Stability of star-shaped RAFT polystyrenes under mechanical and thermal stress

Well-defined three-arm and four-arm star polymers designed via a Z-group approach carrying trithiocarbonate functionalities at the core are prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization featuring molecular weights of Mn{,}SEC = 156 kDa{,} D = 1.16 (3-arm) and Mn{,}SEC = 162 kDa{,} D = 1.15 (4-arm) based on multi-angle laser light scattering (MALLS) detection{,} respectively. The star-shaped polystyrenes are subjected (in bulk) to thermal stress in the temperature range between 140 and 200 [degree]C from 10 minutes up to 96 h. The thermally treated 3-arm and 4-arm star polymers are analyzed via size exclusion chromatography (SEC) to quantify the degradation process at variable temperatures as a function of time under an argon atmosphere. Cleavage rate coefficients of the star polymers are deduced as a function of temperature{,} resulting in activation parameters for the cleavage process{,} i.e. Ea = 131 kJ mol-1; A = 3.93 [times] 1011 s-1 (Mn{,}SEC = 156 kDa{,} D = 1.16{,} 3-arm star) and Ea{,} = 134 kJ mol-1; A = 9.13 [times] 1011 s-1 (Mn{,}SEC = 162 kDa{,} D = 1.15{,} 4-arm star){,} respectively. Processing of the star-shaped polymers is mimicked via a small scale counter rotating twin screw extrusion to achieve nonlinear shear and elongation flow under pressure. Furthermore{,} a rheological assessment via the linear shear deformation region (small amplitude oscillatory shear{,} SAOS) allows for a correlation of the processing conditions with the thermal degradation properties of the star polymers in the melt. Zero shear viscosity ([small eta]0) as a criterion of the degradation process is measured in the rheometer and correlated to the weight-average molecular weight{,} Mw

Unraveling the spontaneous zwitterionic copolymerization mechanism of cyclic imino ethers and acrylic acid

We report a high-resolution electrospray ionization mass spectrometric (HR ESI MS) access route leading to in-depths insight into the spontaneous zwitterionic copolymerization mechanism between cyclic imino ethers (i.e. 2-methyl-2-oxazoline (MeOx), 2-ethyl-2-oxazoline (EtOx) or 2-ethyl-2-oxazine (EtOz)) with acrylic acid (AA), exploiting the characteristic species accumulating during the copolymerization as well as tandem mass spectrometry (MS/MS). We demonstrate preferences in α,ω-end group formation by screening various feed ratios of cyclic imino ethers and acrylic acid (e.g. MeOx:AA = 1:1; MeOx:AA = 2:1; MeOx:AA = 1:2). Critically, a calibration curve – based on AA-MeOx-AA dimer – was established allowing for semi-quantitative determination of the end group ratios with different feed ratios of acrylic acid. The formation of, previously suggested, alternating copolymers was confirmed by MS/MS experiments. Deviations from an ideal alternating composition were found to decrease from MeOx to EtOx to EtOz. The results of (semi-quantitative) HR ESI MS and MS/MS measurements suggest, for the first time presented in such precision, a polymerization mechanism for the spontaneous zwitterionic (alternating) copolymerization indicating optimal monomer ratios and pairings

