Programmed Polymer Folding with Periodically Positioned

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A Programmed Polymer Folding with PeriodicallyPositioned Tetrafunctional Telechelic Precursors by Cyclic Ammonium Salt Units as Nodal Points Kohei Kyoda, Takuya Yamamoto, and Yasuyuki Tezuka J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Journal of the American Chemical Society

A Programmed Polymer Folding with PeriodicallyPositioned Tetrafunctional Telechelic Precursors by Cyclic Ammonium Salt Units as Nodal Points Kohei Kyoda, Takuya Yamamoto+, and Yasuyuki Tezuka*

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan *Address corresponding to this author: [email protected]

+

Current Address; Division of Applied Chemistry, Faculty of Engineering, Hokkaido University,

Sapporo, Hokkaido, 060-8628, Japan.

ABSTRACT: A programmed polymer folding process has been demonstrated by employing a pair of periodically positioned tetrafunctional, linear telechelic poly(THF)s having 5-membered cyclic ammonium salt groups, i.e., N-ethyl or N-phenylpyrrolidinium groups at both chain ends, and N,N-dialkylpyrrolidinium groups at the two interior positions, accompanying two units of a dicarboxylate counteranion to balance the charges, Ia and Ib, respectively. The electrostatic self-assembly and covalent fixation process has subsequently been applied, to cause the ringopening reaction of the pyrrolidinium units by carboxylate counteranions under dilution. 1 ACS Paragon Plus Environment

The

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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obtained doubly cyclized polymer products, IIa from Ia and IIb from Ib, were characterized by 1

H-NMR and by MALDI-TOF mass technique, to indicate the formation of polymeric

constitutional isomers of either manacle-, 8- or -form.

The SEC peak deconvolution analysis

of IIa showed the preferential formation of the manacle-form isomer over the 8- and the -form counterparts, to accord with the polymer folding of Ia, having the equivalent chemical reactivity of the linking groups, directed by the spatial distance between the folding points.

On the other

hand, the relevant SEC analysis of IIb showed the predominant formation of the 8-form isomer, consistent with the polymer folding of Ib promoted by the enhanced chemical reactivity of the Nphenylpyrrolidinium end groups over the interior N,N-dialkylpyrrolidinium groups.

INTRODUCTION The programmed folding of polymer molecules has frequently been encountered in such essential living processes, as DNA packaging/folding1 and protein 3D structure formation,2 achieved after the prolonged chemical evolution.

It has been highlighted, moreover, that a class

of cyclic peptides, cyclotides, having fused-multicyclic structures, formed typically through the folding with the intramolecular S-S bridging between cysteine residues, realized the extraordinary stability and bioactivity.3

Notably, in particular, a topologically intriguing, triply-fused

tetracyclic form of the non-planar K3,3 graph construction, which cannot be embedded in the plane 2 ACS Paragon Plus Environment

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in such a way that its edges intersect only at their endpoints, has been identified in cyclotides from diverse origins.3b In contrast, the effective and programmed folding process of synthetic polymers into designated multicyclic graph constructions have yet been a formidable challenge in polymer chemistry, where the precision control is required in the intramolecular crosslinking reaction between the reactive groups introduced along the linear prepolymer backbone.4

In this respect,

the end-to-end ring closure polymer cyclization and the intramolecular network formation reaction to give single chain polymer nanoparticles are relevant to the polymer folding, and the programmed/controlled polymer folding is crucial for the structure designing and the property tuning of such complex polymer materials.5,6

In polymer diffusion dynamics, moreover,

temporal polymer folding conformations are postulated as “macromolecular individualism”, inspired from the single molecule spectroscopic observation of such isomeric forms in DNA diffusion processes.7 We have so far developed an electrostatic self-assembly and covalent fixation protocol for designing multicyclic polymer structures, by employing linear, star or other branched telechelic precursors having cyclic ammonium salt groups, accompanying plurifunctional carboxylate counteranions, as key intermediates.8,9

The three forms of dicyclic constructions, i.e., θ (fused),

8 (spiro) and manacle (bridged), as well as a trefoil (spiro tricyclic) construction have been

3 ACS Paragon Plus Environment


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effectively produced through this electrostatic self-assembly and the subsequent covalent fixation process.9

Moreover, a triply-fused tetracyclic macromolecular K3,3 graph topology has

successfully been constructed by using a uniform-size dendritic polymer precursor having six cyclic ammonium salt end groups carrying two units of a trifunctional carboxylate counteranions, and subsequent covalent conversion by the ring-opening reaction of cyclic ammonium salt groups under dilution.10 Upon these developments, we show herein a programmed polymer folding process by employing a pair of linear polymer precursors having four linking units, positioned periodically at both chain ends and additionally at two interior positions.

