Early Stage Of Polymerization Research Articles

Taking a Tailored Approach to Material Design: A Mechanistic Study of the Selective Localization of Phase-Separated Graphene Microdomains.

To achieve multifunctional properties using nanocomposites, selectively locating nanofillers in specific areas by tailoring a mixture of two immiscible polymers has been widely investigated. Forming a phase-separated structure from entirely miscible molecules is rarely reported, and the related mechanisms to govern the formation of assemblies from molecules have not been fully resolved. In this work, a novel method and the underlying mechanism to fabricate self-assembling, bicontinuous, biphasic structures with localized domains made up of amine-functionalized graphene nanoplatelets are presented, involving the tailoring of compositions in a liquid processable multicomponent epoxy blend. Kinetics studies were carried out to investigate the differences in reactivity of various epoxy-hardener pairs. Molecular dynamics simulations and in situ optical photothermal infrared spectroscopy measurements revealed the trajectories of different components during the early stages of polymerization, supporting the migration (phase behavior) of each component during the curing process. Confirmed by the phase structure and the correlated chemical maps down to the submicrometer level, it is believed that the bicontinuous phase separation is driven by the change of the miscibility between various building blocks forming during polymerization, leading to the formation of nanofiller domains. The proposed morphology evolution mechanism is based on combining solubility parameter calculations with kinetics studies, and preliminary experiments are performed to validate the applicability of the mechanism of selectively locating nanofillers in the phase-separated structure. This provides a simple yet sophisticated engineering model and a roadmap to a mechanism for fabricating phase-separated structures with nanofiller domains in nanocomposite films.

Self-healing polyurethane composite films loaded with Ni@C nanoparticles

Widely employed as polymers, polyurethanes also struggle to strike a balance between mechanical and self-healing qualities. Extrinsic self-healing can readily result in the production of material defects and stress concentration, which can result in the loss of mechanical property. In this study, self-healing and mechanical properties were combined to create a self-healing polyurethane nanocomposite film (PUHN) with good fatigue resistance. Reversible hydrogen and disulfide bonds were introduced to the polyurethane film, meanwhile, carbon-coated nickel nanoparticles were dispersed into the matrix as fillers during the early stage of polymerization. The results showed that PUHN combines outstanding mechanical properties with excellent self-healing properties, while the tensile strength of PUHN was increased by 38 % to 2.32 MPa due to the small-size effect caused by the introduction of nanoparticles. The introduction of reversible disulfide bonding and the presence of abundant hydrogen bonding give PUHN excellent self-healing properties, which can reach 91.2 % self-healing efficiency after 3 h in an oven at 80℃. Meanwhile, the fatigue resistance of the composite material also shows considerable application potential for use in packaging.

Comprehensive assessment of the estrogenic activity of resin composites

Resin-based dental composites have been developed to restore decayed teeth or modify tooth color due to their excellent physical and chemical properties. Such composites may have intrinsic toxicity due to components released into the mouth during the early stage of polymerization, and afterward as a result of erosion or material decomposition. In addition, resin-based dental composites have potential environmental pollutant by elution of monomers and degradation. Since certain monomers of resin matrices are synthesized from bisphenol A (BPA), which acts as an estrogenic endocrine disruptor, these resin matrices may have estrogenic activity. Therefore, the estrogenic endocrine-disrupting activity of various dental composites should be evaluated. In this study, we evaluated the estrogenic endocrine-disrupting activity of 10 resin composites by using a BRET-based estrogen receptor (ER)α and ERβ dimerization assays and ER transactivation assay. BPA, BisDMA, BisGMA, BisEMA, TEGDMA, HMBP, and DMPA mediated ERα dimerization, and BPA, BisDMA, and DMPA also mediated ERβ dimerization. Except for UDMA and CQ, all the compounds were identified as estrogen agonists or antagonists. In-depth information for the safe use of dental composites was acquired, and it was confirmed how the component of dental composites acts in the ER signaling pathway. Further studies on the low-dose and long-term release of these compounds are needed to ensure the safe use of these resin-based dental composites.

3D Covalent Organic Framework as a Metastable Intermediate in the Formation of a Double-Stranded Helical Covalent Polymer.

