Pushing Synthesis toward the Maximum Generation Range of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Pushing Synthesis toward the Maximum Generation Range of Dendritic Macromolecules Daniel Messmer,† Martin Kröger,‡ and A. Dieter Schlüter*,† †

Polymer Chemistry, Department of Materials, and ‡Polymer Physics, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland

Downloaded via UNIV OF WINNIPEG on July 13, 2018 at 20:38:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The maximum generation gmax of a dendritic molecule denotes the value of the generation number g, above which such a compound cannot be synthesized without defects anymore due to steric constraints. For dendronized polymers (DPs), such a densely packed regime is entered far earlier (gmax ≈ 6) than it is for comparable dendrimers (gmax ≥ 10) because dendritic side chains are confined to a cylindrical rather than a spherical volume. We here report a long sought-after improvement to a key step in the divergent synthesis of high-g DPs which enabled obtaining the polymers of g = 6, 7, and 8. These DPs are of unprecedented dendritic perfection, and the representatives with g > 6 are to our knowledge the first molecules for which gmax has been surpassed. We suggest a straightforward parameter α which allows to assess whether any dendritic molecule is above gmax, given sufficiently efficient chemistry and the possibility of accurately determining the number of defects. Finally, we correlate gel permeation chromatography results and atomic force microscopic images with defect rates.

■

next-generation dendron.5 Percec et al. recently studied the effect of core conformation in aliphatic polyamide dendrimers on the accesibility of higher g dendrimers.6 Interestingly, they found that growth to the next g dendrimer was completely interrupted at a value of g ≪ gmax. This was attributed to backfolding of terminal functional groups, thought to result in a densely packed structure. In all these studies, the concept of limited dendritic growth plays a role, but gmax is usually not discussed explicitly. Whereas dendrimers contain a small, multifunctional core, dendronized polymers (DPs) consist of a polymeric backbone carrying dendrons typically on every repeat unit.7−9 This decoration makes DPs thicker and more rigid than conventional linear polymers.10,11 DPs were therefore coined molecular nanocolloids12 and suggest themselves for the study of gmax effects because gmax is reached at lower g than in spherical dendrimers composed of the same kind of dendrons. This is because a DP’s volume can be approximated by a cylinder, in which the maximum available volume grows only quadratically with g rather than cubically as is the case in dendrimers. gmax is situated even lower for surface-grafted dendritic forests, for which the available volume grows linearly. However, their study is complicated by the immobile nature of grafted layers and the necessity of strictly controlled grafting density in order to generate the desired confinement. We have previously reported on a class of DPs of the type (Figure 1).11 Here, n denotes the number-average PGgNHBoc n

INTRODUCTION The molar mass of dendrimers grows exponentially as a function of the generation number g, while the volume (approximated by a sphere) grows cubically. As was noted by de Gennes in the 1980s, above a certain value of g the presence of defects is therefore unavoidable.1 This value is referred to as gmax and represents the last generation which can theoretically be accessed without defects.2,3 The gmax range of macromolecules is fascinating: It marks the point where synthesis starts to fail and where packing effects begin to dominate. In the ideal case, a dendritic structure beyond gmax cannot take up solvent molecules or other guests and interacts with its environment only via its outermost periphery, thus being reminiscent of a densely cross-linked colloidal particle below its Tg. The investigation of structures at and above gmax is therefore of fundamental interest to both polymer science and synthetic chemistry. A number of studies have investigated the effect of steric congestion on the outcome of chemical reactions in dendrimers. These include the polyamidoamine (PAMAM) class introduced by Tomalia, where a decreasing reactivity of the peripheral functional groups with increasing generation g was observed.2 While these studies were largely qualitative, Tomalia et al. also demonstrated quantitatively using a different dendritic structure that reactivity is highly dependent on spatial constraints: Only cores with sufficient spacing between functional groups afford complete dendrimers in reactions with fairly bulky dendrons.4 Along similar lines, Moore et al. published an impressive mass spectrometric study of shape-persistent phenylacetylene dendrimers, where the size of peripheral groups decided over structural perfection of the © XXXX American Chemical Society

Received: April 27, 2018 Revised: June 22, 2018

A

DOI: 10.1021/acs.macromol.8b00891 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Chemical structures of DPs of generation g = 5 (1−6), g = 6 (7−8), g = 7 (9), and g = 8 (10), denoted by PGgXn , carrying the peripheral functionality X, as well as of the g = 1 (11) and g = 2 (12) dendronization agents. 3TFA of PG5NH (2), the precursor to PG6NHBoc (7), failed due to 500 500 main-chain scission.14 Since this could not be prevented, 3TFA using the g = 2 higher g DPs were synthesized from PG4NH 500 dendronization agent 12 instead of the usually employed g = 1 reagent 11.15,16 This led not only to an increased defect frequency but also to a break in the homologous series of DPs. These issues detrimentally impact the comparability within the DP series and render conclusions about the effects of increasing g tenuous. Consequently, synthetic effort was required that would furnish g > 5 DPs as members of a homologous series and with high structural perfection. We here report a synthetic protocol relying on the allyloxycarbonyl (Alloc) protecting group which achieves these requirements: A homologous series up to g = 8 including NHBoc NHBoc NHBoc PG6500 , PG7500 , and PG8500 with high structural perfection has become available. Thanks to this improvement,

degree of polymerization (Pn) of the polymer backbone, and the superscript denotes the peripheral functionalityin this case Boc-protected amines (see Figure 1 for other representatives). For this series, gmax has been estimated for a span of reasonable densities to lie within 6 ≤ gmax ≤ 7.13 For comparison, the analogous estimate is 12 ≤ gmax ≤ 13 for the PAMAM series. In this study, we will focus on DPs around the estimated gmax value: PG5nNHBoc (1), PG6NHBoc (7), PG7NHBoc (9), and n n NHBoc PG8n (10). Previously prepared representatives of g = 6−8 are unsuited for the discussion of gmax because of their and its lower insufficient structural perfection: While PG5NHBoc 500 g precursors were obtained with very low defect frequencies using a divergent protocol,11 the DPs of g = 6−8 had substantial amounts of defects arising from a detour necessitated by an unexpected issue: The attempted synthesis B


