Molecular Weight Control Research Articles

Systematic Order-Made Synthesis of Sequence-Defined Polyurethanes with Length, Types, and Topologies.

Polyurethanes are industrially and academically important as soft materials. They are conventionally synthesized by a process based on step-growth polymerization; thus, molecular weight and structural control are impossible. However, the development of a synthetic strategy for polyurethanes remains a big challenge in designing soft materials. Herein, we demonstrate a synthetic methodology for generating polyurethanes with selectable lengths and termini characteristics. The multistep synthetic process offered the systematic synthesis of high-molecular weight, regioregular, and α,ω-urethane telechelics. Various oligomers with order-made repeating units revealed the effective length of the polymer properties. To demonstrate the scope of our methodology, it was also applied to the synthesis of block co-oligomers, three-armed star oligomers, and miktoarm star co-oligomers. Thus, our method allows the synthesis of high-molecular-weight oligomers with complete structural and molecular weight control, which is of enormous value to materials science; particularly the study and application of structure-property relationships in polyurethanes.

Polymerization of N-Carboxyanhydride in Cosolvents: The Balance between the Polymerization Rate and Molecular Weight Control

Polymerization of <i>N</i>-Carboxyanhydride in Cosolvents: The Balance between the Polymerization Rate and Molecular Weight Control

Synthesis and properties of biodegradable waterborne polyurethane modified as castor oil

A series of biodegradable waterborne polyurethane (WPU) based on polycaprolactone diol and castor oil is synthesized through the process of the polyaddition reaction. Biodegradable WPUs are designed by using polycaprolactone diol and 4,4′- methylene dicyclohexyl diisocyanate as the main domain of urethane, dimethylol butanoic acid as the anionic internal emulsifier and trimethylamine as the neutralizing agent. Castor oil is introduced as a key component to induce or maximize the biodegradability and biocompatibility of a WPU series. The biodegradable ability of WPUs are changed by the control of Castor oil content and molecular weight (MW). The biodegradation of a WPU series prepared in this study is analyzed in field emission scanning electron microscopy through visual modifications in bio-enzyme evaluation. The effects of Castor oil and MW on the physical properties are evaluated using gel permeation chromatography (GPC), universal test machine (UTM), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and field emission scanning electron microscopy (FE-SEM). The WPU with high level of MW and Castor oil shows enough enhancement in tensile strength and Young's modulus, ranging from 12.28 to 23.47 MPa and from 0.04 to 0.137 MPa, respectively. SEM results point to a significant microphase biodegradation of a WPU series by bio-enzymes. It can be an environmental-friendly candidate to apply various biofields.

High-Activity and High-Selectivity Air-Stable Nickel and Copper Complexes for Copolymerization of Epoxides with Anhydrides.

Ring-opening copolymerization (ROCOP) of epoxides with anhydrides presents a highly promising pathway for the synthesis of diverse polyester copolymers. This process involves the simultaneous opening of the epoxide ring and the anhydride ring, resulting in the formation of copolymer chains with alternating repeating units. Here, we synthesized a series of nickel and copper complexes coordinated by amine-bis(benzotriazole phenolate) ligands (C1NNBiBTP-H2). Different acid-base conditions can result in the synthesis of diverse coordination structures. The six-coordinated mononuclear Ni catalyst synthesized in this study exhibited competitive turnover frequencies (TOF = 450 h-1) compared to previous studies involving copolymerization of epoxides with anhydrides. This indicates the catalyst's high activity in facilitating the copolymerization reaction. The results indicate that the catalyst exhibited a high molecular weight control ability and high polymer selectivity for the ROCOP of epoxides and anhydrides. Thus, this air-stable catalyst has the potential for use in industries.

Efficient biobased oligomeric plasticizers from the renewable biomonomers, glycerol and adipic acid

With the recognition that traditional phthalate plasticizers readily migrate from a polymer matrix into which they have been incorporated, have become widespread environmental pollutants and pose risks to human health, the development of new, effective, nontoxic, nonmigrating plasticizers has gained urgency. A focus has been the generation of plasticizers from renewable, inexpensive, nontoxic biobased precursors. Many small molecule plasticizers have been prepared from readily-available bioprecursors. However, the most promising are branched oligomeric materials. Fully compatible oligomeric plasticizers do not migrate from a polymer matrix. Highly branched materials are effective in increasing free volume and display good plasticizing impact. Using new technology that permits the generation of hyperbranched poly(ester) without gelation and with control of molecular weight and endgroup identity, oligomeric materials have been prepared from the nontoxic biomonomers, glycerol and adipic acid. The monomers are readily available at modest cost. The oligomers may be obtained in a simple one-step process and function as very effective plasticizers for polymeric materials.