The design and development of intricate artificial architectures have been pursued for decades. Helical covalent polymer (HCP) was recently reported as an unexpected topology that consists of chiral 1D polymers assembled through weak hydrogen bonds from achiral building blocks. However, many questions remained about the formation, driving force, and the single-handedness observed in each crystal. In this work, we reveal a metastable, racemic, fully covalently cross-linked, 3D covalent organic framework (COF) as an intermediate in the early stage of polymerization, which slowly converts into single-handed HCP double helices through partial fragmentation and self-sorting with the aid of a series of hydrogen bonding. Our work provides an intriguing example where weak noncovalent bonds serve as the determining factor of the overall product structure and facilitate the formation of a sophisticated polymeric architecture.

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization-Induced Self-Assembly.

Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization‐Induced Self‐Assembly

AbstractPolymerization‐induced self‐assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano‐objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that—unexpectedly—unimer exchange is not necessary to form higher order morphologies during radical RAFT‐mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.

Advancing the Compositional Analysis of Olefin Polymerization Catalysts with High-Throughput Fluorescence Microscopy.

To optimize the performance of supported olefin polymerization catalysts, novel methodologies are required to evaluate the composition, structure, and morphology of both pristine and prepolymerized samples in a resource-efficient, high-throughput manner. Here, we report on a unique combination of laboratory-based confocal fluorescence microscopy and advanced image processing that allowed us to quantitatively assess support fragmentation in a large number of autofluorescent metallocene-based catalyst particles. Using this approach, significant inter- and intraparticle heterogeneities were detected and quantified in a representative number of prepolymerized catalyst particles (2D: ≥135, 3D: 40). The heterogeneity that was observed over several stages of slurry-phase ethylene polymerization (10 bar) is primarily attributed to the catalyst particles' diverse support structures and to the inhomogeneities in the metallocene distribution. From a mechanistic point of view, the 2D and 3D analyses revealed extensive contributions from a layer-by-layer fragmentation mechanism in synergy with a less pronounced sectioning mechanism. A significant number of catalyst particles were also found to display limited support fragmentation at the onset of the reaction (i.e., at lower polymer yields). This delay in activity or "dormancy" is believed to contribute to a broadening of the particle size distribution during the early stages of polymerization. 2D and 3D catalyst screening via confocal fluorescence microscopy represents an accessible and fast approach to characterize the structure of heterogeneous catalysts and assess the distribution of their fluorescent components and reaction products. The automation of both image segmentation and postprocessing with machine learning can yield a powerful diagnostic tool for future research as well as quality control on industrial catalysts.

Polymerization of poorly water-soluble monomers in the presence of ethoxylated polyalkylene glycol Laprol 6003, an ethoxylated product of alcoholate copolymerization of propylene oxide with glycerol

The paper shows the similarity of kinetic regularities of vinyl monomers polymerization in the presence of Laprol 6003, regardless of the degree of solubility of the monomers in water. It is assumed that this feature of polymerization is due to the same mechanism of formation of polymer-monomer particles (PMP) – from monomer microdroplets. The interfacial adsorption layer of PMPs, the main site of polymerization, is formed from a surfactant adsorbed from the bulk of the monomer phase and a polymer formed in the adsorption layer upon initiation of polymerization. The formation of complex structures in the interfacial adsorption layer of particles, the incompatibility of the surfactant and the resulting polymer, and high viscosity cause the formation of a strong interfacial layer that ensures the stability of polymer suspensions from the early stages of polymerization.

Polymer-monomer particles formation and particle size distribution during heterophase polymerization in the presence of water-insoluble organosilicon surfactant

In this work mechanism of polymer particles formation during styrene heterophase polymerization in the presence of waterinsoluble organosilicon surfactant was investigated. Dependency between monomer conversion rate from early stages of polymerization and particle diameters change, molecular weight, particle size and molecular weight distribution was analyzed. The effect of the gel effect and Ostwald maturation on particle diameter and polymer molecular weight was evaluated.

Parallel actin monomers in the 8S complex of actin–INF2

Polymerization and depolymerization of actin play an essential role in eukaryotic cells. Actin exists in cells in both monomeric (G-actin) and filamentous (polymer, F-actin) forms. Actin binding proteins (ABPs) facilitate the transition between these two states, and their interactions with these two states of actin are critical for actin-based cellular processes. Rapid depolymerization of actin is assisted in the brain and/or other cells by its oxidation by the enzyme Mical (yielding Mox-actin), and/or by the binding of Inverted Formin 2 (INF2) – which can also accelerate filaments formation. At their stoichiometric molar ratio INF2 and actin yield the 8S complex (consisting of 4 actin monomers: 2 INF2 dimer molecules). Using biochemical and biophysical methods, we investigate the structural arrangement of actin in the 8S particles and the interaction of INF2 with actin and Mox-actin. To that end, we show 2 D class averages of 8S particles obtained by negative staining electron microscopy. We also show that: (i) 8S particles can seed rapid actin assembly; (ii) Mox-actin and INF2 form 8S particles at proteins ratios similar to those of unoxidized actin; (iii) chemical crosslinkings suggest that actin monomers are in a parallel orientation in the 8S particles of both actin and Mox-actin; and (iv) INF2 accelerates the disassembly of Mox-F-actin. Our results provide better understanding of actin's arrangement in the 8S particles formed during actin depolymerization and in the early polymerization stages of both actin and Mox-actin. Communicated by Ramaswamy H. Sarma