Article

Macromolecules

Scheme 1. Visualization of the Attempted Routes to PG6NHBoc : (A) Classical NHBoc-Based “g + 1” Pathway, Resulting in n Main-Chain Scission at g = 5 for n > 50; (B) NHBoc-Based “g + 2” Route to (Imperfect) PG6NHBoc ; (C) Envisioned Route via n NH2 PG5NHX and PG5 , Where X Is a Protecting Group Suitable for N-Deprotection under Neutral or Basic Conditions n n

dinitrofluorobenzene),20,21 which provides values ≥99.5%a for all steps up to g = 5. Likely due to the greater steric demand of the g = 2 dendronization agent 12, values of only 91−94% 3TFA were achieved in the synthesis of PG6NHBoc from PG4NH 500 500 16 (Scheme 1, route B). The presence of so many defects in the NHBoc resulting polymer, B-PG6500 , renders the gmax concept inapplicable, as the dendrons are not monodisperse. High functional group conversion is therefore a crucial feature of prospective improved synthetic strategies that are powerful enough to meaningfully explore the gmax range. We have hypothesized previously14 that the intermediate 3TFA PG5NH is prone to main-chain scission because of its n polyelectrolytic nature: In the deprotection of NHBoc, 32 ammonium cations are generated per repeat unit. While significant amounts of mass and volume are lost in this reaction, MD simulations of g = 5 DP segments indicate that the steric demand of the dendrons still increases due to the incorporation of tightly associated counterions and due to charge−charge repulsion between unscreened ammonium cations.22 The resulting increased strain may sufficiently weaken the DP backbone for it to undergo scission under the conditions employed during acid-mediated deprotection (a summary of the evidence supporting this hypothesis can be found in the Supporting Information). With this hypothesis in mind, we aimed to develop a 2 synthesis of the free base polyamine PG5NH (3) in order to n avoid charged intermediates and thus main-chain scission (see NH Scheme 1, route C). The polyamines PGgn 2 are soluble in only a small selection of polar solvents (chiefly: MeOH, DMF, and DMSO) and their precipitation is usually irreversible. As 2 intermediate work-up and isolation of PG5NH are therefore n likely to be challenging, the conditions of deprotection should be compatible with dendronization in a one-pot process. Methods for the deprotection of NHBoc that do not rely on protic acids are available (solvent-assisted thermolysis,23 treatment with Lewis acids24 or TBAF25) and were attempted, however, without satisfactory results (see Supporting Information). We thus turned to alternative N-protecting groups, namely Cbz (carboxybenzyl), Fmoc* (2,7-di-tertbutylfluorenylmethyloxycarbonyl), and Alloc (allyloxycarbonyl), as candidates for the implementation of route C. NHCbz is usually cleaved by transition-metal-catalyzed hydrogenolysis, which however failed in the case of the highly sterically congested DPs (see Supporting Information). For the Fmoc* protecting group, the possibility of quantitative deprotection had previously been demonstrated for DPs

a discussion of whether some representatives (g > 6, possibly g > 7) are above gmax has become possible. In such molecules, defects must be present. For dendritic molecules of g ≤ gmax, dendron molar masses determined experimentally using a defect labeling method (Mexp) cannot surpass the maximum molar mass which a dendron in the molecule can have (Mcalc, see Figure 4a). At g > gmax, it is expected that defects become inaccessible due to dense packing and that quantification methods relying on the labeling of defects consequentially provide unrealistically high estimates of molar mass. We therefore set out to explore whether the ratio α = Mexp/Mcalc and its development with g can be used as criteria to judge whether gmax has been surpassed: By first approximation the relationship α ≤ 1 must hold for a dendritic molecule at g ≤ gmax. Values α > 1 are possible only when the efficiency of labeling reactions breaks down due to crowding at g > gmax. We show that such molar mass overestimates can be detected in the recursive analysis17 of the labeling of unreacted amines with Sanger’s reagent. Further, we discuss the applicability and generality of this criterion for determining the onset of gmax in dendritic structures. Finally, to substantiate the claimed higher structural perfection, the novel series reported here is compared to the defect-riddled analogues synthesized in classical routes by GPC and by AFM imaging.

