was no longer present. Removal of the methoxy group was confirmed by 1H NMR, which showed that the corresponding methoxy signal was absent. In addition, a large OH peak was observed at 3425 cm−1 in the IR spectrum. This procedure produced the desired tetra(3,5-dihydroxyphenyl)porphyrin 15 in high purity and a yield approaching 90%. The final synthetic step was acetylation; this was carried by refluxing porphyrin 15 in acetic anhydride for 6 h. Excess acetic anhydride and acetic acid byproduct were removed via vacuum distillation, and the target tetra(3,5-diacetoxyphenyl)porphyrin 16 was obtained in 64% yield after purification. 1H NMR analysis showed a large singlet corresponding to three hydrogens from on the newly introduced acetoxy group at 2.42 ppm. Mass spectrometry gave a molecular ion peak at 1079 which corresponds to that required for the target porphyrin. The next step was to repeat the polymerization using this alternate porphyrin core. The reaction is shown in Scheme 9 and was carried out using the same synthetic and purification methods described for TAPP-HBP 6. After precipitation from tetrahydrofuran and methanol, followed by purification using an alumina column, tetra(3,5-diacetoxyphenyl)porphyrin cored hyperbranched polymer 19 (tetra-(3,5-DAPP-HBP)) was recovered in good yield as a red/brown solid. SEC analysis was carried out using RI and UV detection, with each trace showing a single peak that was directly super imposable on the other. This confirms that the porphyrin was incorporated

ring. In fact, this is exactly the same substitution pattern used by Suslick and Moore on their work on porphyrin cored dendrimers.7 Thus, it was postulated that this alternative porphyrin core may be capable of providing the correct steric environment required for selectivity. Synthesis of Tetra(3,5-diacetoxyphenyl)porphyrin Cored Hyperbranched Polymers. The more substituted target porphyrin 16 could in principle be synthesized in a manner analogous to that used to construct the simple TAPP core 3. Unfortunately the required 3,5-diacetoxybenzaldehyde was not easily available, and its acetylated precursor was prohibitively expensive. However, 3,5-dimethoxybenzaldehyde could be used to assemble tetra-3,5-dimethoxyphenylporphyrin 14, which could in turn be demethylated and finally acetylated to give the target porphyrin, tetra-3,5-diacetoxyphenylporphyrin, tetra-(3,5-DAPP) 16 (Scheme 8). Tetra(3,5dimethoxyphenyl)porphyrin 14 was synthesized via the same Rothemund and Adler−Longo procedure used previously;18 equal molar quantities of 3,5-dimethoxybenzaldehyde and pyrrole were refluxed for 30 min and after work-up and purification gave 3,5-dimethoxyphenylporphyrin 14 in 13% yield. The UV/vis absorption spectrum confirmed the presence of the porphyrin displaying the characteristic Soret band at 420.5 nm and four Q-bands at 514, 548, 588, and 648.5 nm. The 1H NMR spectrum contained peaks corresponding to the β-pyrrolic protons at 8.96 ppm. The ortho and para aromatic protons resonated at 7.43 and 6.93 ppm, respectively, and a large singlet at 3.99 ppm was assigned to the methoxy protons. Finally, a peak corresponding to the highly shielded inner NH hydrogens could be seen at minus 2.81 ppm. The second stage was to demethylate the octamethoxy-functionalized porphyrin; J


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Scheme 9. Synthesis of 3,5-(Diacetoxyphenyl)porphyrin Cored Hyperbranched Polymer Tetra-(3,5-DAPP)-HBP 19 (left) and a Representative Hyperbranched Wedge (right)

Table 5. Mn, Polydispersity, and Number of Repeat Units for Hyperbranched Polymer Zn-3,5-DAPP-HBP 20 Mn

polydispersity

no. of repeat units

Mn

polydispersity

no. of repeat units

1142 3100 3700 5000 6100 6800

1.00 1.81 1.82 1.72 3.12 2.74

0 (core unit) 13 16 24 30 34

8000 9000 10500 11800 12500

2.46 2.33 2.14 1.99 1.87

40 46 55 61 66

evenly across all molecular weights.15 Analysis returned an Mn value of 8100 and a polydispersity of 2.64. 1H NMR confirmed porphyrin incorporation, showing the characteristic deshielded peak from the NH protons at minus 2.03 ppm. As with the 4substituted HBP system, both SEC and NMR indicated very small peaks corresponding to unincorporated porphyrin (equivalent to just 2% of the total product and is completely gone after the fractionation step). Mass spectrometry showed a series of peaks separated by 178 mass units, each corresponding to n monomer residues and a single porphyrin core (744 + n178). Peaks from cyclized and “coreless” polymers were not observed. Probing Dense Packing of Zn-3,5-DAPP-HBP through Ligand Binding Studies. To probe binding first required zinc insertion into the synthesized porphryin 16. This was carried out using zinc acetate dihydrate as shown in Scheme 8. The

