Dendronized polymers (or dendronised polymers) are linear

polymers A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...

to every repeat unit of which dendrons are attached. Dendrons are regularly branched, tree-like fragments and for larger ones the polymer backbone is wrapped to give sausage-like, cylindrical molecular objects. Figure 1 shows a cartoon representation with the backbone in red and the dendrons like cake slices in green. It also provides a concrete chemical structure showing a

polymethylmethacrylate Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylit ...

(PMMA) backbone, the methyl group of which is replaced by a dendron of the third generation (three consecutive branching points). Figure 1. Cartoon representation (left) and a concrete example of a third generation dendronized polymer (right). The peripheral

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...

groups are modified by a substituent X which often is a

protection group A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis. In many ...

. Upon deprotection and modification substantial property changes can be achieved. The subscript n denotes the number of repeat units.

Structure and applications

Dendronized polymers can contain several thousands of dendrons in one

macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. ...

and have a stretched out,

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's phys ...

structure. In this regard they differ from the more or less spherically shaped

dendrimers Dendrimers are highly ordered, branched polymeric molecules. Synonymous terms for dendrimer include arborols and cascade molecules. Typically, dendrimers are symmetric about the core, and often adopt a spherical three-dimensional morphology. The ...

, where a few dendrons are attached to a small, dot-like core resulting in an isotropic structure. Depending on dendron generation, the polymers differ in thickness as the atomic force microscopy image shows (Figure 2). Neutral and charged dendronized polymers are highly soluble in organic solvents and in water, respectively. This is due to their low tendency to entangle. Dendronized polymers have been synthesized with, e.g., polymethylmethacrylate,

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is ...

polyacetylene Polyacetylene (IUPAC name: polyethyne) usually refers to an organic polymer with the repeating unit . The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups. This compound ...

polyphenylene Poly(''p''-phenylene) (PPP) is made of repeating ''p''-phenylene units, which act as the precursor to a conducting polymer Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that Electrical c ...

polythiophene Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have alkyl substituents ...

polyfluorene Polyfluorene is a polymer with formula , consisting of fluorene units linked in a linear chain — specifically, at carbon atoms 2 and 7 in the standard fluorene numbering. It can also be described as a chain of benzene rings linked in ''para'' ...

poly(phenylene vinylene) Poly(''p''-phenylene vinylene) (PPV, or polyphenylene vinylene) is a conducting polymer of the rigid-rod polymer family. PPV is the only polymer of this type that can be processed into a highly ordered crystalline thin film. PPV and its derivat ...

, poly(phenylene acetylene),

polysiloxane A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...

, polyoxanorbornene,

poly(ethylene imine) Polyethylenimine (PEI) or polyaziridine is a polymer with repeating units composed of the amine group and two carbon aliphatic ''CHCH'' spacers. Linear polyethyleneimines contain all secondary amines, in contrast to branched PEIs which contain pr ...

(PEI) backbones.

Molar masses In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, p ...

up to 200,000,000 g/mol have been obtained. Dendronized polymers have been investigated for/as bulk structure control, responsivity to external stimuli, single molecule chemistry, templates for

nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 10 ...

formation, catalysis, electro-optical devices, and bio-related applications. Particularly attractive is the use of water-soluble dendronized polymers for the immobilization of enzymes on solid surfaces (inside glass tubes or microfluidic devices) and for the preparation of dendronized polymer-enzyme conjugates. {{Gallery , width=350 , height=200 , lines=6 , align=center , Image:PG1to4wikipedia.jpg, Figure 2.

Atomic force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the opt ...

height image of co-prepared dendronized polymers of generation one through four (PG1-PG4) reflecting the different thicknesses and apparent persistence lengths for each generation , File:Denpol-enzyme conjugate.png, Figure 3. Schematic representation of a molecular hybrid structure (conjugate) between a dendronized polymer and the two different enzymes HRP (horseradish peroxidase) and SOD (Cu,Zn-superoxide dismutase). PDB (SOD): 1SXA; PDB (HRP): 1ATJ; HRP sugar modification from Gray & Montgomery, ''Carbohydrate Research'', 2006, ''341'', 198-209. Denpol structure: Bertran et al. ''RSC Adv.'', 2013, ''3'', 126-140.

Synthesis

The two main approaches into this class of polymers are the

macromonomer A macromonomer is a macromolecule with one end-group that enables it to act as a monomer. Macromonomers will contribute a single monomeric unit to a chain of the completed macromolecule. Several macromonomers have been successfully synthesized ut ...

route and the attach-to route. In the former, a monomer which already carries the dendron of final size is polymerized. In the latter the dendrons are constructed generation by generation directly on an already existing polymer. Figure 4 illustrates the difference for a simple case. The macromonomer route results in shorter chains for higher generations and the attach-to route is prone to lead to structure imperfections as an enormous number of chemical reactions have to be performed for each macromolecule. Denpol synthesis

History

The name “dendronized polymer” which meanwhile is internationally accepted was coined by Schlüter in 1998. The first report on such a macromolecule which at that time was called “Rod-shaped Dendrimer” goes back to a patent by Tomalia in 1987 and was followed by Percec’s first mentioning in the open literature of a

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

with “tapered side chains” in 1992. In 1994 the potential of these polymers as cylindrical nanoobjects was recognized. Many groups worldwide contributed to this field. They can be found in review articles.A. D. Schlüter, J. P. Rabe ''Angew. Chem. Int. Ed.'' 2000, ''39'', 864-883; A. Zhang, L. Shu, Z. Bo, A. D. Schlüter, ''Macromol. Chem. Phys.'' 2003, ''204'', 328-339; A. D. Schlüter, ''Top. Curr. Chem.'' 2005, ''245'', 151-191; H. Frauenrath ''Prog. Polym. Sci.'' 2005, ''30'', 325-384. B. M. Rosen, C. J. Wilson, D. A. Wilson, M. Peterca, M. R. Imam V. Percec, ''Chem. Rev''. 2009, ''109'', 6275-6540; Y. Chen, X. Xiong, ''Chem. Commun.'' 2010, ''46'', 5049; J. I. Paez, M. Martinelli, V. Brunetti, M. C. Strumia, ''Polymers'' 2012, ''4'', 355.

References

Polymers Soft matter

Structure and applications

Synthesis

History

See also

References