Modern synthetic methods have revolutionized polymer chemistry through the development of new and powerful strategies for the controlled synthesis of complex polymer architectures. 1-5 Many of these developments were spawned by new classes of transition metal catalysts for the synthesis of new polyolefin microstructures, 5 the design of highly efficient families of “living” polymerization strategies for the synthesis of block, graft, and star polymers, 6-12 controlled methods for the synthesis of dendritic macromolecules, 3,13,14

Organocatalytic ring-opening polymerization.

Syntheses and polymerizations of alpha-amino acid N-carboxyanhydrides (NCAs) were reported for the first time by Hermann Leuchs in 1906. Since that time, these cyclic and highly reactive amino acid derivatives were used for stepwise peptide syntheses but mainly for the formation of polypeptides by ring-opening polymerizations. This review summarizes the literature after 1985 and reports on new aspects of the polymerization processes, such as the formation of cyclic polypeptides or novel organometal catalysts. Polypeptides with various architectures, such as diblock, triblock, and multiblock sequences, and star-shaped or dendritic structures are also mentioned. Furthermore, lyotropic and thermotropic liquid-crystalline polypeptides will be discussed and the role of polypeptides as drugs or drug carriers are reviewed. Finally, the hypothetical role of NCAs in molecular evolution on the prebiotic Earth is discussed.

Polypeptides and 100 years of chemistry of alpha-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

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

Syntheses of cyclic polymers including cyclic homo- polymers, cyclic block copolymers, sun-shaped polymers, and tadpole polymers are discussed on the basis of a differentia- tion between synthetic methods and synthetic strategies (e.g., polycondensation, ring-ring equilibration, or ring-expansion polymerization). Furthermore, all synthetic methods are classi- fied as kinetically or thermodynamically controlled reactions. Characteristic properties of cyclic polymers such as smaller hydrodynamic volume, lower melt viscosities, and higher thermostabilities are compared to the properties of their linear counterparts. Furthermore, the nanophase separation of cyclic diblock copolymers is discussed. V C 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 251-284, 2010

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/pola.23755

Cyclic polymers: Synthetic strategies and physical properties

Surface-initiated atom transfer radical polymerization (ATRP) of various hydrophilic methacrylate monomers on submicrometer-sized silica particles in aqueous media at 20 °C leads to polymer-grafted silica particles whose colloid stability depends on the nature of the grafted polymer. These new organic−inorganic hybrid particles have been extensively characterized by thermogravimetry, elemental microanalyses, FT-IR spectroscopy, dynamic light scattering, scanning electron microscopy, and X-ray photoelectron spectroscopy. They are expected to be interesting model colloids for evaluating theories of steric stabilization.

Synthesis of Well-Defined, Polymer-Grafted Silica Particles by Aqueous ATRP

Polylactones. 55. A-B-A triblock copolymers of various polypeptides. Syntheses involving 4-aminobenzoyl-terminated poly(epsilon-caprolactone) as B block.

e-Caprolactone (e-CL) was polymerized with 2,2-dibutyl-2-stanna-1,3-dioxepane or 2,2-dibutyl-2-stanna-1,3,6-trioxocane as initiator All polymerizations were conducted in bulk at monomer/initiator 

Macrocycles. 3. Telechelic polylactones via macrocylic polymerization

Morpholine-2,5-dione and D,L-3-methylmorpholine-2,5-dione were polymerized with 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) as initiator with variation of time and monomer/initiator (M/I)-ratio For comparison a few polymerizations were initiated with Sn(II) 2-ethylhexanoate The DSDOP initiated polymerizations gave slightly higher molecular weights, but the molecular weights were rather low in all cases and did not parallel the M/I-ratio Furthermore, D,L-3-methylmorpholine-2,5-dione was copolymerized with e-caprolactone (e-CL) or with L-lactide 13 C NMR spectra proved the formation of nearly random sequences Therefore, the 1: 1 copolymers were amerphous Higher feed ratios of e-CL or L-lactide yielded higher molecular weights (at constant M/I-ratio) and they yielded crystalline copolyesters as evidenced by DSC-measurements

Polylactones, 45 homo- and copolymerizations of 3-methylmorpholine-2,5-dione initiated with a cyclic tin alkoxide

Polylactones, 46 : Telechelic polylactones having amino acid, dipeptide or penicilline endgroups via macrocyclic polymerization

2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP) was polycondensed with bis(4-chlorothiophenyl) suberat under various reaction conditions, but only moderate molecular weights (Mn ≈ 10000) were obtained. The MALDI-TOF mass spectrosmetry revealed the formation of cyclic oligo- and polyesters in addition to linear species having OH, CO2H, and unreacted 4-chlorothiophenyl ester endgroups. Furthermore, ϵ-Caprolactone (ϵ-CL) was polymerized with DSDOP as the initiator at monomer/initiator (M/I) ratios of 20 and 50. The resulting tin-containing macrocyclic polylactones were reacted with sebacic acid bis(4-thiocresyl)ester at three different temperatures and with different reaction times. Analogous polycondensations were conducted with suberic acid bis(4-chlorothiophenyl) ester. The presence of thioarylester endgroups in the isolated polyesters was checked by 1H NMR spectroscopy. The highest conversions were found at long reaction times (24 or 72 hours), or after the addition of pyridine and N,N-dimethylaminopyridine a...

Macrocycles. 12. ring-opening polycondensations of tin-containing macrocycles with bis(thioarylester)s

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