reaction was complete after 30 min, after which unreacted zinc acetate dihydrate was removed by filtration, followed by flash chromatography to give Zn-tetra-(3,5-DAPP) 17 in 68% yield. Structural confirmation of Zn-tetra-(3,5-DAPP) 17 was provided by 1H NMR, in which the highly shielded peak at minus 2.94 ppm was no longer present and UV/vis analysis, which showed the number of Q-bands had been reduced from four to two. Mass spectrometry showed only a single peak of the molecular ion with M/z of 1141. Metalation of tetra-(3,5DAPP)-HBP 19 to give Zn-tetra-(3,5-DAPP)-HBP 20 is shown in Scheme 9 and was performed in an identical manner to that described above for the porphyrin core 16, with the exception that filtration and trituration (from THF into methanol) was used in place of chromatography for purification. UV/vis spectroscopy showed a strong Soret band at 421.5 nm and that the number of Q-bands had been reduced from four to two. K


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The first few hyperbranched polymers exhibited an increase in Ka with respect to the zinc metalated porphyrin core 17. This increase is again due to the internal surroundings of the hyperbranched polymer, which generate an optimum polarity and electronic environment for the ligands, in turn leading to an increased concentration of ligand and a higher binding affinity. As the molecular weight increases further, steric hindrance takes over and starts to overwhelm any positive electronic effects. The point at which this starts to take is effect is the dense packed limit for this system and is roughly similar for each ligand. This point can be determined by plotting best fit lines through the two distinct (linear) regions of each binding plot. The dense packed limit can be found where these line cross and extrapolating to the Mn value on the x-axis. The dense packed limit for each ligand, along with the reduction in binding affinities are shown in Table 6. On this occasion there does seem to be a relationship between the Mn value that dense packing occurs and the size/ shape of the ligand. As the size of the ligand increases, this corresponds to a decrease in the point at which dense packing takes place. Admittedly, these differences are small, and it could be argued these differences are within experimental error. However, the experiments were repeated a number of times, and the same trend was observed on every occasion. Therefore, although it is unwise to define a precise molecular weight to the dense packing limit, there is a clear trend regarding molecular weight and ligand size. As observed for the more open ZnTAPP-HBP 7, binding is significantly reduced for the larger ligands at higher molecular weight. Furthermore, the extent to which binding is reduced is much greater than that observed for the previous HBP system (i.e., a 42% reduction in binding occurs between the largest ligand and the polymer with Mn 11 800). These binding results are encouraging with respect to any size or shape selective reactions catalyzed at the core of these polymers. Probing Shape and Size Selectivity of the Fe-3,5DAPP-HBP via Catalysis. The binding data clearly show access to the core is restricted at a lower molecular weight for the larger ligands and suggest the 3,5-substituted polymers may have an ability for size selective catalysis. To this end the catalytic potential again probed using the alkene oxidation catalysis as a probe, As before, this requires the free porphyrin and the polymers to be functionalized with iron and then fractionated. Inserting iron into the core was achieved by refluxing the starting material tetra-(3,5-DAPP) 16 with iron(II) chloride and 2,6-lutidine in THF for 4 h (Scheme 8). Subsequent to an acid work-up and purification by column chromatography, the Fe(III) tetra(3,5-diacetoxyphenyl)porphyrin, Fe-tetra-(3,5-DAPP) 18, was obtained in 75% yield. Mass spectrometry displayed a single peak corresponding to the molecular weight of the iron inserted porphyrin complex (MH+ 1133). The UV/vis spectrum showed the Soret band and two Q bands (at 418, 513, and 562 nm), confirming the