Regulating copolymer architecture using photoiniferter polymerization to direct composite morphology

Due to mild conditions and straightforward synthetic routes, photoiniferter polymerization has received increasing use as a facile method for preparation of polymers with highly controlled structure. However, low quantum yields inherent to thiocarbonylthio species result in polymerization speeds that are considerably slower than traditional controlled radical polymerization techniques. To improve polymerization speed while maintaining high level control, it is critical to understand relationships between reaction parameters and ultimate polymer structure. In this work, initiating light intensity, reactant concentration, and monomer pendant groups were manipulated in the photoiniferter polymerization of acrylate monomers to explore the effect of these conditions on reaction kinetics and molecular weight control. Polymerization rate of n-butyl acrylate was significantly enhanced by increasing light intensity rivaling speeds of conventional thermally initiated RAFT polymerization while enabling more precise control over molecular weight. Interestingly, photoiniferter rate asymptotically approached saturation at relatively high light intensities. Moreover, degradation of trithiocarbonate species occurred at greater intensities, ultimately resulting in moderate increases in molecular weight. Increased reactant concentration enabled faster rates with bulk polymerization, possibly due to reduced macro radical mobility and termination. Additionally, comparing photoiniferter polymerization of methyl acrylate and hydroxyethyl acrylate showed that greater polarity considerably enhanced polymerization speeds. Finally, the utility of precisely controlling polymer structure was demonstrated by synthesizing and incorporating a series of amphiphilic block copolymers with increasing molecular weight into a photocurable epoxy resin. The well-defined block copolymer structures enabled controllable phase separation and thermomechanical properties in epoxy composites. This work showed that photoiniferter polymerization speeds could be significantly enhanced using elementary reaction conditions, and that these conditions enable facile production of well-defined copolymers allowing tailored composite morphology and properties.

An Efficient One‐Arrow‐Two‐Hawks Strategy Achieves High Efficiency and Stable Batch Variance for Benzodifuran‐based Polymer Solar Cells

AbstractWith the development of organic solar cells (OSCs), the high‐performance and stable batch variance are becoming a new challenge for designing polymer donors. To obtain high photovoltaic performance, adopting polymers with high molecular weight as donors is an ordinary strategy. However, the high molecular weight need to subtly control the reaction time and state, inevitably caused batch‐to‐batch variations. Herein, a strategy of steric effect is applied to benzodifuran (BDF)‐based polymer by introducing different positions of Cl atom, producing two polymers PBDFCl‐1 and PBDFCl‐2. The more twisted side chains conformation not only achieve the control of moderate molecular weight for PBDFCl‐2, but also easily form molecular stacking through adopting BDF unit and maintain sufficient polymeric crystallinity. Due to the optimized stacking mode and good blend miscibility, PBDFCl‐2‐based device exhibitsa more elegant power conversion efficiency (PCE) of 17.00% compared to PBDFCl‐1‐based device. This is the highest efficiency record for BDF‐based binary OSCs. Meanwhile, the PCE device variation of the different molecular weights for PBDFCl‐2 is little, indicating the reduction of the batch variation. Therefore, smartly using steric effect of Cl atom in strong crystalline BDF unit can form efficient molecular stacking regulations and realize the coordination of high‐performance and stable batch variance.

Thermal pre-processing before extraction of polyhydroxyalkanoates for molecular weight quality control

