Marinos Pitsikalis

Anionic polymerization: High vacuum techniques

Anionic polymerization is a powerful tool for the synthesis of a variety of model materials with well-defined molecular characteristics. However specially designed apparatuses and appropriate high vacuum techniques are needed in order to exclude from the reaction environment all reactive contaminants with the anionic centers. This review describes the basic principles of anionic polymerization as well as detailed experimental methods for the purification of the reagents usually used for the synthesis of model polymeric materials. In addition a few examples of the synthesis of polymers with complex architecture are given.

Synthesis of Well-Defined Polypeptide-Based Materials via the Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides

Since 1906, when Leuchs synthesized the first R-amino acid N-carboxyanhydrides (NCAs),1 later referred to as Leuchs’ anhydrides, a great number of publications dealing with the ring-opening polymerization (ROP) of these monomers (Scheme 1) has accumulated. This interest stems from the wide variety of polypeptides that this polymerization can generate. The synthetic polypeptides produced from the NCAs, although far from being monodisperse or constructed from a precise sequence and composition of R-amino acid residues, possess the ability, as their natural relative-proteins, to form R-helix and -sheet motifs. These secondary structures contribute significantly to the self-assembling character of polypeptide chains, leading to novel supramolecular structures with potential biomedical and pharmaceutical applications.2 As for their natural counterparts, it is important for such synthetic polypeptides to be well-defined with high molecular and structural homogeneity in order to favor their selfassembly into precisely defined nanostructures, a requirement for appropriate functionality. It was not until 1997, when Deming3 reported the first living initiating system for the ROP of NCAs, that the synthesis of well-defined polypeptides was achieved. Following this first report, other alternative living initiating systems or methods have also been developed. These living systems lead to well-defined homo-/copolypeptides and hybrids, with high molecular weight and structural homogeneity. Nevertheless, the earlier studies served as the springboard for developments in the whole area of polypeptide synthesis. Several excellent reviews4 have been dedicated to the ROP of NCAs, elucidating the mechanistic aspects of this polymerization. However, only a few have addressed the synthesis of polypeptide-based materials with different macromolecular architectures.4c,5,6 This review is divided into three parts. The first highlights the mechanistic developments of the ROP of NCAs from the conventional to the living initiating systems/methods; the second is dedicated to the synthesis of polypeptides and polypeptide hybrids with different macromolecular architectures; and the third deals with surface-bound polypeptides. Surface-bound polypeptides were incorporated in the review due to the great interest in biologically active surfaces for medical diagnostics and sensors.7

Nonlinear Block Copolymer Architectures

The synthesis and bulk and solution properties of block copolymers having nonlinear architectures are reviewed. These materials include star-block copolymers, graft copolymers, miktoarm star copolymers, and complex architectures such as umbrella polymers and certain dendritic macromolecules. Emphasis is placed on the synthesis of well-defined, well-characterized materials. Such polymers serve as model materials for understanding the effects of architecture on block copolymer self-assembly, in bulk and in solution.

Asymmetric Star Polymers: Synthesis and Properties

The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies.

Viscoelasticity and self‐diffusion in melts of entangled asymmetric star polymers

The crossover from linear to branched polymer dynamics in highly entangled melts was investigated with a series of asymmetric three-arm stars of poly(ethylene-alt-propylene). Two arms of equal length formed a linear backbone, kept constant through the series, while branches of various length were appended as the third arm. The materials were made by carbanionic polymerization of isoprene and the judicious application of chlorosilane linking chemistry. Subsequent saturation of the polymeric double bonds with deuterium and hydrogen, followed by fractionation, led to a set of structurally matched, nearly monodisperse pairs of deuterium-labeled and fully hydrogenous samples. Dynamic moduli were measured over wide ranges of frequency and temperature. With increasing branch length, the resulting master curves evolve smoothly, but with surprising rapidity, from the relatively narrow terminal spectrum of linear polymers to the much broader spectrum of symmetric stars. The viscosity ηo increases rapidly with branch length, and the diffusion coefficient D, obtained by forward recoil spectrometry, decreases even more rapidly. The product ηoD, however, distinguishes the transition from linear to branched polymer dynamics most clearly: for a backbone with about 38 entanglements, the crossover is already approaching completion for a single mid-backbone branch with only about three entanglements.