SEC analysis confirmed the hyperbranched polymer remained intact after the metalation reaction. The polymer was then fractionated using size exclusion chromatography in the form of a Biobead column; the results are shown in Table 5. As described above for the TAPP-HBP system, UV analysis on the fractions confirmed that the core was evenly distributed across all molecular weights.15 On this occasion, smaller fractions were obtained, enabling more data points to be collected. This also meant that molecular weight differences between fractions would be smaller. This allowed a more accurate analysis of binding data and effects, particularly the onset of dense packing. As the differences between fractions was smaller, there was a large number of fractions between each pseudo-generation, and there was no longer any value in assigning pseudo-generations to the fractions (i.e., between pseudo-generation 3.0 and 4.0 there are five fractions). As before, dense packing was probed by studying the binding of various pyridyl ligands to the fractions of Zn-tetra-(3,5DAPP)-HBP 20 (as well as to the core unit Zn-tetra(3,5,DAPP) 17). The same three ligands were used, and to aid comparison, the data for all ligands are plotted on a single graph. The data have been normalized relative to the binding constants obtained for the porphyrin core as well as being normalized relative to each ligand (Ka values for each ligand are different); the plots are shown in Figure 7.

Figure 7. Plot showing ligand binding affinities vs molecular weight for the Zn-(3,5-DAPP)-HBPs 20. All data normalized for binding to the core porphyrin (Zn-tetra-(3,5-DAPP) 17) and also normalized relative to each ligand. (For reference the Ka values were 14 000, 15 000, and 12 000 M −1 for pyridine, 3,5-lutidine, and 3-phenylpyridine, respectively.)

It is immediately evident the pattern displayed is very similar to the pattern observed for the Zn-TAPP-HBP 7 system described earlier, with each ligand having two distinct regions.

Table 6. Binding Data of Pyridyl Ligands to Zn-Tetra-(3,5-DAPP)-HBP 20 ligand

Ka porphyrin core (M−1)

Ka of HBP with Mn 11 800 (M−1)

onset of dense packing (Mn)

reduction in bindinga (%)

pyridine 3,5-lutidine 3-phenylpyridine

14000 (±1750) 15000 (±1750) 12000 (±1500)

10500 (±1200) 9000 (±1000) 4200 (±500)

7800 (±500) 6800 (±500) 6200 (±500)

25 (±5) 21 (±5) 42 (±5)

a Calculated by comparing the Ka values obtained from the porphyrin core unit 16 and the Zn-3,5-DAPP-HBP fraction with Mn closest to 11 500 (allowing comparison with the pseudo-fourth-generation Zn-TAPP-HBP system discussed above).

L


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iron insertion. Insertion of iron into the bulk hyperbranched polymer 19 was carried out in analogous fashion, with purification achieved by trituration from THF into cold methanol, giving Fe(III) tetra(3,5-diacetoxyphenyl)porphyrin cored hyperbranched polymer 21 (Fe-(3,5-DAPP)-HBP) as shown in Scheme 9. Once more, as for Fe-TAPP-HBP 8, line broadening in the 1H NMR was reduced relative to that of the free porphyrin alone. Crucially, the spectrum showed the disappearance of the highly shielded inner porphyrin protons, confirming the metal insertion was a success. Further confirmation was provided by UV/vis spectrometry which showed a reduction from four Q-bands at 514.5, 548.5, 587.5, and 645 nm to two at 511 and 569.5 nm. SEC analysis of the functionalized polymer confirmed that it had remained intact during the procedure (Mn remained at 8100). The bulk polymer 21 was then fractionated as previously described, and the molecular weights were determined by GPC analysis. Although sufficient material for the catalysis study (plus repeats) was required, it was attempted to maximize the number of data points by collecting relatively small fractions. The molecular weight obtained for each fraction along with the theoretical values calculated for the equivalent dendrimers are shown in Table 7. The data show good agreement between the

Figure 8. Relative yields of all oxidation products obtained from the oxidation of 1-octene 9 and cyclooctene 11 using catalytic experiments using Fe-3,5-DAPP-HBP 21 as the catalyst. Yields quoted relative to those obtained using the G = 0 pseudo-dendrimer (which were 76% and 90%, respectively).