■

RESULTS AND DISCUSSION Alternative Synthetic Pathways. The “standard” diverinvolves two steps gent protocol for the synthesis of PGgNHBoc n (Scheme 1, route A): deprotection of peripheral Boc-protected amines under acidic conditions, usually employing neat trifluoroacetic acid (TFA) to afford the soluble polyelectrolyte NH TFA PGgn 3 , and dendronization by amide bond formation with the active ester 11 under basic conditions to obtain PG(g +1)NHBoc . This simple protocol, adapted from Merrifield-type n solid phase peptide synthesis,18,19 is a very powerful tool to add mass to polymers: For a DP of Pn ≈ 15 000, which is accessible by conventional free-radical polymerization, the dendronization step leading to PG5NHBoc 15 000 adds significantly more mass to a single molecule (>80 MDa) than any other nonbiological process can generate in a controlled manner. This is largely owed to the high functional group conversion achieved: NHBoc cleavage is effectively quantitative, and amide bond formation using N-hydroxysuccinimide esters is very nearly so. We have developed a method of quantifying the degree of functional group conversion in the dendronization step by labeling unreacted amines with Sanger’s reagent (2,4C


Article

Macromolecules synthesized in a convergent manner,26 but the protecting group proved not amenable to divergent synthesis (see Supporting Information). Finally, a successful attempt described below using the Alloc group was made. Synthesis of PG6NHBoc by a “g + 1” Route from 500 . The removal of Alloc is usually achieved with the PG5NHAlloc 500 aid of a Pd0 catalyst such as Pd(PPh3)4 under mild, neutral, or slightly acidic conditions.27,28 The reaction involves an intermediate PdII allyl complex which is susceptible to attack by nucleophiles, including the amines generated by deprotection. To prevent N-allylation, an excess of a scavenging agent is usually added, to directly capture the allylic species and/or to convert the liberated amines into non-nucleophilic, readily decomposable intermediates. Many scavengers are available,29 for some of which30−32 tandem deprotection/amide bond formation has been implemented. Our choice fell on 1,3-dimethylbarbituric acid (DMBA), which has been shown to provide practically quantitative deprotection33 while posing a smaller risk in terms of side reactions than other scavengers. Additionally, the system Pd(PPh3)4/DMBA has been shown to quantitatively cleave allylamines in small molecules.34 This is desirable in the crowded environment of DPs, where N-allylation might well occur even in the presence of a scavenging agent due to confinement in the DP periphery. In a model reaction with a g = 1 dendron, deprotection proceeded smoothly in the presence of Pd(PPh3)4/DMBA. The precise conditions employed were slightly unorthodox in that a large excess of NEt3 was added to fully deprotonate all DMBA (pKa ≈ 4.735) and maintain overall basic conditions. The successive dendronization was undisturbed by the presence of large amounts of DMBA (see Supporting Information for details). Encouraged by this result, the same system was employed in the synthesis of PG6NHBoc (7) following route C. In initial small-scale reactions, precipitation of the polymer from the DMF solution was observed within ca. 1 h of addition of Pd(PPh3)4. A brief solvent screening (see Supporting Information) showed that a mixture of DMSO and NMP (3:2) offers good solubility over the entire course of the reaction sequence. As a second modification, a scavenging agent for Pd (sodium N,N-diethyldithiocarbamate trihydrate36) was added after deprotection. The progress of deprotection and dendronization was monitored qualitatively by testing for precipitation of the reaction mixture in methylene chloride. The observed behavior was in agreement with successful deprotection and dendronization, respectively (see Figure S10). After some optimization of reaction times for the two steps (see Supporting Information), we arrived at the conditions presented in Scheme 2. The duration of deprotection was initially chosen somewhat arbitrarily at 3 days, but later investigations showed that deprotection proceeds fairly rapidly (near-complete disappearance of NHAlloc in 1H NMR within 1 day; see Figures S13 and S14). Gratifyingly, analysis of PG6NHBoc synthesized via route C revealed long chains (C-PG6NHBoc ): While the from PG5NHAlloc 500 500 (Figure TFA-mediated deprotection (route A) of PG5NHBoc 500 2a) to PG5NH3TFA (Figure 2b) resulted in a marked decrease of chain length,14 in the transformation of PG5NHAlloc to C500 PG6NHBoc (Figures 2c and 2d, respectively) chain length was unaffected as judged by visual inspection of AFM images. In GPC, the retention volume decreased, as expected for a successful dendronization without main-chain scission (Figure

Scheme 2. Optimized Conditions for the Deprotection of PG5NHAlloc (6) and Subsequent One-Pot Dendronization to 500 C-PG6NHBoc (7), Showing the Transformations Occurring 500 on a Single Aminea

a

For full structures, refer to Figure 1.

5a). The determination of functional group conversion by labeling with Sanger’s reagent was slightly hampered by the yellow color of the polymers, which was originally ascribed to residual Pd species originating from deprotection. However, the overall Pd content was reasonably low (99.6% (see below for discussion). Syntheses of C-PG7NHBoc and C-PG8NHBoc 500 500 . Initially, it was planned to also apply the successful protocol detailed above to the synthesis of g > 6 polymers. This would have necessitated the synthesis of PG6NHAlloc and higher g analogues. 500 To our surprise, we found that PG6NHBoc , even with the 500 achieved very high structure perfection, does not undergo main-chain scission upon treatment with TFA (see Figure S22). This result is counterintuitive in view of the demonstrated high structural perfection of PG6NHBoc synthe500 sized by route C, which would suggest an even higher strain on the polymer backbone in its deprotection product, C3TFA 3TFA PG6NH , than in PG5NH . It suggested, however, that 500 n the synthesis of DPs of g > 6 might be possible by applying the NHBoc-based protocol (akin to route A). Indeed, we were able to synthesize C-PG7NHBoc (9) without 500 apparent main-chain scission, as demonstrated by AFM imaging (Figure 2e) and GPC (Figure 5a). Another iteration of the protocol from route A afforded C-PG8NHBoc (10), again 500 without apparent main-chain scission (see Figures 2f and 5a for AFM and GPC results, respectively). Functional group conversion as determined by labeling with Sanger’s reagent was high for both steps (99.8% and 99.7% for C-PG7NHBoc and 500 C-PG8NHBoc , respectively). 500 Discussion of Structure Perfection, α, and Implications for gmax. gmax of spherical dendrimers typically lies above g = 10, which also applies to cases of “compact” dendrimers such as the ones reported by Perez-Inestrosa et al.37 and Percec et al.6 (Figure 3). Dendritic molecules at and above their respective gmax have been investigated relatively little, and only a few cases of very high g dendrimers exist.38 We are aware of D