Poly(dl-lactide)-b-poly(N,N-dimethylamino-2-ethyl methacrylate): Synthesis, Characterization, Micellization Behavior in Aqueous Solutions, and Encapsulation of the Hydrophobic Drug Dipyridamole

We synthesized a series of well-defined poly(dl-lactide)-b-poly(N,N-dimethylamino-2-ethyl methacrylate) (PDLLA-b-PDMAEMA) amphiphilic diblock copolymers by employing a three-step procedure: (a) ring-opening polymerization (ROP) of dl-lactide using n-decanol and stannous octoate, Sn(Oct)(2), as the initiating system, (b) reaction of the PDLLA hydroxyl end groups with bromoisobutyryl bromide, and (c) atom transfer radical polymerization, ATRP, of DMAEMA with the newly created bromoisobutyryl initiating site. The aggregation behavior of the prepared block copolymers was investigated by dynamic light scattering and zeta potential measurements at 25 degrees C in aqueous solutions of different pH values. The hydrophobic drug dipyridamole was efficiently incorporated into the copolymer aggregates in aqueous solutions of pH 7.40. High partition coefficient values were determined by fluorescence spectroscopy.

End-functionalized polymers with zwitterionic end-groups

Abstract The synthesis, dilute solution and bulk properties of well-defined polymers with different architectures (linear homopolymers, di- and tri-block copolymers and star homopolymers) having dimethylamine and sulfobetaine end-groups are reviewed. The end functionalized polymers were prepared by means of anionic polymerization using high vacuum techniques. 3-Dimethylaminopropyllithium was used as a functional initiator for the introduction of dimethylamine end-groups. This group was switched to a sulfozwitterionic one by reaction with cyclopropanesultone. The high molecular and compositional homogeneity of these model materials was confirmed by extensive molecular characterization data. Their aggregation behavior in dilute solutions having different polarities was studied by osmometry, viscometry and static and dynamic light scattering and was compared to predictions derived from theoretical models. The end-functionalized polystyrenes and the block copolymers of styrene possess lower degrees of association than the homopolydienes, probably due to the polarizability of the phenyl groups. The bulk properties of the functionalized homopolymers and diblock copolymers, studied by SAXS, rheology and dielectric spectroscopy, revealed new features of self-organization at this low ionic content and extraordinary phase stability at higher temperatures. The adsorption behavior of linear and star homopolydienes was investigated by ellipsometry and by using a surface-force apparatus.

Linking reactions of living polymers with bromomethylbenzene derivatives: Synthesis and characterization of star homopolymers and graft copolymers with polyelectrolyte branches

Anionic polymerization techniques utilizing 1,2,4,5-tetra(bromomethyl)- benzene as the linking agent were employed for the synthesis of four-arm star polymers with poly(tert-butyl methacrylate) (PtBuMA), poly(methyl methacrylate), poly(tert-butylacrylate) (PtBuA), or poly(2-vinylpyridine) (P2VP) branches. This work was extended through the “grafting onto” method, in combination with anionic polymerization techniques, to synthesize graft copolymers consisting of polystyrene backbones and PtBuA, PtBuMA, or P2VP branches. Postpolymerization reactions were performed to produce graft copolymers with polyelectrolyte branches. Crosslinking reactions were observed in some of the graft materials several months after their preparation.

Controlling the self-assembly and dynamic response of star polymers by selective telechelic functionalization

We report on the structure and dynamics of model mono-, di-, and tri-ω-functionalized three-arm star polybutadiene melts. By using x-ray scattering and dynamic rheological measurements, we find that functionalization of the arm ends can lead to distinctly different supramolecular structures and material behavior. The monofunctionalized samples behave like multiarm nonionic star-like dendrimers, whereas the difunctional stars resemble a transient network consisting of highly branched structures with a very broad relaxation spectrum. On the other hand, the trifunctional stars seem to develop an unusually regular structure of dominant intramolecularly aggregated functional groups leading to collapsed star conformations, resembling soft spheres, and a well-defined terminal relaxation. These results suggest that by tailoring the telechelic functionalization of regular star polymers, a route to design and obtain a wealth of controlled supramolecular structures exhibiting a rich and variable dynamics could open.

Radical copolymerization of styrene and alkyl methacrylates: monomer reactivity ratios and thermal properties

Abstract Copolymers containing styrene and alkyl methacrylate ( n -butyl-, n -hexyl-, or stearyl methacrylate) at different compositions have been prepared by radical copolymerization. The monomer reactivity ratios were estimated using the Finemann–Ross, the inverted FR and the Kelen–Tudos graphical methods. Structural parameters of the copolymers were obtained calculating the dyad monomer sequence fractions. The effect of the size of the alkyl methacrylate on the copolymer structure is discussed. The glass transition temperature, T g of the copolymers with butyl and hexyl methacrylate was examined in the frame of several theoretical equations allowing the prediction of these T g values. The best fit was obtained using methods that take into account the monomer sequence distribution of the copolymers. The copolymers of styrene with stearyl methacrylate exhibited the characteristic melting endotherm, due to the crystallinity of the methacrylate sequences and the polystyrene glass transition temperature.