β-cyclodextrin mediated construction of porous helical nanoribbons from oligoaniline derivatives

A novel supramolecular self-assembly nanostructure of porous helical nanoribbons (PHNRs) was developed. PHNRs from oligoaniline derivatives were fabricated through the chemical oxidation of aniline in an i-propanol/water mixture as mediated by β-cyclodextrin (β-CD). The role of β-CD was considered vital through the modulation of the addition time point of β-CD and the molar ratio of β-CD/aniline. In addition, at the early stage of polymerization, the host-guest complex between oligoaniline and β-CD was formed, which was involved in the initial supramolecular assembly process. However, with the reorganization of the oligoaniline assemblies during the polymerization time, the abscission of β-CD from the helical nanoribbons was observed, which eventually induced the formation of PHNRs. We believe the supramolecular host-modulated assembly strategy presented herein will be instructive for the fabrication of porous supramolecular nanostructures.

Synthesis of a conjugated polymer film via interfacial Knoevenagel polymerization and conversion to covalent triazine polymer for photocatalysis

The Knoevenagel condensation was applied to the interfacial polymerization of difunctional aldehyde and trifunctional acetonitrile monomers for the first time. The polymerization was conducted at the interface between water containing a base and an organic phase containing the monomers at room temperature for periods ranging from a day to several weeks. The structural and morphological changes of the polymer were investigated to complete the Knoevenagel condensation and, interestingly, the morphological shape of the polymer was significantly changed from particles to film. Specifically, the polymer showed a particle-type product (about 400 nm) at the early stage of polymerization and was transformed to a film-like structure after a prolonged polymerization time. Scanning electron microscope images revealed that the polymer particles were aggregated and connected to each other to form a film, in which each particle was linked by covalent self-assembly. As one of the proofs of the covalent self-assembly, as a result of the increase in the conjugation length, the polymer's fluorescence wavelength was red-shifted after the film formation. Such a red-shift was also observed across the polymer film from the edge (particle) to the central region (film). To improve the photocatalytic performance under visible light irradiation, a triazine group was introduced using the vinylic nitrile group. Using the triazine-containing polymer, almost 100% photodegradation of rhodamine B dye was attained within 30 min.

Emulsion polymerization using three types of RAFT prepared well-defined cationic polymeric stabilizers based on 2-dimethylaminoethyl methacrylate (DMAEMA): a comparative study

Three types of well-defined stabilizers, based on 2-dimethylaminoethyl methacrylate (DMAEMA), including poly(2-dimethylaminoethyl methacrylate) (PDMAEMA), poly(2-dimethylaminoethyl methacrylate)-block-poly(methyl methacrylate) (PDMAEMA-b-PMMA), and PDMAEMA-co-PMMA, were synthesized through reversible addition-fragmentation chain transfer (RAFT) solution polymerization and used as well-defined cationic stabilizers in emulsion (co)polymerization of styrene and/or methyl methacrylate (MMA). Because of different characteristics of MMA and styrene, using varying ratios of these monomer as well as using different structures of stabilizers allowed us to better understanding of stabilizers’ behavior during polymerization course. The results showed that PDMAEMA-b-PMMA was more robust against changes in monomer composition, meaning less fluctuations in particle size distribution and kinetics plots. Also, more than 85% monomer conversion was reached and the resulting polymer particles, ranging in diameter from 75 to ~135 nm. Using PDMAEMA-b-PMMA and adding only 5 mol% MMA as a comonomer in emulsion polymerization of styrene resulted in producing highly uniform particles with size polydispersity of (0.04), as confirmed by TEM and DLS. This narrow size polydispersity was attributed to generation of more positively charged amphiphilic oligomers at early stages of polymerization and thus, faster nucleation.