Past studies have repeatedly demonstrated the technical feasibility to produce polyhydroxyalkanoate (PHA) using bacterial biomass of mixed microbial cultures (MMCs). Commercial quality grades of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, can be produced with control of average monomer composition. However, demonstration of PHBV production and recovery with quality control of molecular weight (MW) distribution has been lacking in the research literature. Towards this goal, a workflow has been developed for characterizing molecular weight control by thermal treatment pre-processing of dried PHA-rich biomass before solvent extraction. Dimethyl carbonate (DMC) was a suitable solvent in this workflow in the routine evaluation of extractable PHA. From assessments of DMC extraction using differential scanning calorimetry, 125 °C was selected for nominally 100 percent extraction yield independent of polymer 3-hydroxyvalerate (3HV) content (2 to 41 wt.% 3HV) and molecular weight (100 to 1400 kDa). Intrinsic viscosity measurements of PHBV in DMC at 60 °C was used for molecular weight monitoring. Mark-Houwink constants, α (0.738 ± 0.010) and LogK (-2.016 ± 0.025), were estimated for a PHBV co-polymer blend having 36 wt.% 3HV. A model of random scission supported that weight average molecular weight (Mw) was a more robust metric, compared to number average molecular weight (Mn), for assessing the polymer scission rates. During isothermal heat treatment for a given biomass batch, interpreted scission rate was reproducible and commonly, but not always, constant in time. Scission rates between biomass batches were also variable. Measured properties of the polymer in the biomass (thermal stability, biomass PHA content, PHBV grade, initial moisture content) could not be correlated to this observed batch-to-batch variation of scission rate. Molecular weight loss before extraction did not influence the melting temperatures of the co-polymer blends of PHBV evaluated over a wide sub-eutectic range of average 3HV content. Molecular weight changes for these PHBV co-polymer blends were considered to have likely influenced the nature of blend 3HV distribution, and consequently, crystallization behaviour. Molecular weight loss effects on crystallization behaviour at constant PHBV average 3HV wt.% content could then have contributed to the observed variability for glass transition temperatures and melting enthalpies. However, a reproducible correlation between this variability and MW change was not observed.

Favorable Propylene-Incorporated Terpolymerization of Ethylene with CO Mediated by Cationic [P,O]-Pd and Ni Complexes.

Commercial polyketone materials are generally produced by palladium-catalyzed terpolymerization of ethylene and α-olefin with carbon monoxide (CO), and rare examples were reported regarding the incorporation of propylene into an ethylene/CO copolymer chain using a cost-effective nickel catalyst. In this study, we have developed a series of [P,O]-type cationic Pd and Ni complexes supported by a diphosphazane monoxide (PNPO) platform, and the electronic and steric effect on phosphine, amine, and phosphine oxide moieties is systematically investigated for terpolymerization in terms of activity, propylene/CO (C3) incorporation, and molecular weight control. It is observed that the melting temperature (Tm) is proportional to the number of C3 incorporations present in the polymer chain, and the incorporated propylene does not affect the degradation temperature substantially, thus broadening the processing temperature window of the resultant polyketones. Notably, in comparison with dppp-type catalysts, PNPO catalysts exhibited a higher preference for propylene consumption, which is of great importance for making more efficient use of α-olefin resources.

Anionic Polymerization of the Terpene-Based Diene β-Ocimene: Complex Mechanism Due to Stereoisomer Reactivities

Motivated by the need for sustainable, bio-based materials, the living anionic polymerization of the under-explored terpene monomer β-ocimene (Oc) was investigated for the first time. Homopolymers with Mn up to 50 kg mol–1 of the Oc isomeric mixture (E:Z = trans:cis = 70:30) were synthesized in cyclohexane and analyzed with respect to molecular weight control, dispersity, microstructure, and glass transition temperature, Tg. Employing styrene as a comonomer, diblock copolymers, and a series of statistical copolymers with Mn up to 20 kg mol–1 with varying comonomer compositions offered the opportunity to tailor the glass transition of the copolymers. Real-time 1H nuclear magnetic resonance (NMR) kinetics indicated a remarkably divergent reactivity of the trans and cis isomers. This unveiled the unique observation that the homopolymerization of Oc is in fact a copolymerization of the cis and trans isomers, which one might name as “stereo-copolymerization” (rtrans = 3.16; rcis = 0.32). Kinetic studies of the statistical copolymerization of the Oc isomeric mixture with styrene revealed an astonishingly contradictory reactivity of the two isomers (rtrans < rcis). The reactivity differences of the cis and trans isomers in the polymerization were utilized to isolate the individual isomers for the first time. Subsequently, they were independently homo- and copolymerized with styrene. The complex mechanism of these polymerizations and the rather high polymer dispersities (Đ ≈ 1.6–2) are discussed using various kinetic models supported by density functional theory modeling. The surprisingly different behavior of the two isomers with styrene was validated experimentally via a 1H NMR-monitored chemical titration.

Aggregation state tuning via controlling molecular weights of D–A1–A2 type polymer donors for efficient organic photovoltaics

A polymer donor with a D–A1–A2 type structure was developed, whose aggregation state was tuned by molecular weight control, finally leading to over 15% power conversion efficiency.

A guide to modern methods for poly(thio)ether synthesis using Earth-abundant metals.