Amphiphilic block copolymers featuring a reversible hetero Diels-Alder linkage

The present article reports the preparation of a novel class of switchable amphiphilic diblock copolymers with a temperature switchable linkage. Reversible addition fragmentation chain transfer (RAFT) polymerization was used to synthesize the individual blocks: for the preparation of the non-polar block{,} i.e. poly(isoprene-co-styrene) (P(I-co-S)) (9200 g mol-1 [less-than-or-equal] Mn [less-than-or-equal] 50 000 g mol-1{,} 1.22 [less-than-or-equal] D [less-than-or-equal] 1.36){,} a chain transfer agent (CTA{,} 3-((2-bromo-2-methylpropanoyl)oxy)propyl 2-(((dodecylthio)carbonothioyl)thio)-2-methylpropanoate) carrying a bromine group was employed{,} ready for subsequent cyclopentadienyl (Cp) transformation. For the preparation of the polar block{,} triethylene glycol methyl ether acrylate (TEGA) was polymerized (6600 g mol-1 [less-than-or-equal] Mn [less-than-or-equal] 35 000 g mol-1{,} 1.12 [less-than-or-equal] D [less-than-or-equal] 1.30) using a RAFT agent carrying a phosphoryl Z-group{,} which is able to undergo hetero Diels-Alder (HDA) ligation with Cp moieties. Both building blocks were conjugated at ambient temperature in the presence of ZnCl2 as catalyst yielding the amphiphilic block copolymer P(I-co-S)-b-PTEGA (16 000 g mol-1 [less-than-or-equal] Mn [less-than-or-equal] 68 000 g mol-1{,} 1.15 [less-than-or-equal] D [less-than-or-equal] 1.32). To investigate the bonding/debonding capability of the HDA linkage{,} high temperature nuclear magnetic resonance (HT-NMR) spectroscopy{,} high temperature dynamic light scattering (HT-DLS) and high temperature size exclusion chromatography (HT-SEC) were carried out{,} evidencing that efficiently switchable amphiphilic block copolymers were generated (>4 cycles)

Macrocyclization efficiency for poly(2-oxazoline)s and poly(2-oxazine)s

The unique material properties of cyclic polymers have made them attractive biomaterials, prompting the need for more efficient cyclization protocols. Due to their chemical versatility, poly(2-alkyl-2-oxazoline)s (PAOx) and poly(2-oxazine)s (POzi) provide potent candidates in this field. However, little to no comprehensive data on the cyclization process of these materials is available, especially not for POzi. In this study, we investigate the impact of a number of reaction parameters (batch volume, catalyst equivalents, molecular weight, and polymer addition method) on the macrocyclization efficiency of PAOx and POzi using copper(i)-catalysed azide-alkyne cycloaddition (CuAAC). It was found that POzi show more efficient conversion to the desired monocyclic species compared to PAOx, possibly due to increased chain flexibility of POzi. Considering macrocyclization efficiency, up to 33x more cyclic POzi could be synthesized per unit volume of solvent, compared to previous protocols for related PAOx, indicating potential for the future scale-up of macrocyclic POzi

Post-Functionalization of Polymers via Orthogonal Ligation Chemistry

This chapter highlights the current status of critically selected post-functionalization techniques of polymers via orthogonal ligation chemistry, reports the major characteristics of the specific transformation chemistry, and provides the reader useful synthesis-relevant and characterization information. It focuses on the most widely used 1,3-dipolar cycloadditions and Diels-Alder (DA) reactions in polymer chemistry. The chapter then overviews the various ways in which thiols can be used to achieve polymeric post-functionalization. Emphasis is placed upon the ways in which the polymeric materials are prepared to include the required functionality for conjugation and the conditions with which the actual conjugation is carried out. The chapter highlights some elegant examples for the preparation of functional polymers by ring-opening metathesis polymerization (ROMP) via post-modification. It also provides a selection of recent methods in which acyclic diene metathesis (ADMET) can be used for polymeric post-functionalization. Finally, the chapter explains the Pd-catalyzed coupling reactions.</p

Polymer Interfaces: Synthetic Strategies Enabling Functionality, Adaptivity, and Spatial Control

Polymer interfaces are ubiquitous in Nature and technology. Equipping artificial polymer-based interfaces with highly defined functions requires advanced macromolecular chemistry and powerful chemical tools. In this Perspective, we explore the nature of anisotropic - i.e., spatially resolved - polymer interfaces prepared via top-down and bottom-up approaches with selected examples from the recent literature. These range from self-assembly driven systems based on single polymer chains and block copolymers to lithographic encoding able to span wide spatial dimensions of patterning. Based thereon, we formulate the in our opinion required advances in polymer chemistry that will contribute significantly to preparing the next generation of structured interfaces. Among others, this includes the to-date limited orthogonality of parallelly executed ligation reactions as well as limits in λ-orthogonally addressable pericyclic ligation chemistry. Finally, we propose some long-term visions for not yet existing - however currently sought - technology that could drive interactive and adaptive polymer interface construction to new levels. These include the spatially resolved encoding of interfaces with molecular precision and the introduction of programmable properties to interfaces of varying shape and chemical complexity. © 2016 American Chemical Society