Interior design of hierarchical micro/nanostructures for enhancing energy storage ability of polyanilines through frozen interfacial polymerization

Large specific surface area and hierarchically porous structure play critical roles in the preparation of high–performance electrodes for supercapacitor that strongly depended on the morphologies of electroactive materials. To impart large specific surface area and high ionic permeability, three kinds of hierarchical micro/nanostructures, i.e. sea urchin–like, flower–like and carpenterworm–like nanostructures, of polyaniline (PANI) products were constructed through simple frozen interfacial polymerization without any templates, special chemical regents or organic solvents. The formation of hierarchical micro/nanostructured PANIs were significantly affected by the interfacial area between aniline and oxidant ice layers and the adding order of aniline and oxidant, more detailedly, the number of the nanofibrous seeds formed at the early stage of polymerization and their ambient aniline concentration as well as the release rates of aniline and oxidant. Furthermore, the PANI with sea urchin–like micro/nanostructure exhibited the highest charge storage ability among all the electrode materials, more clearly, the specific capacitance of 716.8 F g–1 at a current density of 1 A g–1, rate retention of 96.9% from 1 to 20 A g–1, and cycling retention of 73.6% after 2000 cycles. The remarkable electrochemical performance can be ascribed to unique self–assembly of highly ordered nanofibers, making it possible to give full play to the advantages of PANI nanofibers including large electrode/electrolyte contact area, high charge transport along the nanofibers, accommodate volume changes extending in every direction during cycling, and fast electron transport and ion diffusion through porous structure. It is believed that such interior design and eco–friendly fabrication can offer an effective strategy to promote the electroactive properties of the PANI, and further development in the sensor– and energy–related fields.

GTP-dependent formation of straight tubulin oligomers leads to microtubule nucleation.

Nucleation of microtubules (MTs) is essential for cellular activities, but its mechanism is unknown because of the difficulty involved in capturing rare stochastic events in the early stage of polymerization. Here, combining rapid flush negative stain electron microscopy (EM) and kinetic analysis, we demonstrate that the formation of straight oligomers of critical size is essential for nucleation. Both GDP and GTP tubulin form single-stranded oligomers with a broad range of curvatures, but upon nucleation, the curvature distribution of GTP oligomers is shifted to produce a minor population of straight oligomers. With tubulin having the Y222F mutation in the β subunit, the proportion of straight oligomers increases and nucleation accelerates. Our results support a model in which GTP binding generates a minor population of straight oligomers compatible with lateral association and further growth to MTs. This study suggests that cellular factors involved in nucleation promote it via stabilization of straight oligomers.

GTP-Dependent Formation of Straight Oligomers Leads to Nucleation of Microtubules

Microtubule (MT) nucleation is essential for cellular activities, but its mechanism is not known because of the difficulty involved in capturing rare stochastic events in the early stage of polymerization. in cells, MTs are nucleated at tubulin concentrations significantly lower than those required for spontaneous nucleation in vitro. The high efficiency of nucleation is due to the synergistic effects of various cellular factors, but the underlying mechanism has not been clarified yet. Here, combining negative stain electron microscopy and kinetic analysis, we demonstrate that the formation of single-stranded straight oligomers with critical size is essential for nucleation in vitro.

Influence of Metal-Alkyls on Early-Stage Ethylene Polymerization over a Cr/SiO2 Phillips Catalyst: A Bulk Characterization and X-ray Chemical Imaging Study.

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization since its invention in 1953. The uniqueness of this catalyst is related to its ability to produce broad molecular weight distribution (MWD) PE materials as well as that no co‐catalysts are required to attain activity. Nonetheless, co‐catalysts in the form of metal‐alkyls can be added for scavenging poisons, enhancing catalyst activity, reducing the induction period, and tailoring polymer characteristics. The activation mechanism and related polymerization mechanism remain elusive, despite extensive industrial and academic research. Here, we show that by varying the type and amount of metal‐alkyl co‐catalyst, we can tailor polymer properties around a single Cr/SiO2 Phillips catalyst formulation. Furthermore, we show that these different polymer properties exist in the early stages of polymerization. We have used conventional polymer characterization techniques, such as size exclusion chromatography (SEC) and 13C NMR, for studying the metal‐alkyl co‐catalyst effect on short‐chain branching (SCB), long‐chain branching (LCB) and molecular weight distribution (MWD) at the bulk scale. In addition, scanning transmission X‐ray microscopy (STXM) was used as a synchrotron technique to study the PE formation in the early stages: allowing us to investigate the produced type of early‐stage PE within one particle cross‐section with high energy resolution and nanometer scale spatial resolution.