The formation of the three

polymeric isomers of dicyclic constructions, i.e., manacle-, 8- and -forms, is expected by the double folding of four linking points in the linear polymer precursor. (Scheme 1)

Thereby, we

have attempted to disclose key parameters in the polymer folding process, in contrast to the hypothetical random combination between the reactive groups located along the linear polymer segment, which is frequently assumed upon the formation of single polymer nanoparticles4,5 and of the crosslinked polymer products.6


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Scheme 1. Construction of Dicyclic Polymer Topologies through the Folding of A Tetrafunctional Telechelic Precursor

RESULTS AND DISCUSSION We have prepared a pair of telechelic precursors (Ia and Ib), either having four periodicallypositioned N,N-dialkylpyrrolidinium units, Ia, with the equivalent chemical reactivity each other, or alternatively having a more reactive pair of N-phenylpyrrolidinium units at both chain end groups, Ib, in place of N-ethylpyrrolidinium end groups in Ia.

(Scheme 2)

Thereupon, two

units of difunctional carboxylate, in particular biphenyldicarboxylate, counteranions, were introduced through the ion-exchange reaction to balance the charges.

Subsequently, the

electrostatic self-assembly and the subsequent covalent fixation was applied to complete the selective covalent conversion reactions at the prescribed temperatures, i. e., at 70 oC for the N-



phenylpyrrolidinium groups and 110 oC for both the N-ethylpyrrolidinium and the N,Ndialkylpyrrolidinium counterparts.8,9

Scheme 2. Periodically Positioned Tetrafunctonal Telechelic Poly(THF)s of (top) Equivalent and (bottom) Distinctive Reactivity

The three polymeric isomers of manacle-, 8- and -forms are expected to be produced in equal contents, in case the random folding, i.e., the statistical linking of the folding points of Ia, takes place.

On the other hand, the manacle-form isomer could become predominant, in case

the linking between the spatially closer points is promoted.

(Scheme 3)


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Scheme 3. A Programmed Polymer Folding with A Tetrafunctional Telecheic Polymer Precursor Having Chain-End/Interior Units of the Equivalent Reactivity by Two Units of Difunctional Counteranions

In contrast, the two end groups in Ib having the higher chemical reactivity are expected to be connected each other in the initial step of the polymer folding process, resulting in the prevailed



formation of the 8-shaped isomer.

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While, the manacle- and -shaped counterparts could also be

produced when the two counteranions are allowed to react at the two end units in Ib, prior to the intramolecular polymer folding or cyclization process. (Scheme 4) Thus, we describe first on the synthesis and characterization of the two precursors, Ia and Ib, followed by the polymer folding to give the corresponding doubly cyclized products, IIa and IIb, respectively.

The subsequent SEC deconvolution analysis has revealed the distinctive

compositional features in the polymer folding products IIa and IIb.

Thereupon, we show the

key directing parameters in the non-enzymatic polymer folding undergoing on synthetic linear polymers, either the spatial distance between the lining groups or the chemical reactivity of each linking groups in the linear polymer precursors.

Scheme 4.

A Programmed Polymer Folding with A Tetrafunctional Telechelic Polymer

Precursor Having Chain-End/Interior Units of Distinctive Reactivity by Two Units of Difunctional Counteranions


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1. Preparation of periodically positioned tetrafunctional poly(THF) precursors having cyclic ammonium salt groups. A pair of periodically positioned tetrafunctional telechelic poly(THF)s, Ia and Ib, having 5-membered cyclic ammonium salt groups, at both chain ends and at the two interior positions, were prepared by the click linking reaction using a telechelic poly(THF) having alkyne-modified dialkylpyrrolidinium salt groups (1) and another telechelic poly(THF) having commonly an azide group and additionally either an N-ethyl or N-phenylpyrrolidinium salt group (2a and 2b, respectively).

(Scheme 2)

Thus, a telechelic poly(THF) having alkyne-appended dialkylpyrrolidinium salt groups (1) has been prepared by the end-capping reaction of a bifunctionally living poly(THF) with a pyrrolidine derivative having an alkyne unit, 1-(2-(2-propyn-1-yloxy)ethyl)pyrrolidine (see Figure S1 and S2, for 1H- and 13C-NMR spectra, respectively).