Polyethers and polythioethers have a long and storied history dating back to the start of polymer science as a distinct field. As such, these materials have been utilized in a wide range of commercial applications and fundamental studies. The breadth of their material properties and the contexts in which they are applied is ultimately owed to their diverse monomer pre-cursors, epoxides and thiiranes, respectively. The facile polymerization of these monomers, both historically and contemporaneously, across academia and industry, has occurred through the use of Earth-abundant metals as catalysts and/or initiators. Despite this, polymerization methods for these monomers are underutilized compared to other monomer classes like cyclic olefins, vinyls, and (meth)acrylates. We feel a focused review that clearly outlines the benefits and shortcomings of extant synthetic methods for poly(thio)ethers along with their proposed mechanisms and quirks will help facilitate the utilization of these methods and by extension the unique polymer materials they create. Therefore, this Feature Article briefly describes the applications of poly(thio)ethers before discussing the feature-set of each poly(thio)ether synthetic method and qualitatively scoring them on relevant metrics (e.g., ease-of-use, molecular weight control, etc.) to help would-be poly(thio)ether-makers find an appropriate synthetic approach. The article is concluded with a look ahead at the future of poly(thio)ether synthesis with Earth-abundant metals.

Synthesis of block copolymer of vinyl acetate and methyl acrylate by cobalt-mediated radical polymerization in a packed column system: simultaneous control of molecular weight, separation, and purification

Synthesis of block copolymer of vinyl acetate and methyl acrylate by cobalt-mediated radical polymerization in a packed column system: simultaneous control of molecular weight, separation, and purification

Cost-Effective Control of Molecular Weight in Ultrasound-Assisted Emulsion Polymerization of Styrene.

This paper focuses on the determination of economically most feasible conditions to obtain polystyrene with various target molecular weights through ultrasound-assisted emulsion polymerization. Briefly, batch polymerizations of styrene have been performed by ultrasound-assisted emulsion polymerization process using different reaction feed compositions. Polymerization rates were calculated using the monomer conversions at various reaction times. Also, molecular weights of the synthesized polymers, as well as the Mark-Houwink constants, were determined by intrinsic viscosity and gel permeation chromatography measurements. It was found that the polydispersity index of the polymers is ranging from 1.2 to 1.5, and the viscosity average molecular weights are in between 100000-1500000 g/mol depending on the reaction conditions. Finally, model equations were also developed for response variables, and the most economical ways of reaching various target molecular weights were interpreted by response surface methodology based multi objective optimization.

Polymer Molecular Weight Determination via Fluorescence Lifetime.

Control of polymer molecular weight is critical for tailoring structure-function properties; however, traditional molecular weight characterization techniques have limited ability to determine the molecular weight of polymers in real time without sample removal from the reaction mixture, with spatial resolution, and of insoluble polymers. In this work, a fluorescence lifetime imaging microscopy (FLIM) method was developed that overcomes these limitations. The method is demonstrated with polynorbornene and polydicyclopentadiene, polymers derived from ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The polymer Mw, ranging from 35 to 570 kg/mol as determined by gel-permeation chromatography, was quantitatively correlated with the fluorescence lifetime. The revealed correlation then enabled time-resolved measurement of Mw during an ongoing ROMP reaction, requiring only 1 s per measurement (of a 45 μm × 45 μm polymer sample area), and provided spatial resolution, resulting in simultaneous characterization of polymer morphology. To provide the fluorescence signal, the initial reaction solutions contained a very low doping of a reactive norbornene monomer labeled with fluorescent boron dipyrromethene (BODIPY), such that 1 in every 107 monomers contained a fluorophore. The resulting FLIM visualization method enables the rapid determination of the molecular weights of growing polymers without removal from the reaction mixture and regardless of polymer solubility.

Cobalt -mediated radical polymerization of vinyl acetate in a packed column system: simultaneous effective control of molecular weight, separation, and purification

Cobalt -mediated radical polymerization of vinyl acetate in a packed column system: simultaneous effective control of molecular weight, separation, and purification

Sulfur-Free Radical RAFT Polymerization of Methacrylates in Homogeneous Solution: Design of exo-Olefin Chain-Transfer Agents (R-CH2 C(=CH2 )Z).

In this work, the development of exo-olefin compounds (R-CH2 C(=CH2 )Z) as chain-transfer agents for the sulfur-free reversible addition-fragmentation chain transfer (RAFT) radical polymerization of methacrylates in homogeneous solution is described. A series of exo-olefin compounds with a methyl methacrylate (MMA) dimer structure as the R group and a substituted α-methylstyrene unit as the -CH2 C(=CH2 )Z (Z: Ph-Y) group were synthesized and used for the radical polymerization of MMA in toluene and PhC(CF3 )2 OH. These compounds underwent transfer of the CH2 C(=CH2 )Z group via addition-fragmentation of the propagating methacryloyl radical. More electron-donating (Y) substituents, such as methoxy and dimethylamino groups, produced polymers with narrower molecular weight distributions. A continuous monomer addition method further improved molecular weight control and enabled the synthesis of colorless, sulfur-free, multiblock copolymers of methacrylates in homogeneous solutions.

