Ozcan Altintas

Single Chain Folding of Synthetic Polymers by Covalent and Non-Covalent Interactions: Current Status and Future Perspectives

The present feature article highlights the preparation of polymeric nanoparticles and initial attempts towards mimicking the structure of natural biomacromolecules by single chain folding of well-defined linear polymers through covalent and non-covalent interactions. Initially, the discussion focuses on the synthesis and characterization of single chain self-folded structures by non-covalent interactions. The second part of the article summarizes the folding of single chain polymers by means of covalent interactions into nanoparticle systems. The current state of the art in the field of single chain folding indicates that covalent-bond-driven nanoparticle preparation is well advanced, while the first encouraging steps towards building reversible single chain folding systems by the use of mutually orthogonal hydrogen-bonding motifs have been made.

Constructing star polymersvia modular ligation strategies

Branched polymers result in a more compact structure in comparison to linear polymers of identical molecular weight, due to their high segment density which affects the crystalline, mechanical, and viscoelastic properties of the polymer. Star polymers constitute the simplest form of branched macromolecules where all of the chains—or arm segments—of one macromolecule are linked to a centre defined as the core. Over recent years, modular ligation reactions—some of which adhere to click criteria—have enabled the synthesis of a variety of star polymersvia efficient polymer–polymer conjugations. While the modified Huisgen [3 + 2] dipolar copper catalyzed azide and alkyne cycloaddition (CuAAC) has been widely employed for macromolecular star synthesis, Diels–Alder and hetero Diels–Alder reactions offer alternative pathways which allow for similarly efficient macromolecular conjugations. Moreover, combinations of these protocols afford the synthesis of more complex star polymer structures which previously had not been achievable.

Single chain self-assembly: preparation of α,ω-donor–acceptor chains via living radical polymerization and orthogonal conjugation

Alpha,omega-hydrogen donor/acceptor functional polymer strands are prepared via a combination of living radical polymerization and orthogonal conjugation and subsequently self-assembled as single chains to emulate--on a simple level--the self-folding behaviour of natural biomacromolecules.

Synthesis of Terpolymers by Click Reactions

Well-defined polymeric structures were easily generated through living polymerization systems, in particular, living radical polymerizations. The polymeric precursors with orthogonal functionalities were subsequently clicked together with single or double (combinatorial) click reactions, such as the copper-catalyzed azide-alkyne cycloadditions (CuAAC) and Diels-Alder reactions, to create a wide variety of linear and nonlinear terpolymers.

Synthesis of an ABCD 4-Miktoarm Star Quaterpolymer Through a Diels-Alder Click Reaction

An ABCD 4-miktoarm star quaterpolymer with A = poly(ε-caprolactone) (PCL), B = poly(tert-butyl acrylate) (PtBA), C = polystyrene (PS) and D = poly(methyl methacrylate) (PMMA) arms was prepared using the Diels–Alder reaction strategy. Firstly, PCL with anthracene (PCL-Anth) and PtBA with furan-protected maleimide (PtBA-MI) end-functionalities were synthesized separately via ring-opening polymerization of ε-CL and atom transfer radical polymerization of tBA, respectively. These homo-polymers were linked via the Diels–Alder click reaction in toluene at 100°C in order to give PCL-b-PtBA co-polymer. Next, this block co-polymer is utilized successively as macroinitiator in nitroxide-mediated radical polymerization of styrene and in free radical photo-polymerization of MMA in order to achieve the PCL-PtBA-PS-PMMA 4-miktoarm star quaterpolymer.