Article

Macromolecules

3TFA Figure 2. AFM height images of (a) PG5NHBoc , (b) PG5NH obtained after NHBoc deprotection (route A), (c) PG5NHAlloc , (d) C-PG6NHBoc , (e) 500 ∼50 500 500 NHBoc NHBoc C-PG7500 , and (f) C-PG8500 .

only two cases where gmax has formally been reached: Astruc’s carbosilane dendrimers39 (gmax ≈ 8−9, prepared up to g = 9) and Majoral’s polyphosphazene dendrimers40 (gmax ≈ 12−13, prepared up to g = 12). In the former case, the authors found defects to occur fairly early in the course of their divergent synthesis. In the latter case, the knowledge of the efficiency of the dendronization chemistry is limited to the accuracy of 31P NMR spectroscopy: Even though rigorously applied, that method cannot compete with the sensitivity of UV/vis spectroscopy. It is commendable that the authors undertook a very serious effort in characterizing their structures by MALDI-TOF mass spectrometry, an attempt which unfortunately was not met with success.41 For many other dendritic

structures, defect characterization is lacking entirely or a significant buildup of defects occurs already at early stages: For instance, commercial PAMAM samples show significant structural imperfections as early as g = 5,42 while the gmax of these dendrimers lies at gmax ≈ 13 (Figure 3). For the case of “aliphatic amide” dendrimers, Percec et al. even reported a complete absence of reactivity of peripheral groups above g = 4−6, the exact threshold depending on the core of the dendrimer.6 Again gmax ≈ 11 is higher. De Gennes dense packing1 is often invoked in such cases. While seminal to the field of dendritic matter, his original self-consistent field approach to the radial density distribution in dendrimers disregards the possibility of backfolding. This essential feature E


Article

Macromolecules

In B-PG6NHBoc a total of approximately 1.1 MDa in molar mass 500 is missing compared to a perfect 500mer (see Table S5 and ref 36); this corresponds to α = 0.90. Comparatively, the losses , where the calculated lack in are far smaller in C-PG6NHBoc 500 molar mass amounts to only 140 kDa, corresponding to α = 0.99 (Figure 4b). This significant improvement is owed to the superior functional group conversion in the step from g = 5 to g = 6. However, C-PG7NHBoc and C-PG8NHBoc with the 500 500 apparent molar masses Mexp derived from Sanger labeling cannot exist: They surpass Mcalc for both DP generations (Figure 4b), thereby resulting in values of α > 1. For route B, the course of αwhich deviates significantly from the ideal α = 1directly mirrors the defects introduced . The detected functional group in the synthesis of B-PG6NHBoc 500 and conversion decreases again in the syntheses of B-PG7NHBoc 500 B-PG8NHBoc , to such an extent that α > 1 for the latter polymer. 500 Because of the prior strong deviation from the ideal, this does however not mean that gmax has merely been shifted to higher values: The polydispersity resulting from the high defect frequency below the calculated gmax is not corrected during the synthesis of the higher g DPs; it is merely masked by the is therefore not increasing steric congestion. B-PG8NHBoc 500 strictly dendritic and instead resembles a fairly densely substituted hypergrafted polymer in some respects. For route C, on the other hand, the observed behavior exactly follows the expectation for a dendritic system of high structure perfection approaching and surpassing gmax: The number of Sanger-labeled defects is low throughout (X ≈ 100%), making it unclear at which point defects occur exactly. Only the comparison with Mcalc therefore delivers an indication as to where labeling fails to account for defects. This comparison is possible because gmax is fairly well-defined: For reasonable assumptions of the packing density ρpackingb and the extended radius Rmax, gmax varies by no more than ±1 (see and CSupporting Information). As α > 1 for C-PG7NHBoc 500 , we propose that these DPs represents the first PG8NHBoc 500 dendritic structures which have surpassed gmax. For g > 6, packing effects have become apparent, restricting the reliability of even small labeling agents. This has been enabled by the use of highly efficient amide bond formation as the linking chemistry, which results in high structural perfection below gmax. Another important factor is the

Figure 3. “gmax plots” for PAMAM dendrimers (red), aliphatic amide (green). The radii Rmax (solid lines) dendrimers (blue), and PGgNHBoc n and Rpacking (dashed lines) correspond to maximally extended and densely packed dendrons, respectively (see Supporting Information for details). The correspondingly colored arrows mark the location of gmax (ρpacking = 1.25 g cm−3, reported as the lowest g for which Rpacking ≤ Rmax; see Supporting Information) and the colored boxes mark the g range at which either reduced reactivity or decreased perfection has been reported (see text).