(Scheme 2)

The 1H-NMR

spectrum of 1, listed in Figure 1 (top), showed the methyne and methylene signals due to the propynyl unit at 2.56 ppm and at 4.21 ppm, respectively, together with the overflowed signals of poly (THF) main chain at around 1.6-1.7 ppm, and at around 3.4-3.5 ppm.

Moreover, the

ring methylene signals due to the pyrrolidinium unit is visible at around 3.7 to 4.0 ppm and at around 2.1 to 2.4 ppm, respectively.



Figure 1: 300 MHz 1H-NMR spectra of (top) a telechelic poly(THF) having alkyne-modified dialkylpyrrolidinium salt groups (1), (middle) a telechelic poly(THF)s having an azide and an N-ethylpyrrolidinium salt group (2a), and (bottom) a periodically positioned tetrafunctional telechelic poly(THF)s having 5-membered cyclic ammonium salt groups, at both chain ends and at the two interior positions (Ia)

(CDCl3). 10 ACS Paragon Plus Environment

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Figure 2: 300 MHz 1H-NMR spectra of (top) a telechelic poly(THF) having alkyne-modified dialkylpyrrolidinium salt groups (1), (middle) a telechelic poly(THF)s having an azide and an N-phenylpyrrolidinium salt group (2b) and (bottom) a periodically positioned tetrafunctional telechelic poly(THF)s having 5-membered cyclic ammonium salt groups, at both chain ends and at the two interior positions (Ib)

(CDCl3). 11 ACS Paragon Plus Environment


The ring opening reaction of the pyrrolidinium groups in 1 was then performed by benzoate anions (tetrabutylammonium benzoate) in toluene under reflux for 6 h, to further confirm the formation of 1 having ionic end groups through the quantitative covalent conversion with the alkyne unit intact.

Thus, the 1H-NMR spectrum of the covalent conversion product from 1

(Figure S3) showed the shift of the N-adjacent methylene signals to 2.7-3.0 ppm, together with the benzoate ester methylene signal at 4.35 ppm.

Furthermore, the MALDI-TOF mass

spectrum of the covalent conversion product from 1 (Figure S4) showed the resolved peaks, where the peak at m/z = 3490.9, assumed to be the adduct with Na+, corresponds to the covalent conversion product from 1 possessing the expected chemical structure with a DPn of 40; (C4H8O) × 40 + C36H48N2O6 plus Na+ equals 3490.07.

SEC showed the peak molecular weight of 4400

with PDI of 1.10. A pair of telechelic poly(THF)s, having commonly an azide end group and additionally either an N-ethyl or N-phenylpyrrolidinium salt end group (2a and 2b, respectively), were then prepared by the end-capping reaction of a living poly(THF), prepared with an azido-benzoyl chloride/silver hexafluoroantimonate (AgSbF6) initiator system, with either N-ethylpyrrolidine (for 2a) or N-phenylpyrrolidine (for 2b), respectively.

The ring-opening reaction of N-

ethylpyrrolidinium groups by benzoate anion undergoes at 110 oC under reflux in toluene, while


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the N-phenylpyrrolidinium groups exhibit the enhanced reactivity at 70 oC in toluene.8,9 (Scheme 2) The 1H NMR spectra of 2a and 2b (Figure 1 (middle) and Figure 2 (middle), respectively) showed commonly the signals due to the azidobenzoate ester unit from the initiator at 4.33, 7.06 and 8.03 ppm, in addition to the methyl signal from the N-ethylpyrrolidinium unit at 1.38 ppm for 2a, and to the phenyl signals from the N-phenylpyrrolidinium unit at around 7.5-7.7 ppm for 2b. The covalent conversion of 2a and 2b, both having an ionic end group, was then performed through the heating treatment with benzoate anions (tetrabutylammonium benzoate) in toluene under reflux for 6 h.

The 1H NMR spectra of the covalent derivatives showed the ester

methylene signal commonly at 4.34 ppm by the ring-opening reactions by benzoate anion (Figures S5 and S6), and the shift of the N-phenyl unit signals toward 6.6-6.7 and 7.18 ppm for the covalent conversion product from 2b (Figure S6).