Cobalt(II) complexes of α-diimine derived from cycloalkylamines as controlling agents for organometallic mediated radical polymerization of vinyl acetate

A series of cobalt(II)-α-diimine complexes derived from cycloalkylamines (cycloalkyl = cyclopentyl (1a), cyclohexyl (1b), cycloheptyl (1c), and cyclooctyl (1d)) were synthesized: [CoCl2(Pent-DAB)] (2a), [CoCl2(Hex-DAB)] (2b), [CoCl2(Hept-DAB)] (2c), and [CoCl2(Oct-DAB)] (2d). The α-diimines 1a-d were characterized by FTIR, UV–Vis, and 1H- and 13C NMR spectroscopy, while their respective cobalt(II) complexes (2a-d) were characterized by elemental analysis, FTIR, UV–Vis, EPR, and computational studies. The ability of complexes 2a-d to mediate the vinyl acetate (VAc) polymerization initiated by AIBN, according to a cobalt-mediated radical polymerization (CMRP) mechanism, was explored. A mechanistic investigation of radical trapping based on the combination of theoretical and experimental results for CMRP using 2a-d was conducted. This study shows that the activation/deactivation equilibrium exerted by 2a-d occurs by only one of the faces of the complexes via reversible-termination mechanism. The molecular weight increased in direct proportion to the monomer conversion in the early stages of polymerization for complexes 2a and 2b, while uncontrolled polymerization was observed for 2c and 2d. The molecular weights indicate that the steric effect operated by the cycloalkyl substituents plays an important role in the polymerization control, and that a greater steric hindrance disfavors the dormant state for CMRP equilibrium.

Effect of rapid high-intensity light-curing on polymerization shrinkage properties of conventional and bulk-fill composites

ObjectivesTo compare the effect of high-intensity (3 s with 3440 mW/cm2) and conventional (10 s with 1340 mW/cm2) light-curing on shrinkage properties and degree of conversion of conventional and bulk-fill resin composites, including two composites specifically designed for high-intensity curing. MethodsReal-time linear shrinkage and shrinkage force of 1.5 mm thick composite specimens were measured for 15 min after the start of light-curing using custom-made devices. From the shrinkage force data, maximum shrinkage force rate and time to achieve maximum shrinkage force rate were determined. Degree of conversion was measured using Fourier transform infrared spectrometry. ResultsFlowable composites showed significantly higher linear shrinkage compared to sculptable composites (1.93–2.91 % vs. 1.15–1.54 %), as well as significantly higher shrinkage forces (18.7–24.4 N vs. 13.5–17.0 N). Degree of conversion amounted to 45.8–60.1 %. For high-intensity curing, degree of conversion was significantly lower in three out of seven composites, whereas shrinkage forces were either increased, decreased, or unchanged compared to conventional curing. For high-intensity curing, maximum shrinkage rates were 6–61 % higher, whereas times to achieve maximum shrinkage force rate were 15–53 % shorter compared to conventional curing. Composites specifically designed for high-intensity curing showed shrinkage parameters comparable to other investigated composites. ConclusionShrinkage behavior under conditions of high-intensity light-curing was material-dependent. Shrinkage force kinetics were more strongly affected by high-intensity curing than absolute values of linear shrinkage and shrinkage force. Clinical significanceDespite being attractive for its convenience, high-intensity curing can lead to considerably faster development of shrinkage forces in the early stage of polymerization.

Isoprene polymerizations catalyzed by TiCl4/MgCl2 type Ziegler-Natta catalysts with different titanium contents

The isoprene polymerization kinetics by using TiCl4/MgCl2 type Ziegler-Natta catalysts (TMC) with different titanium contents were studied for better understanding the possible active-specie structures of heterogeneous Ziegler-Natta (Z-N) catalysts for conjugated diene polymerizations. The catalytic features including the catalytic activity (CA), number of active centers ([C*]/[Ti]), rate constant for polymerization (kp), stereoregularity of these TMC catalysts were studied. The low titanium content catalyst TMC-1 (Ti = 0.09 wt.%) with more isolated mononuclear titanium species showed higher CA and yielded cis-1,4-/trans-1,4- mixed microstructure polyisoprene with low molecular weight (Mw) in the initial polymerization stage. The medium high titanium content catalyst TMC-2 (Ti = 2.38 wt.%) with more clustered mononuclear titanium species showed lower CA and yielded cis-1,4-/trans-1,4- mixed microstructure polyisoprene with high Mw in the early stage of polymerization. The high titanium content catalyst TMC-3 (Ti = 3.76 wt.%) presented lowest CA and yielded high trans-1,4- polyisoprene with higher Mw.