Alexander Sunder

Controlling the Growth of Polymer Trees: Concepts and Perspectives For Hyperbranched Polymers

This article summarizes basic principles and recent progress in the field of cascade-branched polymers. Methods for the preparation of macromolecules with hyperbranched structures are presented and compared concerning the extent of control over molecular weights and polydispersity. Step-growth and recently developed chain-growth strategies as well as enzyme and transition metal catalyzed polymerizations are discussed with respect to mechanism and future potential.

Molecular Nanocapsules Based on Amphiphilic Hyperbranched Polyglycerols.

Polar dyes can be solubilized in apolar media-molecular nanocapsules with hydrophilic interiors have been prepared (see schematic representation) using polyglycerols with narrow polydispersity and simple esterification with fatty acids. These unimolecular micelles offer attractive potential for a variety of applications ranging from controlled drug release to the design of microreactors and catalysts.

Hyperbranched Polyether Polyols: A Modular Approach to Complex Polymer Architectures

In recent years, highly branched polymers have gained widespread attention due to their unique properties, which differ significantly from their linear counterparts. Great emphasis in this area has been placed on polymers with treelike or acascade-typeo branching, with a branch-onbranch topology. These macromolecules typically exhibit compact, globular structures in combination with a high number of functional groups. Since the conformation of such polymers is restricted by their molecular architecture, in contrast to linear polymer chains, entanglements are neglectable. Currently, there is rapidly growing interest in this type of materials for numerous applications. The low viscosity in combination with the high functionality may be useful for a wide variety of possible uses, ranging from functional crosslinkers, additives, and rheology modifiers to components in adhesives, advanced coatings, structured hydrogels, and dental composites. Such polymers might also be of interest in nanotechnology, for example, as building blocks for nanoscale reaction compartments, as a template for nanoporous materials with low dielectric constants, or for the fabrication of defined hybrid particles (e.g., biomineralization techniques). Other fields considered are biochemistry and biomedicine, where such macromolecules could act as carriers, either highly loaded for diagnostic purposes (e.g., for magnetic resonance imaging (MRI) of blood vessels) or as host compartments for controlled drug-release. Finally, their use as homogeneous supports for recyclable catalysts and for supported organic and biochemical syntheses has been suggested. Commonly, the perfectly branched dendrimers have been discussed in this context. However, dendrimers have to be prepared in tedious multistep syntheses, which obviously is a limiting factor for most applications. In contrast to dendrimers, the less structurally perfect (i.e., randomly branched) hyperbranched polymers synthesized via one-step reactions have been considered as possible alternative, since structural perfection may not be a strict prerequisite for many applications. So far, hyperbranched polymers have been regarded as the apoor cousinso of dendrimers because they commonly possess broad polydispersity (often exceeding Mw/Mn = 5!). In addition, their randomly branched architecture (Fig. 1) has been thought to be unsuitable for the construction of complex polymer architectures with, for instance, core±shell topology or defined cavities that would permit the entrapment and release of guests. Furthermore, due to intramolecular cyclization during the synthesis, hyperbranched polymers possess no defined single focal point, such as dendrimer segments (adendronso) that permits further monofunctionalization or attachment to a core as well as polymer chains.

Multi-arm star block copolymers based on ɛ-caprolactone with hyperbranched polyglycerol core

Multi-arm star block copolymers based on e-caprolactone have been prepared by a core-first approach using propoxylated hyperbranched polyglycerol P(G52PO3) as a polyfunctional initiator. Various monomer/initiator ratios were employed in the Sn-catalyzed polymerization in order to vary the length of the caprolactone arms (DPn(arm) = 10 to 50). The molecular weights of the block copolymers were in good aggreement with the calculated values. Careful NMR analysis revealed that the monomer/initiator ratio not only controls the arm length, but also influences the degree of functionalization. Poly(e-CL) stars with 52 arms and absolute molecular weights between 69 200 and 360 400 g/mol have been prepared. SEC measurements showed that the narrow polydispersity of the core molecule (Mw/Mn = 1.24) could be maintained upon block copolymerization. The resulting star polymers exhibited polydispersities between 1.21 and 1.33.

Control of the molecular weight of hyperbranched polyglycerols

Hyperbranched polyglycerols exceeding M n > 15,000 g/mol (DPn > 270) have been prepared, using a modified version of the synthetic protocol reported earlier. In the optimized process polydispersities recorded by SEC remained narrow, with M w /M n < 1.7 over a large molecular weight range. The drastic increase of viscositity in the course of the reaction, was found to be responsible for the increased fraction of cycles (MALDI-TOF MS) at high molecular weights.

Addition-fragmentation free radical polymerization in the presence of olefinic dithioethers as chain transfer agents

Thioethers of 2-methylenepropane-1,3-thiol were prepared as a new class of symmetrically substituted chain transfer agents for addition-fragmentation-type free radical polymerization of styrene. Ester- and hydroxy-functional thioethers were prepared to introduce two functional end groups during free radical polymerization. Chain transfer constants were measured and compared to those of the corresponding disulfides.

HYPERVERZWEIGTE POLYETHERPOLYOLE MIT FLUSSIGKRISTALLINEN EIGENSCHAFTEN

Die Anbindung von Mesogenen als Endgruppen an hyperverzweigtes Polyglycerin (Polymerisationsgrad 22–45; siehe schematische Darstellung, die starren Mesogene sind als Stabchen und die flexiblen Alkylketten als Linien wiedergegeben) fuhrt zu engverteilten flussigkristallinen Polymeren, deren flussigkristallines Verhalten allein durch die mesogenen Endgruppen induziert wird.