The MALDI-TOF mass spectra of the

covalent conversion product from 2a (Figure S7) and from 2b (Figure S8), showed the resolved peaks, where the peaks at m/z = 2602.1, and at m/z = 2674.0, assumed to be the adducts with H+ for the former and with Na+ for the latter, respectively, correspond to the respective covalently converted products possessing the expected chemical structure with a DPn of 30; (C4H8O) × 30 + C24H30N4O4 plus H+ equals 2601.75, for the former, and with a DPn of 30; (C4H8O) × 30 +



C28H30N4O4 plus Na+ equals 2672.78, for the latter, respectively. SEC showed 2a having the peak molecular weight of 2900 with PDI of 1.10 and 2b having the peak molecular weight of 3100 with PDI of 1.10. The subsequent click coupling reaction was carried out between one unit of bifunctional alkyne-ended precursor, 1, and two units of the azide-ended counterpart, either 2a or 2b, to produce a pair of periodically positioned tetrafunctional telechelic poly(THF)s having 5membered cyclic ammonium salt groups, at both chain ends and at the interior positions, Ia and Ib, respectively. (Scheme 2)

Thus, the reactions of 1 with a slightly excess amount of either

2a or 2b were conducted in the presence of copper sulfate hydrate and sodium ascorbate in THF/water (4/1 in vol/vol).

The 1H-NMR spectra of the products Ia and Ib (Figure 1 (bottom)

and Figure 2 (bottom), respectively) showed the elimination of the propynyl signals in 1 to generate the triazole unit signals at 4.78/4.78 and 8.38/8.40 ppm in Ia and in Ib, while other signals from the precursors 1/2a or 1/2b remained unchanged. The effective click coupling reaction was further confirmed after the covalent conversion of the ionic products Ia and Ib by benzoate anions.

The 1H-NMR spectra of the covalent

conversion derivatives from Ia and from Ib, isolated by means of the preparative SEC fractionation technique, (Figure S9 and in Figure S10, respectively) showed the methyl signals due to the N-ethyl unit at 1.37 ppm for the former and the phenyl signals due to the N-phenyl


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unit at around 6.6-6.7 and at 7.18 ppm for the latter, respectively. Moreover, the benzoate ester methylene signal was observed at around 4.3-4.4 ppm, to confirm the ring-opening reactions of the N-ethylpyrrolidinium unit in Ia and of the N-phenylpyrrolidinum unit in Ib, as well as the N,N-dialkylpyrrolidinium units present in both Ia and Ib. The SEC comparison of the covalent derivative of Ia with the relevant precursors, 1 and 2a, and of Ib with the corresponding precursors, 1 and 2b, were summarized in Figure 3 (left and right), respectively.

The peak molecular weight for Ia was 9800, with a minor trace having

a peak molecular weight of 3000, corresponding to the precursor, 2a, and the peak molecular weight for Ib was 9500, with a minor trace having the peak molecular weight of 3100, corresponding to the precursor 2b.

The covalently converted products from Ia and from Ib

were then isolated by the preparative SEC technique for the subsequent MALDI TOF mass analyses.



Figure 3: SEC traces of (left, from top to bottom) 1 and 2a with Ia, and (right, from top to bottom) 1 and 2b with Ib (Broken lines and solid lines show those obtained before and after SEC fractionation, respectively. THF as aneluent, 1.0 mL/min.)

The MALDI-TOF mass spectra of the covalent derivatives from Ia and from Ib were presented in Figure 4, top and bottom, respectively, to show commonly the resolved peaks, where the peaks at m/z = 7971.7, and at m/z = 8089.1, assumed to be the adducts with H+ for the former and with Na+ for the latter, respectively, correspond to the respective products possessing the expected chemical structure with a DPn of 90; (C4H8O) × 90 + C84H108N10O14 plus H+ equals 7972.50 for the former, and with a DPn of 90; (C4H8O) × 90 + C92H108N10O14 plus Na+ equals 8090.58 for the latter, respectively.


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Figure 4: MALDI-TOF MS spectra of (top) the covalent conversion product from Ia and (bottom) from Ib (Linear mode, matrix: dithranol with sodium trifluoroacetate. DPn denotes the number of monomer units in the product.)

2. The programmed polymer folding of periodically-positioned tetrafunctional poly(THF) precursors having cyclic ammonium salt groups. 17 ACS Paragon Plus Environment


The electrostatic self-assembly and covalent conversion protocol8,9 was applied for the subsequent polymer folding of Ia and of Ib, through the ring-opening reaction of cyclic ammonium salt units located along the poly(THF) segment by the two units of dicarboxylate anions under dilution.

