Computer Simulation of Fifth Generation Dendronized Polymers: Impact of Charge on Internal Organization Oscar Bertran,* ,† Baozhong Zhang, ‡ A. Dieter Schlü ter, ‡ Martin Krö ger, ‡ and Carlos Alema ́ n* ,§,∥ † Departament of Applied Physics, EEI, Universitat Polite ̀ cnica de Catalunya, Pç a Rei 15, Igualada 08700, Spain ‡ Department of Materials, Institute of Polymers, Swiss Federal Institute of Technology, ETH Zurich, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland § Departament of Chemical Engineering, ETSEIB, Universitat Polite ̀ cnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain ∥ Centre for Research in Nano-Engineering, Universitat Polite ̀ cnica de Catalunya, Edifici C′, C/Pasqual i Vila s/n, Barcelona E-08028, Spain ABSTRACT: The internal organization of a fifth-generation dendronized polymer (PG5) has been investigated by atomistic molecular dynamics simulations in a vacuum. This study reveals an exceptional behavior of PG5 within the homologous series of g-generation PGg polymers. Three molecular configurations, which present a heterogeneous distribution of dendrons and an amount of backfolding lower than PG4 and PG6, have been obtained for PG5. The highest stability and closest agreement with available experimental data corresponds to a helical conformation characterized by a pitch of about 30 Å, thickness of 105 Å, and average density of 0.861 g/cm 3 . While small angle neutron scattering studies of PG5 in solution show a constant radial density distribution around the backbone, we here in our vacuum studies find a cylindrical volume element of sharply reduced density reminiscent of a pore. This neutral PG5 was compared with its charged deprotected analogue, dePG5 in water, to see in as much the positive charges in the periphery of this macromolecule would affect its conformational behavior. During deprotection of PG5, the tert-butyloxycarbonyl protected amine groups are converted into ammonium, mimicking the experimental situation during a divergent synthesis procedure. The repulsive interactions among the positively charged ammonium groups are responsible for a huge (∼30%) reduction of the average density and a small (∼1%) increase of elongation of the helical backbone, which results in a structure with a spongy appearance. Also here, we find a reduced dendron density near the backbone which is compensated for by the pore being filled with water. ■ INTRODUCTION Dendronized polymers (DPs) consist of regularly branched fragments (dendrons) densely attached along a linear polymer backbone. Steric repulsions among neighboring dendrons, increasing in strength with their generation number, g, compel polymer main chains to extend from random coils to weakly bent, rod-like cylinders typically found for DPs. 1−5 Accordingly, the rigidity, diameter, and properties of DPs can be tuned by varying g. 6 DPs represent an important class of single molecular nanomaterials with important potential applications, as for example in catalytic, drug delivery, and biosensors systems. 7−12 On the other hand, atomistic modeling of the internal structure of DPs is a very challenging task 13−19 because of their huge dimensions and intrinsic conformational complexity, which reflect crowding and excluded volume interactions. Current computational facilities allow studies of these chemical systems, which were unaffordable a few years ago, enabling one to get microscopic information that often remains exper- imentally inaccessible. Within this context, we recently 19 used conformational search and growth procedures combined with molecular dynamics (MD) simulations to model the structure and properties of a homologous series of neutral DPs (Figure 1a), whose repeat units are regularly branched dendrons of generations g =1−7, denoted PG1−PG7 (where PGg refers to a DP made of g-generation dendrons). The backbone of DPs with g ≤ 4 was found to display an elongated shape (Figure 1b), while PG6 exhibited a helical conformation (Figure 1b). Furthermore, we predicted that the existence of defect-free DPs with g ≥ 7 is precluded because of their stiffness and related strain onto their backbone. Both properties were seen to reflect packing constraints. Other calculated properties for these polymers were the fractal dimensionality, the local density profiles, the thickness, and the diffusion and load of small molecules inside DP structures. Experimental estimations, 6,20 when available, were in good agreement with theoretical predictions. Despite the synthesis and structural properties of PG5 (Scheme 1) were recently reported, 21 this DP was excluded from our previous modeling study. 19 This was because preliminary simulations evidenced its seemingly exceptional behavior within the homologous series, indicating that the studies required for their thorough characterization are more extensive and elaborated for PG5 than for the other polymers of the series. Indeed, PG5 is the largest synthetic linear Received: March 18, 2013 Revised: April 17, 2013 Published: May 6, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 6007 dx.doi.org/10.1021/jp402695g | J. Phys. Chem. B 2013, 117, 6007−6017 For your personal use only. Not for redistribution related contributions available from the author(s) at www.complexfluids.ethz.ch