of dendritic structures was later considered by Muthukumar et al.43 and since confirmed computationally44−46 and experimentally.47 We suspect that more than inherent packing effects, inefficient bond formation chemistry, and the g dependence of solvent quality are often the culprits responsible for the formation of defects. The occurrence of defects at comparatively low g results in polydisperse dendrons and α ≪ 1 (Mexp ≪ Mcalc) far below the calculated gmax, as is the case in the DPs previously synthesized by route B, rendering discussion of gmax difficult in all these cases. However, the DPs in the homologous series g = 1−8 available from route C are essentially defect-free according to the labeling with Sanger’s reagent. This for the first time permits a well-founded discussion of Mexp and α. The recursive analysis of defects as determined by Sanger labeling17,21 reveals large differences between routes B and C:

Figure 4. (a) Above gmax, the maximum achievable molar mass Mcalc (α = 1.0) is not the mass of the isolated dendron, but restricted by the volume accessible to dendrons at maximum extension (assuming a cylinder of ρpacking = 1.25 g cm−3). (b) Values of α for the two DP series derived using routes B and C. F


Article

Macromolecules

Figure 5. Normalized GPC traces (DRI signal) of DPs of g = 5−8, where PG6NHBoc was synthesized (a) by route C and (b) by route B. The curves 500 in each graph were measured in a single run.

Figure 6. (a) AFM height image of coprepared PG5NHAlloc and C-PG6NHBoc , with one clearly identifiable representative each of PG5NHAlloc and C500 500 PG6NHBoc highlighted (see Supporting Information for a more in-depth analysis). Geometric models of (b) straight and (c) undulated cylinders as possible conformations for high-g DPs. The undulated cylinder possesses a higher volume at identical backbone length; R < Rmax.

provide at least cursory information about the behavior above gmax. By first approximation, one would expect that DPs take on a straight cylindrical shape as they approach gmax. However, CPG6NHBoc was found to appear more strongly undulated and 500 corrugated by visual inspection of many AFM images than its precursor PG5NHAlloc (Figure 5a; also compare Figure 2c vs 2d 500 and Figure S18). This is supported by copreparations, in which two populations are clearly distinguishable by eye. For PG5NHBoc and B-PG6NHBoc , differences are far less distinct, as 500 500 NHBoc B-PG6500 does not appear strongly corrugated (see Supporting Information for further images and analyses). NHBoc NHBoc NHBoc Like C-PG6500 , C-PG7500 and C-PG8500 appear strongly undulated (see Figure 2e,f and Figure S23). Previously observed instances of corrugation16 had been ascribed to defects introduced at the g = 6 level being propagatedan explanation which does not hold in view of the very much improved structure perfection of C-PG6NHBoc . 500 However, geometric considerations provide a possible explanation: For g ≤ gmax, it is possible for the dendrons to pack into a space such that R < Rmax, where Rmax denotes the maximum possible extension of the dendrons.3 In this regime, it is possible for a chain to take on a helical or undulated rather than straight cylindrical shape (Figure 6b,c). The former two

availability of Sanger’s reagent as an efficient, chemoselective defect probe which permits a reliable assessment of structure perfection and therefore a meaningful comparison with theoretical structures. In any dendritic system where structure perfection is high and for which accurate quantification of defects is possible, we therefore propose α to serve as a meaningful measure of whether gmax has been achieved. GPC and AFM Imaging. The GPC retention volumes of DPs of g = 5−8 in route C consistently decrease, in concert with the expected increase in molar mass (Figure 5a). This is not the case in route B: DPs synthesized using a “g + 2” step (B-PG6NHBoc , B-PG8NHBoc ) have a retention volume which is 500 500 equal to or even slightly above that of the DPs one generation lower, which were derived from the same precursors using dendronization agent 11 (see Figure 5b). This observation is peculiar as the molar mass of B-PG6NHBoc , even considering 500 defects, still surpasses that of PG5NHBoc by ca. 80%. Likely, this 500 observation is related to the defects introduced by the use of the bulky dendronization agent 12, which leaves large solventaccessible gaps.16 A similar effect can be observed when dendronization is halted prematurely in route C (see Figure S11). The GPC results therefore support the improvement in structure perfection indicated by Sanger labeling data and G


Article

Macromolecules

Nevertheless, by employing highly efficient bond formation reactions coupled with the monitoring and quantification of defects exemplified by the present DP system, dendritic systems can be pushed beyond gmax, as demonstrated for CPG7NHBoc and C-PG8NHBoc . In DPs this is facilitated by the low 500 500 value of gmax. Accessing this regime for dendrimers requires many more steps, as gmax is usually situated far higher, and careful synthetic optimization as major defects often appear already at g ≪ gmax.

options provide a larger volume for the same length of the internally straight DP backbone, and it therefore seems possible that a shape transformation to e.g. a helix could take place in proximity to gmax ≈ 6, where DPs are very tightly packed. Whether or not this is thermodynamically favorable is beyond the scope of this publication, as the interplay between bulk and solvent interactions is complex. In any case, this reasoning does not hold true when gmax is surpassed, i.e., when R ≈ Rmax: A straight, cylindrical shape is to be expected. That and C-PG8NHBoc indicates this is not the case for C-PG7NHBoc 500 500 sufficient mobility for internal rearrangement and/or solvent swelling; i.e., the DP structure is apparently not hermetically sealed against ingress, although the results discussed previously indicate that reagents such as 11 or Sanger’s reagent are too large to enter and react with free amines.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00891. Summary of the current evidence base for the hypothesis of charge-induced main-chain scission; calculation of gmax for a number of dendrimers; materials and methods; syntheses of new polymers and their small-molecule precursors; additional experimental details; supplemenNHAlloc NHBoc , C-PG6500 , Btary AFM images of PG5500 NHBoc NHBoc , C-PG7 , and C-PG8 (PDF) PG6NHBoc 500 500 500