Thus, the dicarboxylate counteranions were introduced through the ion-

exchange reaction by the repeated precipitation of the acetone solution of either Ia or Ib into aqueous solution containing an excess amount of disodium biphenyl dicarboxylate precooled in an ice-bath.

The 1H NMR spectra of the ion-exchange products, Ia/dicarboxylate and

Ib/dicarboxylate, (Figure 5, top, and Figure 6, top, respectively) showed commonly the phenyl signals assignable to biphenyl dicarboxylate counteranions at around 7.5-7.7 and at 8.0-8.2 (for Ia/dicarboxylate) / 8.11 (for Ib/dicarboxylate) ppm. 1

The ion-exchange yields, estimated by the

H NMR, were 56 % for Ia/dicarboxylate and 70 % for Ib/dicarboxylate after the three and four

times repeated precipitation treatments, respectively.


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Figure 5: 300 MHz 1H NMR spectra of (top) the ion-exchange products from Ia and (bottom) the polymer folding product (IIa) obtained after the heating treatment at 110oC.



Figure 6: 300 MHz 1H NMR spectra of (top) the ion-exchange products from Ib, (middle) the product after the subsequent heating treatment at 70oC, and (bottom) the final polymer folding product (IIb) obtained after the further heating treatment at 110oC.


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The subsequent polymer folding was conducted with Ia/dicarboxylate by heating to reflux in toluene (110 oC) at the dilution of 0.2 g/L for 6 h, to cause the ring-opening reaction of the pyrrolidinium units by carboxylate anions.

The 1H NMR spectrum of the product, IIa, isolated

after the purification by means of the preparative SEC fractionation technique, (Figure 5, bottom) showed the biphenyl dicarboxylate signals at 7.68 and 8.08 ppm, as well as the methyl signals from the N-ethyl unit at 1.37 ppm, relevant to the covalent conversion product of Ia with benzoate (Figure S9). On the other hand, the two-step polymer folding process was applied for Ib/dicarboxylate, first by heating at 70 oC in toluene at the dilution of 0.2 g/L for 3 h, to cause the quantitative ring-opening reaction of the N-phenylpyrrolidinium salt groups at the chain ends, followed by the heating at the higher temperature of 110 oC for 6 h, to allow the quantitative ring-opening reaction of the remaining N,N-dialkylpyrrolidinium units at the interior positions in the poly(THF) segment. The 1H NMR spectrum of the product recovered after the first heating step (Figure 6, middle) showed the N-phenyl signals, formed specifically by the ring-opening of the Nphenylpyrrolidinium unit by the carboxylate counteranion, at around 6.6-6.7 and at 7.18 ppm, while the signals due to the N,N-dialkylpyrrolidinium unit at the interior positions remained



intact at this stage.

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And the signal intensity of the ester methylene protons at 4.3-4.4 ppm, by

reference to those of the triazole unit protons at 4.78 ppm, confirmed the selective covalent conversion of the N-phenylpyrrolidinium unit by the biphenyl diacrboxylate counteranions. Notably, on the other hand, it was undistinguishable by the NMR technique whether the ring opening reaction took place either by a single unit of biphenyl dicarboxylate anion (cyclization), or alternatively by the two separate dicarboxylate counteranions (end-capping).

(Scheme 4)

The 1H NMR spectrum of the product obtained after the subsequent heating treatment at 110 oC for 6 h, and isolated by the preparative SEC fractionation technique showed the phenyl signals from the biphenyldicarboxylate units at 7.68 and 8.09 ppm (Figure 6, bottom), relevant to the covalent conversion product of Ib with benzoate (Figure S10).

The signal intensity of

the ester methylene protons by reference to those of the triazole unit, further confirmed the selective covalent conversion of the N,N-dialkylpyrrolidinium units at the interior positions by the biphenyldiacrboxylate. The SEC traces of the polymer folding products, IIa and IIb, are shown in Figure 7 (bottom, left and right, respectively).

The noticeable reduction of the 3D sizes (hydrodynamic volume)

was commonly observed along with the polymer folding of Ia and Ib.

Thus, the SEC peak

molecular weight of the product IIa was 7900, corresponding to 0.81 times of that of the linear precursor analogue, Ia (9800), and the SEC peak molecular weight of the product IIb was 7300,


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corresponding to 0.76 times of that of the linear precursor analogue of Ib (9500), respectively. Additionally, the traces for to the unreacted precursor 2a and 2b having the peak molecular weights of 2,900 and of 3200 correspond, respectively, were observed in the recovered crude products.