of the larger cyclic alkene 11 shows that there is initially a small decrease in activity for each polymer as size is increased. However, there is a notable decrease in yield for the G3.5 and G4 polymers, where the activity drops to 76% and 72% of the value obtained for the core unit. Therefore, in both cases a point is reached when access to the catalytic core becomes more hindered, either through dense packing of the polymer backbone/shell or dense packing specifically around the central porphyrin core. Significantly, this point occurs a generation earlier when the larger alkene substrate 11 is used. Overall, the catalytic data concur well with the binding evidence that showed access to the porphyrin core is being restricted as the pseudo-generation increases, and this restriction is more pronounced for larger species. This was once again encouraging with regard to possible shape and size selectivity reactions. To probe the possibility of a selective reaction, a mixed experiment was carried out using a 1:1 mixture of the linear alkene 9 and the cyclic alkene 11. This experiment was performed as described for the Fe-TAPP-HBP system described above, and once again the reactions were followed by measuring the yield of epoxides, 10 and 12 (Scheme 5). The data are again normalized with respect to core porphyrin (Fetetra-(3,5-DAPP) 18) and the reactivity difference between the linear and cyclic alkenes taken into account as before (Figure 9). Although the data appear encouraging and the graph does indicate a very small shift in selectivity (toward the less reactive linear species), the change is less than 10%; therefore, once again it may be argued the data are within the error limits of the experiment. It had been hoped that moving the growth/ initiation points to the 3,5-position on the phenyl rings on the porphryin core would reduce the space around the porphyrin core sufficiently to achieve selectivity, and with respect to the zinc metalated polymers and ligand binding this was indeed the case. However, no definitive shape or size selectivity could be observed when using the iron metalated polymers to simultaneously catalyze the epoxidation of the two alkenes. Although only a modest change in selectivity was observed when the initiation/growth points were moved to the 3,5positions, the results are an improvement over those obtained using the 4-substituted system studied earlier. In a final effort to improve selectivity, we proposed to move the initiation/growth

Table 7. Theoretical Values for Pseudo-Generations vs the Values Obtained for Fractions of Fe(III) 3,5-DAPP-HBP 21a molecular weight

repeat unitsa

pseudo-generation

theoretical

obtained

theoretical

obtained

0 (Fe-HBP-17) 1.5 2.5 3.0 3.5 4.0

1133 3983 8271 11134 16862 22590

1133 3500 7750 11500 16500 21500

0 15 37 56 83 120

0 13 37 59 86 117

a

Half-generations were assigned to hyperbranched polymers when the number of terminal groups and the molecular weights fell between those for the full generations.

values obtained for the fractionated polymer and the theoretical values. The obtained molecular weights are all close enough to ensure valid comparison using pseudo-generations. The catalytic reactions were carried out using the same method and concentrations described for Fe-TAPP-HBP 8, again using UV spectroscopy to ensure that the porphyrin concentrations were constant for all polymer fractions tested. The experiments independently studied the effect of catalyst size on the oxidation reactions of the linear alkene 9 and the cyclic alkene 11 (Figure 8). The yields of all oxidation products for each polymer catalyst are normalized with respect to the porphyrin core, Fe-tetra-(3,5-DAPP) 18. As expected, the porphyrin cored hyperbranched system was capable of catalyzing the oxidation of both alkenes. For the linear alkene 9 the activity of G1 to G3.5 never drops below 90% of the value obtained for the core unit. This is within the 5% error limits, and we can conclude that the reaction remains fairly constant for these polymeric catalysts. However, there is a decrease in activity for the larger G4.0 polymer, where the recorded yield has dropped to around 82% of the value obtained for the core unit. Therefore, it can be concluded that the catalytic reaction involving the smaller linear alkene 9 occurs without hindrance until the largest polymer is used (Mn > 21 000). The oxidation M


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using an analogous method to that of the substituted 3,5substituted diacetoxyphenylporphyrin, tetra(3,5-DAPP) 16. This dictated that the synthesis must begin with 2,6dimethoxyporphyrin, which was synthesized from the corresponding 2,6-dimethoxybenzaldehyde, which, in turn was obtained by reacting 1,3-dimethoxybenzene with butyllithium and dimethylformamide (Scheme 10).40 After purification, 2,6Scheme 10. Synthesis of 2,6-Dimethoxybenzaldehyde 22

dimethoxybenzaldehyde 22 was isolated as a pale yellow solid in a 90% yield. The 1H NMR spectrum shows a highly deshielded peak corresponding to the aldehyde proton at 10.46 ppm. The three aromatic protons are also easily distinguished in the spectrum in the 6.55−7.44 ppm range as well as a large singlet at 3.88 ppm corresponding to the six protons from the methoxy hydrogens. Although it was possible to synthesize the 2,6-substituted porphyrin using the procedure described previously, the yield was very low (

Probing Dense Packed Limits of a Hyperbranched Polymer through

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