■

CONCLUSIONS AND OUTLOOK In summary, we attempted the implementation of several Nprotecting group strategies in DPs of g = 5, designed to avoid 3TFA . Among the protecting the unstable intermediate PG5NH n groups used, NHAlloc proved amenable to Pd0-catalyzed cleavage under basic conditions, affording the free base NH intermediate PG5500 2. The procedure uses widely available reagents and simple conditions and may therefore find more general use in postpolymerization modification. proceeded with Successive dendronization to C-PG6NHBoc 500 ≥99.6% conversion of functional groups, and no main-chain scission was observed. The resulting polymer proved capable of and Ccontinued divergent growth, affording C-PG7NHBoc 500 by the previously employed NHBoc-based protocol. PG8NHBoc 500 This marks a significant improvement over previous methods for the synthesis of g > 5 DPs and has granted access to the first series of DPs of g = 1−8 which are all members of a truly homologous series with low detected defect frequency. The successful avoidance of main-chain scission provides a path to single molecules of well-defined structure with GDa molar 2 masses. Simultaneously, the accessibility of PG5NH 500 has opened new avenues for the investigation of main-chain scission, promising to shed some light on the causes behind this synthetic barrier. Comparisons with theoretically perfect structures indicate and C-PG8NHBoc are in a reactant-centered that C-PG7NHBoc 500 500 view above the gmax of this DP system, in that access to the interior is denied to small entities such as 11 and Sanger’s reagent, as evidenced by α = Mexp/Mcalc > 1. AFM imaging showed deviations from the ideal cylindrical shape which support the supposed undetected defects: While these defects apparently affect the DP shape and provide significant peripheral mobility, they are not addressable by the aforementioned reagents anymore. It is however difficult to assess how much this is caused by size segregation alone and how much desolvation penalties and steric hindrance prohibiting certain reaction trajectories contribute. This illustrates the clash between theoretical predictions for dendritic systems and the synthetic limitations which arise in reality: While the present synthetic tools do allow for the generation of near-perfect structures up to gmax ≈ 6, truly dense objects forbidding access even to solvent are possibly not accessible by synthetic chemistry: Solution-phase synthesis requires some solvent swelling of the DPs, after all. This agrees with the apparent corrugation found in AFM images.

■

AUTHOR INFORMATION

Corresponding Author

*(A.D.S.) E-mail [email protected]. ORCID

Daniel Messmer: 0000-0002-2242-4280 Martin Kröger: 0000-0003-1402-6714 A. Dieter Schlüter: 0000-0001-9975-9831 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS First and foremost, the authors thank the late Prof. Avraham (Avi) Halperin for his interest in and support of research into the gmax range of dendritic structures; his contributions will be sorely missed. Prof. N. D. Spencer (D-MATL, ETH Zürich) is gratefully acknowledged for access to AFM instruments. Dr. H. Yu (Wuhang University) is thanked for preliminary experiments. We further thank Dr. S. Mantellato (D-BAUG, ETH Zürich) for conducting ICP-OES measurements and advice regarding sample preparation. The authors thank ETH Zürich for financial support.

■

ADDITIONAL NOTES Functional group conversion values given in this publication were calculated from labeling data (see Supporting Information, Tables S4 and S5) using the method published by Zhang et al.17 The values reported here are larger than those published previously, as this recursive method takes into account defects detected at lower g. b The value of ρpacking = 1.25 g cm−3 used in Figures 3, and 4 is derived from bulk density measurements of PGgNHBoc . Also see 500 ref 12. a

■

REFERENCES

(1) de Gennes, P. G.; Hervet, H. Statistics of “Starburst” Polymers. J. Phys., Lett. 1983, 44 (3), 351−360. (2) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Starburst Dendrimers: Molecular-Level Control of Size, Shape, Surface