We assume the partial loss of the folding products together with the side products

having ionic species, formed due to the incomplete ion-exchange by the dicarboxylate counteranions to Ib, in particular, took place during the sample work-up (silica-gel and reprecipitation treatments) before the SEC measurements, to bring the apparent but conspicuous SEC peak for the low-MW side product (Figure 7, (bottom, right), dotted line).

Thus, the

polymer folding products IIa and IIb were isolated by the preparative SEC technique to remove these lower molecular weight fractions for the subsequent MALDI TOF mass measurements (Figure 7, (bottom), solid lines).



Figure 7: SEC traces of (left) the linear precursor, Ia, (top) and the folding product, IIa, (bottom) before (dotted line) and after (solid line) the SEC fractionation, and of (right) the linear precursor Ib, (top) and the folding product IIb, (bottom) before (dotted line) and after (solid line) the SEC fractionation. (THF as an eluent, 1.0 mL/min.)

MALDI-TOF mass spectra of the isolated IIa and IIb (Figure 8, top and bottom, respectively) showed the resolved peaks, where the peak at m/z = 7994.5, and at m/z = 8084.7, assumed to be the adduct with Na+, correspond to the respective IIa and IIb, possessing the expected chemical structure with a DPn of 90; (C4H8O) × 90 + C84H104N10O14 plus Na+ equals 7992.56, for the former and with a DPn of 90; (C4H8O) × 90 + C92H104N10O14 plus Na+ equals 8086.32, for the latter, respectively.

Notably, moreover, the reduction of 4 mass units was

confirmed for IIa and for IIb in comparison with the linear covalent conversion derivatives from


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Ia and from Ib by four benzoate counteranions, shown before in Figure 3 (Note; the detected mass of Ia with H+, and others with Na+ adducts).

Figure 8: MALDI-TOF MS spectra of (top) the polymer folding products IIa and (bottom) IIb (Linear mode, matrix: dithranol with sodium trifluoroacetate. DPn denotes the number of monomer units in the product.)



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3. SEC deconvolution analysis of the polymer folding products from the periodicallypositioned tetrafunctional poly(THF) precursors. The obtained polymer folding products, IIa and IIb, are trustfully comprised of three dicyclic constitutional isomers having either manacle-, 8- or -forms, possessing the distinct hydrodynamic volumes or 3D sizes each other. (Scheme 1)

Notably, the relative 3D sizes of the

three polymeric isomers have been quantitatively estimated by reference to the SEC peak molecular weight of the relevant linear counterpart, and those were reported as 0.89 for the manacle-, 0.69 for the 8- and 0.57 for the -form, respectively.9

The SEC comparison of the

current polymer folding products with their linear precursors, i.e., from Ia to IIa and from Ib to IIb, are shown in Figure 9, top (left and right, respectively).

The extent of the 3D size reduction

along with the double polymer folding was observed to be 0.81 for the former and 0.76 for the latter, respectively, consistent with formation of the cyclized polymer products.

It is remarkable,

moreover, that the extent of the 3D size reduction along with the polymer folding was smaller from Ia to IIa than from Ib to IIb, and that the SEC peak profile was noticeably broader for IIa in comparison with that for IIb.

(Figure 9, top)

Upon these observations, the polymer folding product IIa is presumed to include the manacle-form isomer as a major component together with marginal contents of the two other isomers.

In contrast, the product IIb is likely comprised of the 8-form isomer as a predominant


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component.

Thus, the relative contents of the three isomer components in IIa and in IIb were

estimated by means of the SEC deconvolution analysis,11 performed by taking into account for the relative sizes of the three polymeric isomers against the linear counterpart, namely 0.89 for the manacle-, 0.69 for the 8- and 0.57 for the -form, respectively.9

(See Supporting Information

for the deconvolution procedure by Excel Solver)

Figure 9: SEC comparison of (top, left) the linear precursor Ia (broken line) and the polymer folding product IIa (solid line), and (top, right) the linear precursor Ib (broken line) and the polymer folding product IIb (solid line), and (bottom) the deconvolution results into three isomeric components in IIa (left) and in IIb (right) (manacle-isomer in violet, 8-isomer in red and -isomer in green, respectively).

The relative contents of the three polymeric isomer components in the product IIa were thus determined to be 60:33:7 for the manacle-, the 8- and the -form, respectively (Figure 9 27 ACS Paragon Plus Environment


(bottom, left)).