Single‐Chain Self‐Folding of Synthetic Polymers Induced by Metal–Ligand Complexation

The controlled folding of a single polymer chain is for the first time realized by metal- complexation. α,ω-Bromine functional linear polymers are prepared via activators regenerated by electron transfer (ARGET) ATRP (M¯n,SEC = 5900 g mol(-1) , Đ = 1.07 and 12 000 g mol(-1) , Đ = 1.06) and the end groups of the polymers are subsequently converted to azide functionalities. A copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is carried out in the presence of a novel triphenylphosphine ligand and the polymers to afford homotelechelic bis-triphenylphosphine polymeric-macroligands (MLs) (M¯n,SEC = 6600 g mol(-1) , Đ = 1.07, and 12 800 g mol(-1) , Đ = 1.06). Single-chain metal complexes (SCMCs) are formed in the presence of Pd(II) ions in highly diluted solution at ambient temperature. The results derived via (1) H and (31) P{(1) H} NMR experiments, SEC, and DLS unambiguously evidence the efficient formation of SCMCs via metal ligand complexation.

Star and miktoarm star block (co)polymers via self- assembly of ATRP generated polymer segments featuring Hamilton wedge and cyanuric acid binding motifs

Hamilton wedge (HW) end-functionalized poly(styrene) (PS–HW, Mn = 5400 g mol−1, PDI = 1.06), HW mid-chain functionalized poly(styrene) (PS–HW–PS, Mn = 4600 g mol−1, PDI = 1.04), cyanuric acid (CA) end-functionalized poly(styrene) (PS–CA, Mn = 3700 g mol−1, PDI = 1.04) and CA end-functionalized poly(methyl methacrylate) (PMMA–CA, Mn = 8500 g mol−1, PDI = 1.13) precursors were successfully synthesized via a combination of atom transfer radical polymerization (ATRP) and copper catalyzed azide–alkyne cycloaddition (CuAAC). The precursor polymers were characterized viasize exclusion chromatography (SEC) and 1H NMR with respect to both molecular weight and structure. Supramolecular homopolymer (PS–HW·PS–CA), block copolymer (PS–HW·PMMA–CA), star polymer (PS–HW–PS·PS–CA) as well as miktoarm star polymer (PS–HW–PS·PMMA–CA) were formed in solution in high yields at ambient temperature (association close to 89% for PS–HW·PS–CA, 90% for PS–HW–PS·PS–CA and 98% for PS–HW–PS·PMMA–CA) via H-bonding between the orthogonal recognition units, HW and CA. The formation of supramolecular polymers was confirmed via1H NMR at ambient temperature in deuterated methylene chloride (CD2Cl2) solution.

Reversible single-chain selective point folding via cyclodextrin driven host–guest chemistry in water

In the present communication we introduce a new platform technology for the reversible folding of single polymer chains in aqueous environment on the basis of cyclodextrin (CD) host-guest chemistry and controlled radical polymerization protocols. The single-chain folding of adamantyl-β-CD α-ω-functionalized poly(N,N-dimethylacrylamide) and its reversion at elevated temperatures were monitored by DLS and nuclear Overhauser enhancement spectroscopy (NOESY).

Macromolecular surface design: photopatterning of functional stable nitrile oxides.

The efficient trapping of photogenerated thioaldehydes with functional shelf-stable nitrile oxides in a 1,3-dipolar cycloaddition is a novel and versatile photochemical strategy for polymer end-group functionalization and surface modification under mild and equimolar conditions. The modular ligation in solution was followed in detail by electrospray ionization mass spectrometry (ESI-MS). X-ray photoelectron spectroscopy (XPS) was employed to analyze the functionalized surfaces, whereas time-of-flight secondary-ion mass spectrometry (ToF-SIMS) confirmed the spatial control of the surface functionalization using a micropatterned shadow mask. Polymer brushes were grown from the surface in a spatially confined regime by surface-initiated atom transfer radical polymerization (SI-ATRP) as confirmed by TOF-SIMS, XPS as well as ellipsometry.

Entropy-Driven Selectivity for Chain Scission: Where Macromolecules Cleave

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high-temperature (HT) NMR spectroscopy of self-assembled supramolecular diblock systems as well as temperature-dependent size-exclusion chromatography (TD SEC) of covalently bonded Diels-Alder step-growth polymers.