H


Article

Macromolecules

Impact of Charge on Internal Organization. J. Phys. Chem. B 2013, 117 (19), 6007−6017. (23) Choy, J.; Jaime-Figueroa, S.; Jiang, L.; Wagner, P. Novel Practical Deprotection of N-Boc Compounds Using Fluorinated Alcohols. Synth. Commun. 2008, 38 (21), 3840−3853. (24) Olah, G. A.; Narang, S. C. Iodotrimethylsilanea Versatile Synthetic Reagent. Tetrahedron 1982, 38 (15), 2225−2277. (25) Jacquemard, U.; Bénéteau, V.; Lefoix, M.; Routier, S.; Mérour, J.-Y.; Coudert, G. Mild and Selective Deprotection of Carbamates with Bu4NF. Tetrahedron 2004, 60 (44), 10039−10047. (26) Al-Hellani, R.; Schlüter, A. D. A Series of First- and SecondGeneration Dendronized Polymers with Orthogonally Protected Amine Groups in the Periphery. Macromolecules 2006, 39 (26), 8943−8951. (27) Minami, I.; Ohashi, Y.; Shimizu, I.; Tsuji, J. PalladiumCatalyzed Reaction of Allyl Carbamates; Allylation of Carbonucleophiles, and Protection-Deprotection of Amines. Tetrahedron Lett. 1985, 26 (20), 2449−2452. (28) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Amino AcidProtecting Groups. Chem. Rev. 2009, 109, 2455−2504. (29) Guibé, F. Allylic Protecting Groups and Their Use in a Complex Environment Part II: Allylic Protecting Groups and Their Removal through Catalytic Palladium π-Allyl Methodology. Tetrahedron 1998, 54 (13), 2967−3042. (30) Thieriet, N.; Alsina, J.; Giralt, E.; Guibé, F.; Albericio, F. Use of Alloc-Amino Acids in Solid-Phase Peptide Synthesis. Tandem Deprotection-Coupling Reactions Using Neutral Conditions. Tetrahedron Lett. 1997, 38 (41), 7275−7278. (31) Thieriet, N.; Gomez-Martinez, P.; Guibé, F. Tandem Deprotection-Coupling of Nα-Alloc-Amino Acids by Use of Ternary Systems Pd Cat./PhSiH3/Carboxy-Activated Amino Acid. Tetrahedron Lett. 1999, 40 (13), 2505−2508. (32) Roos, E. C.; Bernabe, P.; Hiemstra, H.; Speckamp, W. N.; Kaptein, B.; Boesten, W. H. J. Palladium-Catalyzed Transprotection of Allyloxycarbonyl-Protected Amines: Efficient One-Pot Formation of Amides and Dipeptides. J. Org. Chem. 1995, 60 (6), 1733−1740. (33) Gomez-Martinez, P.; Dessolin, M.; Guibé, F.; Albericio, F. NαAlloc Temporary Protection in Solid-Phase Peptide Synthesis. The Use of Amine−borane Complexes as Allyl Group Scavengers. J. Chem. Soc., Perkin Trans. 1 1999, 31 (20), 2871−2874. (34) Garro-Helion, F.; Merzouk, A.; Guibe, F. Mild and Selective Palladium(0)-Catalyzed Deallylation of Allylic Amines. Allylamine and Diallylamine as Very Convenient Ammonia Equivalents for the Synthesis of Primary Amines. J. Org. Chem. 1993, 58 (22), 6109− 6113. (35) Korotkikh, N. I.; Cowley, A. H.; Moore, J. A.; Glinyanaya, N. V.; Panov, I. S.; Rayenko, G. F.; Pekhtereva, T. M.; Shvaika, O. P. Reaction of 1-Tert-Butyl-3,4-Diphenyl-1,2,4-Triazol-5-Ylidenes with a Malonic Ester. Org. Biomol. Chem. 2008, 6 (1), 195−199. (36) Gallagher, W. P.; Vo, A. Dithiocarbamates: Reagents for the Removal of Transition Metals from Organic Reaction Media. Org. Process Res. Dev. 2015, 19, 1369−1373. (37) Ruiz-Sanchez, A. J.; Mesa-Antunez, P.; Barbero, N.; Collado, D.; Vida, Y.; Najera, F.; Perez-Inestrosa, E. Synthesis of All-Aliphatic Polyamide Dendrimers Based on a 3,3′-Diaminopivalic Acid Scaffold. Polym. Chem. 2015, 6 (16), 3031−3038. (38) Lim, J.; Kostiainen, M.; Maly, J.; da Costa, V. C. P.; Annunziata, O.; Pavan, G. M.; Simanek, E. E. Synthesis of Large Dendrimers with the Dimensions of Small Viruses. J. Am. Chem. Soc. 2013, 135 (12), 4660−4663. (39) Ruiz, J.; Lafuente, G.; Marcen, S.; Ornelas, C.; Lazare, S.; Cloutet, E.; Blais, J.-C.; Astruc, D. Construction of Giant Dendrimers Using a Tripodal Building Block. J. Am. Chem. Soc. 2003, 125, 7250− 7257. (40) Lartigue, M.-L.; Donnadieu, B.; Galliot, C.; Caminade, A.-M.; Majoral, J.-P.; Fayet, J.-P. Large Dipole Moments of PhosphorusContaining Dendrimers. Macromolecules 1997, 30 (23), 7335−7337. (41) Blais, J.-C.; Turrin, C.-O.; Caminade, A.-M.; Majoral, J.-P. maldi tof Mass Spectrometry for the Characterization of Phosphorus-

Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter. Angew. Chem., Int. Ed. Engl. 1990, 29 (2), 138−175. (3) Zhang, B.; Wepf, R.; Kröger, M.; Halperin, A.; Schlüter, A. D. Height and Width of Adsorbed Dendronized Polymers: Electron and Atomic Force Microscopy of Homologous Series. Macromolecules 2011, 44 (17), 6785−6792. (4) Swanson, D. R.; Huang, B.; Abdelhady, H. G.; Tomalia, D. A. Unique Steric and Geometry Induced Stoichiometries Observed in the Divergent Synthesis of Poly(Ester-Acrylate/Amine) (PEA) Dendrimers. New J. Chem. 2007, 31 (7), 1368−1378. (5) Kawaguchi, T.; Walker, K. L.; Wilkins, C. L.; Moore, J. S. Double Exponential Dendrimer Growth. J. Am. Chem. Soc. 1995, 117, 2159− 2165. (6) Jishkariani, D.; Macdermaid, C. M.; Timsina, Y. N.; Grama, S.; Gillani, S. S.; Divar, M.; Yadavalli, S. S.; Moussodia, R.-O.; Leowanawat, P.; Berrios Camacho, A. M.; et al. Self-Interrupted Synthesis of Sterically Hindered Aliphatic Polyamide Dendrimers. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (12), E2275−E2284. (7) Schlüter, A. D.; Rabe, J. P. Dendronized Polymers: Synthesis, Characterization, Assembly at Interfaces, and Manipulation. Angew. Chem., Int. Ed. 2000, 39 (5), 864−883. (8) Frauenrath, H. Dendronized Polymersbuilding a New Bridge from Molecules to Nanoscopic Objects. Prog. Polym. Sci. 2005, 30 (3−4), 325−384. (9) Schlüter, A. D.; Halperin, A.; Kröger, M.; Vlassopoulos, D.; Wegner, G.; Zhang, B. Dendronized Polymers: Molecular Objects between Conventional Linear Polymers and Colloidal Particles. ACS Macro Lett. 2014, 3 (10), 991−998. (10) Grebikova, L.; Kozhuharov, S.; Maroni, P.; Mikhaylov, A.; Dietler, G.; Schlüter, A. D.; Ullner, M.; Borkovec, M. The Persistence Length of Adsorbed Dendronized Polymers. Nanoscale 2016, 8, 13498−13506. (11) Zhang, B.; Wepf, R.; Fischer, K.; Schmidt, M.; Besse, S.; Lindner, P.; King, B. T.; Sigel, R.; Schurtenberger, P.; Talmon, Y.; et al. The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object. Angew. Chem., Int. Ed. 2011, 50 (3), 737−740. (12) Schlüter, A. D.; Halperin, A.; Kröger, M.; Vlassopoulos, D.; Wegner, G.; Zhang, B. Dendronized Polymers: Molecular Objects between Conventional Linear Polymers and Colloidal Particles. ACS Macro Lett. 2014, 3 (10), 991−998. (13) Zhang, B.; Wepf, R.; Kröger, M.; Halperin, A.; Schlüter, A. D. Height and Width of Adsorbed Dendronized Polymers: Electron and Atomic Force Microscopy of Homologous Series. Macromolecules 2011, 44 (17), 6785−6792. (14) Yu, H.; Schlüter, a. D.; Zhang, B. Main-Chain Scission of a Charged Fifth-Generation Dendronized Polymer. Helv. Chim. Acta 2012, 95 (12), 2399−2410. (15) Yu, H.; Schlüter, A. D.; Zhang, B. Synthesis of Dendronized Polymers by a “n + 2” Approach. Macromolecules 2012, 45 (21), 8555−8560. (16) Yu, H.; Schlüter, A. D.; Zhang, B. Synthesis of High Generation Dendronized Polymers and Quantification of Their Structure Perfection. Macromolecules 2014, 47 (13), 4127−4135. (17) Zhang, B.; Yu, H.; Schlüter, A. D.; Halperin, A.; Kröger, M. Synthetic Regimes Due to Packing Constraints in Dendritic Molecules Confirmed by Labelling Experiments. Nat. Commun. 2013, 4, 1993. (18) Merrifield, R. B. Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. J. Am. Chem. Soc. 1963, 85 (14), 2149−2154. (19) Erickson, B. W.; Merrifield, R. B. Solid-Phase Peptide Synthesis. In The Proteins; 1976; pp 255−527. (20) Sanger, F. The Free Amino Groups of Insulin. Biochem. J. 1945, 39 (5), 507−515. (21) Shu, L.; Göossl, I.; Rabe, J. P.; Schlüter, A. D. Quantitative Aspects of the Dendronization of Dendronized Linear Polystyrenes. Macromol. Chem. Phys. 2002, 203 (18), 2540−2550. (22) Bertran, O.; Zhang, B.; Schlüter, A. D.; Kröger, M.; Alemán, C. Computer Simulation of Fifth Generation Dendronized Polymers: I


Article

Macromolecules Containing Dendrimers. Scope and Limitations. Anal. Chem. 2000, 72 (20), 5097−5105. (42) van Dongen, M. A.; Desai, A.; Orr, B. G.; Baker, J. R.; Banaszak Holl, M. M. Quantitative Analysis of Generation and Branch Defects in G5 Poly(Amidoamine) Dendrimer. Polymer 2013, 54 (16), 4126− 4133. (43) Lescanec, R. L.; Muthukumar, M. Configurational Characteristics and Scaling Behavior of Starburst Molecules: A Computational Study. Macromolecules 1990, 23 (8), 2280−2288. (44) Boris, D.; Rubinstein, M. A Self-Consistent Mean Field Model of a Starburst Dendrimer: Dense Core vs Dense Shell. Macromolecules 1996, 29, 7251−7260. (45) Ballauff, M.; Likos, C. N. Dendrimers in Solution: Insight from Theory and Simulation. Angew. Chem., Int. Ed. 2004, 43 (23), 2998− 3020. (46) Bertran, O.; Zhang, B.; Schlüter, A. D.; Halperin, A.; Kröger, M.; Alemán, C. Computer Simulation of Dendronized Polymers: Organization and Characterization at the Atomistic Level. RSC Adv. 2013, 3 (1), 126−140. (47) Rosenfeldt, S.; Dingenouts, N.; Ballauff, M.; Werner, N.; Vögtle, F.; Lindner, P. Distribution of End Groups within a Dendritic Structure: A SANS Study Including Contrast Variation. Macromolecules 2002, 35, 8098−8105.

J


Pushing Synthesis toward the Maximum Generation Range of

Recommend Documents