Conformation of Tunable Nanocylinders: Up to Sixth-Generation Dendronized Polymers via Graft-Through Approach by ROMP

Well-defined dendronized polymers (denpols) bearing high-generation dendron are attractive nano-objects as high persistency provides distinct properties, contrast to the random coiled linear polymers However, their syntheses via graft-through approach have been very challenging due to their structural complexity and steric hindrance retarding polymerization. Here, we report the first example of the synthesis of poly(norbornene) (PNB) containing ester dendrons up to the sixth generation (G6) by ring-opening metathesis polymerization. This is the highest generation ever polymerized among dendronized polymers prepared by graft-through approach, producing denpols with molecular weight up to 1960 kg/mol. Combination of size-exclusion chromatography, light scattering, and neutron scattering allowed a thorough structural study of these large denpols in dilute solution. A semiflexible cylinder model was successfully applied to represent both the static and dynamic experimental quantities yielding persistent length (lp), cross-sectional radius (Rcs), and contour length (L). The denpol persistency seemed to increase with generation, with lp reaching 27 nm (Kuhn length 54 nm) for PNB-G6, demonstrating a rod-like conformation. Poly(endo-tricycle[4.2.2.0]deca-3,9-diene) (PTD) denpols exhibited larger persistency than the PNB analogues of the same generation presumably due to the higher grafting density of the PTD denpols. As the dendritic side chains introduce shape anisotropy into the denpol backbone, future work will entail a study of these systems in the concentrated solutions and melts.


I.2.1. Synthesis of NB-G6
Scheme S1. Synthesis of the macromonomer NB-G6 NB-G5 and 1 were prepared according to the literature procedure 1 . 1.18g (0.26 mmol) of NBG5 was deprotected in excess methanol (6.5ml, 0.04M) with catalytic amount of ptoluenesulfonic acid (4.9mg, 10 mol%). After stirring 2 days, the reaction was quenched by excess amount of triethyl amine and the solvents were dried by rotary evaporator and high vacuum and used without further purification. To the mixture, dichloromethane (2.6mL, 0.1 M) and triethylamine (1.7 mL, 48 equiv) were added. Then isopropylidene-2,2bis(oxymethyl)propionic anhydride(1) (4.12 g, 48 equiv) and 4-dimethylaminopyridine (DMAP) (3.1 mg, 10 mol %) were added at room temperature, and the reaction mixture was stirred for 7 days. After completion of the reaction, saturated NaHCO 3 (10 mL) aqueous solution was added and the reaction stirred for 1 h. The mixture was washed with NaHCO 3 (10 mL) solution. The organic layer was extracted with ethyl acetate and dried with anhydrous MgSO 4 and the solvent was removed on a rotary evaporator. The product was purified by column chromatography with ethyl acetate−hexane mixture (5:1 in volumetric ratio). The separated product solutions were collected and concentrated to yield final product.

I.2.2. Synthesis of TD-G5
Scheme S2. Synthesis of the macromonomer TD-G5 TD-G4 was prepared according to the literature 2 . 1.02g (0.42 mmol) of TD-G4 was deprotected in excess methanol (10.5ml, 0.04M) with catalytic amount of p-toluenesulfonic acid (3.98mg, 5 mol%). After stirring 24 hours, the reaction was quenched by excess amount of triethyl amine and the solvents were dried by rotary evaporator and high vacuum and used without further purification. To the mixture, dichloromethane (4.2mL, 0.1 M) and triethylamine (1.4 mL, 24 equiv) were added. Then isopropylidene-2,2bis(oxymethyl)propionic anhydride(ref) (3.33 g, 24 equiv) and 4-dimethylaminopyridine (DMAP) (5.1 mg, 10 mol %) were added at room temperature, and the reaction mixture was stirred for 2 days. After completion of the reaction, saturated NaHCO 3 (10 mL) aqueous solution was added and the reaction stirred for 1 h. The mixture was washed with NaHCO 3 (10 mL) solution. The organic layer was extracted with ethyl acetate and dried with anhydrous MgSO 4 and the solvent was removed on a rotary evaporator. The product was purified by column chromatography with ethyl acetate−hexane mixture (4:1 in volumetric ratio). The separated product solutions were collected and concentrated to yield final product. 1.55g, 80%. 1

I.3. Synthesis of PNB-G6 and PTD-G5
I.3.1. General Procedure 2-mL sized screw-cap vial with septum was charged with monomer and a magnetic bar. The vial was purged with argon four times, and degassed THF was added purged with Ar, and then dissolved in dry and degassed solvent. The initiator solution was added at once to the monomer solution under vigorous stirring. After c.a. 12h, the polymerization was quenched with by excess ethyl vinyl ether. The concentrated reaction mixture was then precipitated and the polymer was collected and dried under reduced pressure.