Page 28 of 35

The obtained isomer composition is largely deviated from the equal isomer

contents of 33:33:33, expected by the random combination of the linking units in Ia.

Rather, the

obtained composition apparently indicates an alternative process, where the polymer folding of Ia, having the identical chemical reactivity, proceeds by the preferred linking between the two adjacent positions of the spatially closer distance.

Notably, this is in accord with the previous

reports on the kinetics of polymer cyclization reactions,12-14 estimated by the photo-quenching of pyrene-ended polymers of different chain lengths to give the equation of K = N-d , d=1.0-1.5, where K is the rate constant and N is the chain length of the polymer precursor. By the spatially directed polymer folding process, the manacle form isomer is expected to be produced exclusively, in case the first fixation step of the dicarboxylate unit takes place at either of the chain ends, as schematically shown in Scheme 3.

Moreover, the manacle form

isomer is produced together with the 8-form counterpart with the equal probability when the first fixation step occurs at the interior position.

On the other hand, the formation of the -form

isomer is suppressed by the spatially directed polymer folding, as the -form isomer is produced by the combination of the two linking units of the non-adjacent positions in the linear precursor. For the product IIb, in contrast, the composition of the three polymeric isomers was determined to be 11:84:5 for the manacle-, 8- and -form, respectively. (Figure 9, (bottom, right)) The predominant 8-form isomer formation from Ib is consistent with the chemically directed


Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


polymer folding process, where the linking of the N-phenylpyrrolidinium chain ends takes place first by a single diphenylcarboxylate counteranion, followed by the covalent conversion of the remaining two N,N-dialkylpyrrolidinium units located at the interior positions by a second counteranion.

(Scheme 4)

The observed minor portions of the 8-form (11%) with further smaller  -form isomer (5%) in IIb is accounted for by the covalent linking by the two separate biphenyl dicarboxylate counteranions onto the two chain ends at the first step, followed by the cyclization with the N,Ndialkylpyrrolidinium units at the interior positions. (Scheme 4)

The preferred production of the

manacle-form isomer than the -form counterpart is consistent with the spatially directed polymer folding observed in the polymer folding of Ia.

CONCLUSIONS A programmed polymer folding has been demonstrated by employing a pair of periodically positioned tetrafunctional, linear telechelic poly(THF)s having 5-membered cyclic ammonium salt groups, at both chain ends and at the interior positions, accompanying two dicarboxylate counteranions to balance the charges.

The polymer folding has been achieved through the

electrostatic self-assembly and the subsequent covalent fixation process, to cause the ring-opening reaction of cyclic ammonium salt units by carboxylate counteranions under dilution, to complete



the covalent linking.

Page 30 of 35

The obtained doubly-cyclized polymer folding products were

characterized by 1H-NMR and by MALDI TOF mass technique to substantiate the formation of polymeric constitutional isomers of either manacle-, 8- or -forms.

The subsequent SEC

deconvolution analysis allowed to estimate the composition of the three polymeric isomers, to show that the polymer folding process is directed either by the spatial distance between the functional points, by which the manacle-form isomer is produce preferentially over 8- and -form counterparts, or by the chemical reactivity, by which the 8-form isomer is obtainable from the telechelic precursor having more reactive N-phenylpyrrolidinium end groups than the N,Ndialkylpyrrolidinium interior groups. Thereupon, this study could provide, for the first time, important insights on the key structural/chemical parameters to direct polymer folding process, to achieve the programmed but non-enzymatic polymer folding with synthetic polymer systems to produce single polymer nanoparticles as well as crosslinked polymer network products.

ASSOCIATED CONTENT Supporting Information Experimental section, and 1H and 13C NMR of 1-(2-(2-propyn-1-yloxy)ethyl)pyrrolidine, and 1H NMR of the covalent conversion products of 1, 2a, 2b, Ia, Ib, MALDI-TOF mass of 1, 2a, 2b. SEC deconvolution data by Excel Solver.

This material is available free of charge via the


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Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail

[email protected]; [email protected];

Tel +81 3 3727 8017 ORCID Yasuyuki Tezuka: 0000-0001-5264-9846 Takuya Yamamoto: 0000-0001-9716-8237

Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS

This work was supported partly by KAKENHI (17H04878 and 18H04470 to T.Y., and 17H06463 to Y.T.)



