Hermis Iatrou

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

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.

Asymmetric caging in soft colloidal mixtures

The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge scientists, on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario, namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties.

Morphology of miktoarm star block copolymers of styrene and isoprene

The morphologies of several styrene–isoprene miktoarm star copolymers have been examined by small angle x‐ray scattering and electron microscopy. Three of these polymers were of the three‐armed A2B type, and six were four‐armed A3B miktoarms. The ordered microphase‐separated morphologies displayed by these polymers were seen to be highly dependent on the architecture of the chains, and were quite different from those that occur in the corresponding linear block copolymers. These results can be quite well explained by a theory of Milner which is based on the bending energy of the microphase interface and the ability of the chains to stretch away from the interface between the microphases. We speculate that the small differences between the observations and theoretical predictions are due to the effects of the compression of chains in small gaps between adjacent domains.

Complex macromolecular chimeras.

By combining two living polymerizations, anionic and ring opening (ROP), the following novel multiblock multicomponent linear and miktoarm star (micro-star) polymer/polypeptide hybrids (macromolecular chimeras) were synthesized: Linear, PBLL-b-PBLG-b-PS-b-PBLG-b-PBLL; 3micro-stars, (PS)2(PBLG or PBLL), (PS)(PI)(PBLG or PBLL); 4micro-stars, (PS)2[P(alpha-MeS)](PBLG or PBLL), (PS)2(PBLG or PBLL)2 [PS, polystyrene; PI, polyisoprene; P(alpha-MeS), poly(alpha-methylstyrene); PBLG, poly(gamma-benzyl-L-glutamate); and PBLL, poly(-tert-butyloxycarbonyl-L-lysine)]. The procedure involves (a) the synthesis of end- or in-chain amino-functionalized polymers, by anionic polymerization high vacuum techniques and appropriate linking chemistry and (b) the use of the amino groups for the ROP of alpha-amino acid carboxyanhydrides (NCAs). Molecular characterization revealed the high molecular weight and compositional homogeneity of the macromolecular chimeras prepared. The success of the synthesis was based mainly on the high vacuum techniques used for the ROP of NCAs, ensuring the avoidance of unwanted polymerization mechanisms and termination reactions.

Tailoring the flow of soft glasses by soft additives.

We examine the vitrification and melting of asymmetric star polymer mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a single glass, in which the small component is fluid in the glassy matrix of the big one, and a double glass, in which both components are vitrified. Addition of small-star polymers leads to melting of both glasses, and the melting curve has a nonmonotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems.

Micellization in pH-sensitive amphiphilic block copolymers in aqueous media and the formation of metal nanoparticles.

Dynamic light scattering, potentiometric titration, transmission electron microscopy and atomic force microscopy have been used to investigate the micellar behaviour and metal-nanoparticle formation in poly(ethylene oxide)-block-poly(2-vinylpyridine), PEO-b-P2VP, poly(hexa(ethylene glycol) methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate), PHEGMA-b-PDEAEMA, and PEO-b-PDEAEMA amphiphilic diblock copolymers in water. The hydrophobic block of these copolymers (P2VP or PDEAEMA) is pH-sensitive: at low pH it can be protonated and becomes partially or completely hydrophilic leading to molecular solubility whereas at higher pH micelles are formed. These micelles consist of a P2VP or PDEAEMA core and a PEO or PHEGMA corona, respectively, where the core forming amine units can incorporate metal compounds due to coordination. The metal compounds (e.g., H2PtCl6, K2PtCl6) can either be introduced in a micellar solution, where they are incorporated within the micelle core via coordination with functional groups, or can be added to a unimer solution at low pH, where they lead to a metal-induced micellization. In these micellar nanoreactors, metal nanoparticles nucleate and grow upon reduction with sizes in the range of a few nanometers as observed by TEM. The effect of the metal incorporation method on the characteristics of the micelles and of the synthesized nanoparticles is investigated.

Surface mobility and slip of polybutadiene melts in shear flow

Surface mobility and wall slip of entangled polybutadiene melts were studied with attenuated-total-reflectance infrared spectroscopy at stresses characteristic of the sharkskin, spurt, and melt-fracture regimes. Small-scale slip, accompanied by an apparent decrease in transverse mobility, occurs in the sharkskin regime, but at a stress above the visual onset of sharkskin in capillary viscometry. Simulations cannot distinguish between a cohesive mechanism and a lubrication mechanism that might follow from a stress-induced phase transition, but an adhesive failure seems to be excluded. The near-surface length scale is of the order of four to six times the equilibrium root-mean-square end-to-end distance, and the estimated slip velocity is insensitive to molecular weight. Strong slip occurs in the spurt regime, either at the wall or within one radius of gyration. Substantial apparent slip occurs with a fluorocarbon surface, but the mechanism does not appear to be an adhesive failure; there seems to be a substantial decrease in the friction coefficient of chains over a distance of order 300 nm or more from the fluorocarbon surface, and the transverse chain mobility in this region appears to be enhanced rather than retarded. Overall, the results of this study indicate that the influence of the wall extends farther into the sheared melt than would be expected from the chain dimensions, except in the case of strong slip.

Effect of Chain Topology on the Self-Organization and Dynamics of Block Copolypeptides: From Diblock Copolymers to Stars

The effect of chain topology on (i) the peptide secondary structure, (ii) the nanophase self-assembly, and (iii) the local segmental and global peptide relaxations has been studied in a series of model diblock and 3-arm star copolypeptides of poly(epsilon-carbobenzyloxy-L-lysine) (PZLL) and poly(gamma-benzyl-L-glutamate) (PBLG) with PZLL forming the core. Diblock copolypeptides are nanophase separated with PBLG and PZLL domains comprising alpha-helices packed in a hexagonal lattice. Star copolypeptides are only weakly phase separated, comprising PBLG and PZLL alpha-helices in a pseudohexagonal lattice. Phase mixing has profound consequences on the local and global dynamics. The relaxation of the peptide secondary structure speeds up, and the helix persistence length is further reduced in the stars, signifying an increased concentration of helical defects.