I.3.2. Optimization for PNB-G6 and PTD-G5
Scheme S3. Polymerization of NB-G6     ------1661 1.24 a M w and Ɖ were determined by SEC-MALLS in THF. b estimated by the relative integration of the denpol peak to the macromonomer peak. c corresponds to entry 11 in table 1 in the main text. d corresponds to entry 12 in table 1 in the main text. e High molecular weight fraction of the denpols obtained from entry 2 by preparative-SEC.

III.1. General Information
We have used the following parameters to characterize the synthesized denpols: molecular weight M, contour length L, monomer size b 0 persistence length l p , gyration radius R g , cross section radius R cs , hydrodynamic radius R h . We have used a combination of different experimental techniques to access these static and dynamic parameters in dilute denpol solutions, in THF and chloroform (CF). SEC-MALLS data were carefully analyzed in order to provide the radius of gyration R g as a function of molecular weight M within individual fractions. Small Angle Neutron Scattering (SANS) was employed to measure the form factor over a broad wave vector range. The representation of the SANS patterns by the Kholodenko model 3 led to the estimation of L K , l p,K , and R cs . Static light scattering (SLS) was used to measure fraction solutions and obtained average radius of gyration R g , second virial coefficient A 2 , and average weight-averaged molecular weight M w . The hydrodynamic radius R h was deduced from the relaxation functions recorded by dynamic light scattering (DLS) analysis.

III.2. Size Exclusion Chromatography with Multi-Angles Laser Light Scattering (SEC-MALLS)
SEC-MALLS data provided independent measurements of both R g and the corresponding M w in real time during the elution process. The relationship between R g vs M w is utilized to evaluate the persistence length of the denpols, using the Benoit-Doty model as described in section II. . Samples in 0.004-0.006 wt% THF or CF were filtered with a 0.2 μm PTFE filter before injection. Flow rate was 1.0 mL/min and temperature of column was maintained at 35 ℃. The 8-angle MALLS records the wave vector dependent intensity I(q) that leads to the radius of gyration R g (typical data can be found in Figure S14. Values of I/c are then used to calculate the molecular weight M w ("absolute" calibration of the GPC). The data was analyzed using the ASTRA software. Values of dn/dc needed to obtain the molecular weight M w were measured separately from batch mode measurements of polymer solutions at different concentrations, and are provided in supporting information (Table S3). The full fraction samples were also measured by SLS.
MALLS and SLS results were found to be in good agreement (Table S6).

III.3. Small Angle Neutron Scattering (SANS)
SANS experiments were performed on the PA20 spectrometer of the LLB, CEA Saclay. We used wavelengths ranging from 0.5 to 12 nm (dispersion of 10%) with 3 sample-detector configurations, to provide broad wave vector coverage from 0.02 to 4 nm -1 . For the SANS experiments, denpol solutions were prepared in deuterated d8-THF. Standard procedure for signal treatment and normalization were done using. Care was taken to subtract the incoherent background coming from the hydrogenated denpols. The data analysis was done using SASfit software 4 . In particular, the intensity patterns were represented by the Kholodenko form factor 3 described in section II.2.5. In Figure S15 (a) scattering intensity from SANS and SLS for pTD-Gn denpols was normalized by the forward scattering intensity as a function of wave-vector (q) in dilu te solution in d8-THF (c≅5 g/l). In Figure S15 (b) the scattering intensity is normaliz ed by the forward scattering intensity and the wave-vector as a function of the wavevector in log-lin plot. The red dashed lines correspond to the best fit obtained with t he Kholodenko model  Figure S16. Comparison of normalized scattering intensity (q.I(q)/I0) versus q, for two denpols with same dendron generation (G4) but PNB and PTD backbone.
In Figure S16 the purple dash lines correspond to the best fit obtained with the Kholodenko model.

III.4. Dynamic and static light scattering (DLS/SLS)
Light scattering measurements were performed following the same procedure as described earlier 5 . Briefly, the scattered intensity was measured at 25 °C and at several scattering angles (θ) ranging from 20° to 150°. The intensities were normalized using the dn/dc values provided in SI to obtain static characteristics (Mw, R g and A 2 ). The autocorrelation function of the scattered intensity g 2 (q,t) was recorded using an ALV-5000/E correlator and analysed with routine REPES and CONTIN treatments to get the two dynamic quantities, translational diffusion coefficient , D 0 , and hydrodynamic radius,

III.5. Worm like chain models
The denpols conformation was represented by a wormlike chain (WLC) model also known as Kratky-Porod. The semi-flexible chain is characterized by persistence length l p and its contour length L. The WLC model can be retrieved from the freely jointed chain of N monomers with length b 0 and a fixed angle  between consequent monomers. Then , b is known as the Kuhn segment length.