α,ω-Reactive Building Blocks Based on a Dual Functional RAFT Agent for Thermal and Light-Induced Ligation

A dual functional chain transfer agent (CTA) capable of highly efficient sequential thermal and photo-induced ligation, generating alpha,omega-functional polymers, is introduced. The newly designed CTA (1-(4-((2-formyl-3-methyl phenoxy)methyl)phenyl)ethyl (diethoxyphosphoryl)methane dithioate) fuses thermally triggered hetero Diels-Alder chemistry with rapid light-induced photoenol chemistry. The versatility of the CTA is exemplarily demonstrated via the preparation of an amphiphilic triblock quaterpolymer poly-(isoprene-co-styrene)-block-poly(ethyl acrylate)-block-poly(ethylene oxide) (P(I-co-S)-b-PEA-b-PEO). Subsequent to the homopolymerization of ethyl acrylate (PEA), a Cp-functional poly(isoprene-co-styrene) (P(I-co-5)) is conjugated with the electron-deficient C=S double bond (dienophile) of the CTA end group, generating a P(I-co-S)-b-PEA diblock terpolymer. The triblock quaterpolymer P(I-co-S)-b-PEA-b-PEO is generated by photoligation of a macromolecular dienophile, i.e., the fumarate-terminated poly(ethylene oxide) (PEO-fum) to the photoenol-functional P(I-co-S)-b-PEA. The new dual functional ligation RAFT agent constitutes a technology platform for generating alpha,omega-reactive building blocks from one single chain transfer agent

Orange-Light-Induced Photochemistry Gated by pH and Confined Environments

We introduce a new photochemically active compound, i.e., pyridinepyrene (PyPy), entailing a pH-active moiety that effects a significant halochromic shift into orange-light (λ = 590 nm) activatable photoreactivity while concomitantly exerting control over its reaction pathways. With blue light (λ = 450 nm) in neutral to basic pH, a [2 + 2] photocycloaddition can be triggered to form a cyclobutene ring in a reversible fashion. If the pH is decreased to acidic conditions, resulting in a halochromic absorption shift, photocycloaddition on the small-molecule level is blocked due to repulsive interactions and exclusive trans-cis isomerization is observed. Through implementation of PyPy into the confined environment of a single-chain nanoparticle (SCNP) design, one can overcome the repulsive forces and exploit the halochromic shift for orange light (λ = 590 nm)-induced cycloaddition and formation of macromolecular three-dimensional (3D) architectures.</p

Multiple morphologies, phase transitions, and cross-linking of crew-cut aggregates of polybutadiene-block-poly(2-vinylpyridine) diblock copolymers

We describe detailed investigations on the self-assembly of polybutadiene-block-poly(2-vinylpyridine) diblock copolymers into so-called crew-cut micellar aggregates in aqueous media. Three analogous diblock copolymers with different short segments of poly(2-vinylpyridine) were synthesized via anionic polymerization. The self-assembled aggregates are studied in dioxane/water mixtures by means of dynamic light scattering and transmission electron microscopy. Depending on the added volume fraction of water, spherical micelles, branched cylindrical micelles, and vesicles can be found. These aggregates can be cross-linked in a facile fashion using a UV photoinitiator. The structures of the crew-cut aggregates remain intact during cross-linking, yielding stabilized polymeric nanoparticles. A transfer of these nanoparticles into common, only slightly selective solvents, in which no aggregates or different type of aggregates would exist without cross-linking, is possible. The cross-linked nanoparticle structures remain unchanged during this process. Moreover, a new pathway for the phase transition of cylindrical micelles to vesicles was found and is elucidated.</p