Sulfur‐Free Radical RAFT Polymerization of Methacrylates in Homogeneous Solution: Design of exo‐Olefin Chain‐Transfer Agents (R−CH2C(=CH2)Z)

AbstractIn this work, the development of exo‐olefin compounds (R−CH2C(=CH2)Z) as chain‐transfer agents for the sulfur‐free reversible addition‐fragmentation chain transfer (RAFT) radical polymerization of methacrylates in homogeneous solution is described. A series of exo‐olefin compounds with a methyl methacrylate (MMA) dimer structure as the R group and a substituted α‐methylstyrene unit as the −CH2C(=CH2)Z (Z: Ph−Y) group were synthesized and used for the radical polymerization of MMA in toluene and PhC(CF3)2OH. These compounds underwent transfer of the CH2C(=CH2)Z group via addition‐fragmentation of the propagating methacryloyl radical. More electron‐donating (Y) substituents, such as methoxy and dimethylamino groups, produced polymers with narrower molecular weight distributions. A continuous monomer addition method further improved molecular weight control and enabled the synthesis of colorless, sulfur‐free, multiblock copolymers of methacrylates in homogeneous solutions.

Independent control of molecular weight, concentration, and stiffness of hyaluronic acid hydrogels

Hyaluronic acid (HA) hydrogels have been used for a multitude of applications, perhaps most notably for tissue engineering and regenerative medicine, owing to the versatility of the polymer and its tunable nature. Various groups have investigated the impact of hydrogel parameters (e.g. molecular weight, concentration, stiffness, etc) in vitro and in vivo to achieve desired material performance characteristics. A limitation in the literature to date has been that altering one hydrogel parameter (a ‘manipulated variable’) to achieve a given hydrogel characteristic (a ‘controlled variable’) changes two variables at a time (e.g. altering molecular weight and/or concentration to investigate cell response to stiffness). Therefore, if cell responses differ, it may be possible that more than one variable caused the changes in observed responses. In the current study, we leveraged thiol-ene click chemistry with a crosslinker to develop a method that minimizes material performance changes and permitted multiple material properties to be independently held constant to evaluate a single variable at a time. Independent control was accomplished by tuning the concentration of crosslinker to achieve an effectively constant stiffness for different HA hydrogel molecular weights and polymer concentrations. Specific formulations were thereby identified that enabled the molecular weight (76–1550 kDa), concentration (2%–10%), or stiffness (∼1–350 kPa) to be varied while the other two were held constant, a key technical achievement. The response of rat mesenchymal stem cells to varying molecular weight, concentration, and stiffness demonstrated consistent upregulation of osteocalcin gene expression. The methodology presented to achieve independent control of hydrogel parameters may potentially be adopted by others for alternative hydrogel polymers, cell types, or cell culture medium compositions to minimize confounding variables in experimental hydrogel designs.

SiO2‐Supported Ni(II) and Fe(II) Catalysts Bearing Sodium‐Sulfonate Group for Olefin Polymerization†

Comprehensive SummaryHeterogeneous catalysts have been dominant for industrial polyolefin production, offering many distinct advantages, such as controlling the morphology of the polymer and preventing reactor fouling. In this contribution, we designed several types of imine ligands bearing sodium‐sulfonate ionic groups and the corresponding nickel/iron catalysts. These catalysts could be immobilized on SiO2 with strong affinity and high catalyst loadings. The SiO2‐supported catalysts behaved better than their homogeneous counterparts during ethylene polymerization with extremely high activity (2.6 × 104 kg·mol−1·h−1) and high thermal stability. In addition, the supported nickel catalysts generated free‐flowing polyethylene with high molecular weight (Mn up to 2.65 × 103 kg·mol–1) and excellent morphology control, indicating no reactor fouling due to the leaching of catalyst. Moreover, for copolymerization, immobilized nickel catalysts showed a higher tolerance of polar monomers, and the efficient copolymerization of ethylene with undecylenic acid was achieved with high activity (450 kg·mol−1·h−1) and a high incorporation ratio (up to 12.3%).