III.5.1. Gyration radius and Benoit-Doty relation
In the SEC-MALLS experiments, the scattered intensity is measured as a function of elution time and an instantaneous molecular weight M is obtained. Then, the chemical contour length L=(M/M 0 ) b 0 is computed from the monomer molar mass, M 0 , calculated from the macromonomer chemical formula (values available in Table S3). The monomer size b 0 is obtained from the chemical structure. As in our previous work, we employed b 0 =0.49±0.04 nm for pNB and b 0 =0.37±0.01 nm for pTD chains.
The mean-square-radius of gyration of monodisperse worm like chains is a function of the persistence length l p (values available in Table S5) and the contour length L, as given by the Benoit-Doty relation: 4 (1) Retrieving the Gaussian coil limit at L/l p >>1 and R g 2 =L.l p /3, and the rod limit at L/l p <<1 and R g 2 = L 2 /12. In the case of thick polymer a cross section radius is needed for a better descrption. In the simple case of a rigid sphero-cylinder of length L and cross section radius R cs : (2)

III.5.2. Form factor and Kholodenko model 3
The wide q-range of the SANS experiments allow access to the full form-factor P 0 (q) of the polymer chain. The WLC model does not result into a simple P 0 (q) expression. Instead, Kholodenko has introduced an empirical form factor model that is correct in both the flexible chain and the rod limits 3 , and is now well accepted for semi-flexible chains 6 .
where 1 ( , ) and 2 ( , ) are 1 st kind of modified Bessel function, x=3L K /(2l p , K ), L K and l p,K parameters are the contour length and persistence length, respectively, with K subscript being used to denote the values obtained from fitting the SANS spectra to the Kholodenko form factor with the ones.
In the case of a thick WLC a term corresponding to the cylindrical cross-section form factor is included to the overall form factor with the form: where R cs is the cross-section radius and 1 ( ) is 1 st kind of Bessel function.
The overall thick WLC Kholodenko form factor is: ( , , , ) = ( , , ). ( , ) By construction it recovers the Gaussian coil form factor in the limit L>>l p and R cs <<1 and the rod or cylinder limit in the case l p >>L.

III.5.3. Hydrodynamic and Winkler model 7-8
Though not analytically, the diffusion coefficient D (and the corresponding hydrodynamic radius R h ) can be computed for semi-flexible chain. Compared to the WLC static form factor, an extra parameter accounting for the friction between monomer and solvent is needed. The drag, which is associated to the monomer length, is envisaged as a hydrodynamic crosssection radius, R hcs, in the case of thick macromolecules. In this case, the diffusion coefficient reads 7-8 : where d is the hydrodynamic diameter (2.R hcs ) of the chain and s indicates distances along the chain.

Polydispersity effect
In the case of length polydispersity, scattering intensities provide measures of z-averaged gyration radius and weight-average molar weight M w , or similarly weight average contour length. L w = (M w /M0)b 0 . Winkler addressed the importance 9 of polydispersity in worm-like chains and has showed that the effect is less pronounced for the dynamic R h than the static R g

III.6. Atomic Force Microscopy (AFM)
The AFM images were collected on a Bruker NanoScope V Multimode 8 device at ambient temperature in tapping mode using non-contact mode silicon tips from Nanoworld (Pointprobe ® tip, NCHR type) with spring constant of 42 N m -1 and tip radius of ≤ 8 nm. 10 μL of the sample solution (0.01 mg/mL in chloroform) was used and then was place on freshly cleaved mica. Then the mica substrate was spin-coated using Spin Coater ACE-200 at a speed 3000 rpm during 15 s. The scanning speed was at a line frequency of 1.0 Hz, and the original images were sampled at a resolution of 512 x 512 pixels.

III.7. Schematic in TOC
The denpols shown in the TOC are realistic representation obtained simulating the Freely Rotating Chain model and using the measured values of l p and Nb o . They were turned into cylinders using R CS .A Python code was written where a monomer is added in 3D space with a random velocity vector. Then, a second monomer is added with a certain bond angle and a dihedral angle which is chosen randomly in the range of 0-180 o . The bond angle is calculated from the relation: ) with the persistence length (known a priori from measurements), 0 the monomer size (in simulation is set to unity) and the bond angle. With this, we obtain the Cartesian coordinates of each monomer. To obtain the 3D models of the polymers, these coordinates were input in Blender where the thickness of the molecule was added as well, with respect to the aspect ratio.