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b) Dabrowski-Tumanski, P.; Sulkowska, J. I., Topological knots and links in

proteins, Proc. Nat. Acad. Sci., USA, 2017, 114, 3415-3420 3. a) de Veer, S. J.; Weidmann, J.; Craik, D. Cyclotides as Tools in Chemical Biology. Acc. Chem. Res., 2017, 50, 1557-1565. b) Craik, D. J., Seamless proteins tie up their loose ends. Science, 2006, 311, 1563-1564. c) Yin, Y.; Fei, Q.; Liu, W.; Li, Z.; Suga, H.; Wu, C. Chemical and Ribosomal Synthesis of Topologically Controlled Bicyclic and Tricyclic Peptides Scafolds Primed by Selenoether Formation, Angew. Chem., Int. Ed., 2019, 58, 4880-4885. 4. Mavila, S; Eivgi, O.; Berkovich, I.; Lemcoff, N. G., Intramolecular Cross-Linking Methodologies for the Synthesis of Polymer Nanoparticles,

Chem. Rev. 2016, 116, 878-

961. 5. a) Rothfuss, H; Knöfel, N. D.; Roesky, P. W.; Barner-Kowollik, C., Single-Chain Nanoparticles as Catalytic Nanoreactors, J. Am. Chem. Soc., 2018, 140, 5875–5881. b) ter


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Huurne, G. M.; Voets, I. K.; Palmans, A. R. A.; Meijer, E. W., Effect of Intra- versus Intermoleciular Cross-Linking on the Supramolecular Folding of a Polymer Chain, Macromolecules 2018, 51, 8853-8861. c) Formanek, M.; Moreno, A. J., Single-Chain Nanoparticles under Homogeneous Shear Flow, Macromolecules 2019, 52, 1821-1831. 6. a) Wang, J.; Lin, T.-S.; Gu, Y.; Wang, R.; Olsen, B. D.; Johnson, J. A., Counting Secondary Loops Is Required for Accurate Prediction of End-Linked Polymer Network Elasticity, ACS Macro Lett. 2018, 7, 244-249. b) Zhang, K.; Lackey, M.; Cui, J.; Tew, G. N., Gels Based on Cyclic Polymers, J. Am. Chem. Soc., 2011, 133, 4140-4148. 7. Wang, Z.-G., Polymer Conformation—A Pedagogical Review, Macromolecules 2017, 50, 9073-9114. 8. a) Tezuka, Y., Topological Polymer Chemistry Designing Complex Macromolecular Graph Constructions, Acc. Chem. Res., 2017, 50, 2661-2672.

b) Topological Polymer Chemistry:

Progress of cyclic polymers in syntheses, properties and functions, Tezuka, Y. Ed., World Scientific, Singapore, 2013. 9. Oike, H.; Imaizumi, H.; Mouri, T.; Yoshioka, Y.; Uchibori, A.; Tezuka, Y., Designing Unusual Polymer Topologies by Electrostatic Self-Assembly and Covalent Fixation, J. Am. Chem. Soc., 2000, 122, 9592-9599.



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10. Suzuki, T.; Yamamoto, T.; Tezuka, Y., Constructing a Macromolecular K3,3 Graph through Electrostatic Self-Assembly and Covalent Fixation with a Dendritic Polymer Precursor, J. Am. Chem. Soc., 2014, 136, 10148-10155. 11. Attempts to resolve the three isomer components by means of liquid chromatography techniques have not successful so far. 12. Mita, I.; Horie, K., Diffusion-controlled Reactions in Polymer Systems, J. Macromol. Sci.Rev., Macromol. Chem. Phys., 1987, C27, 91-169. 13. Martin, J. E.; Eichinger, B. E., Dimensions of Intramolecularly Cross-Linked Polymers: 1. Theory, Macromolecules, 1983, 16, 1345-1350. 14. Martin, J. E.; Eichinger, B. E., Dimensions of Intramolecularly Cross-Linked Polymers: 2. Dilute

Solution

Thermodynamic

Parameters

and

Macromolecules, 1983, 16, 1350-1358.


Photon

Correlation

Results,

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Graphic Abstract for

A Programmed Polymer Folding with Periodically-Positioned Tetrafunctional Telechelic Precursors by Cyclic Ammonium Salt Units as Nodal Points Kohei Kyoda, Takuya Yamamoto+, and Yasuyuki Tezuka*

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan *Address corresponding to this author: [email protected] +

Current Address; Division of Applied Chemistry, Faculty of Engineering, Hokkaido University,

Sapporo, Hokkaido, 060-8628, Japan.


Programmed Polymer Folding with Periodically Positioned

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