Series Of Statistical Copolymers Research Articles

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.

Log Poct/SA Predicts the Thermoresponsive Behavior of P(DMA-co-RA) Statistical Copolymers

Polymers that exhibit a lower critical solution temperature (LCST) have been of great interest for various biological applications such as drug or gene delivery, controlled release systems, and biosensing. Tuning the LCST behavior through control over polymer composition (e.g., upon copolymerization of monomers with different hydrophobicity) is a widely used method, as the phase transition is greatly affected by the hydrophilic/hydrophobic balance of the copolymers. However, the lack of a general method that relates copolymer hydrophobicity to their temperature response leads to exhaustive experiments when seeking to obtain polymers with desired properties. This is particularly challenging when the target copolymers are comprised of monomers that individually form nonresponsive homopolymers, that is, only when copolymerized do they display thermoresponsive behavior. In this study, we sought to develop a predictive relationship between polymer hydrophobicity and cloud point temperature (TCP). A series of statistical copolymers were synthesized based on hydrophilic N,N-dimethyl acrylamide (DMA) and hydrophobic alkyl acrylate monomers, and their hydrophobicity was compared using surface area-normalized octanol/water partition coefficients (Log Poct/SA). Interestingly, a correlation between the Log Poct/SA of the copolymers and their TCPs was observed for the P(DMA-co-RA) copolymers, which allowed TCP prediction of a demonstrative copolymer P(DMA-co-MMA). These results highlight the strong potential of this computational tool to improve the rational design of copolymers with desired temperature responses prior to synthesis.

Grafting Density Governs the Thermoresponsive Behavior of P(OEGMA-co-RMA) Statistical Copolymers.

Thermoresponsive copolymers that exhibit a lower critical solution temperature (LCST) have been exploited to prepare stimuli-responsive materials for a broad range of applications. It is well understood that the LCST of such copolymers can be controlled by tuning molecular weight or through copolymerization of two known thermoresponsive monomers. However, no general methodology has been established to relate polymer properties to their temperature response in solution. Herein, we sought to develop a predictive relationship between polymer hydrophobicity and cloud point temperature (TCP). A series of statistical copolymers were synthesized based on hydrophilic oligoethylene glycol monomethyl ether methacrylate (OEGMA) and hydrophobic alkyl methacrylate monomers and their hydrophobicity was compared using surface area-normalized partition coefficients (log Poct/SA). However, while some insight was gained by comparing TCP and hydrophobicity values, further statistical analysis on both experimental and literature data showed that the molar percentage of comonomer (i.e., grafting density) was the strongest influencer of TCP, regardless of the comonomer used. The lack of dependence of TCP on comonomer chemistry implies that a broad range of functional, thermoresponsive materials can be prepared based on OEGMA by simply tuning grafting density.

Anion-conducting polysulfone membranes containing hexa-imidazolium functionalized biphenyl units

Poly(arylene ether sulfone)s containing randomly distributed biphenyl units tethered with precisely six imidazolium cations are designed and prepared with the aim to facilitate ionic clustering and conductivity of anion exchange membranes (AEMs). A series of statistical copolymers with different cationic contents are synthesized via K2CO3-mediated polycondensations of 2,2′,3,3′,5,5′-hexamethyl-4,4′-dihydroxybiphenyl, bisphenol-A and 4,4′-dichlorodiphenylsulfone. After near quantitative benzylic brominations, the copolymers are functionalized with N-methylimidazolium (NIM), 1,2,4,5-tetramethylimidazolium (4IM) and trimethylammonium (QA) cations, respectively. Small angle X-ray scattering of AEMs cast from solution shows that all the different hexa-functionalized moieties induce distinct phase separation. This is especially efficient in the NIM materials, which may be because the less bulky nature of this cation in comparison with 4IM. Thus, at a given water uptake the AEMs containing NIM reach a significantly higher conductivity than those with 4IM ions. In addition, AEMs containing any of the two hexa-imidazolium moieties reach higher conductivities than corresponding materials with hexa-QA moieties, which probably results from the delocalized charge of the former cations which promotes ionic dissociation despite the very high local ionic concentrations. Introducing biphenyl units tethered with precisely six imidazolium cations along a copolymer backbone may be a viable synthetic strategy towards efficient AEMs for different electrochemical energy applications.

Polyfluorenes containing tetraphenylthiophene segments: Synthesis, photophysics, and electroluminescence

Abstract2,5‐Bis(4‐bromophenyl)‐3,4‐diphenylthiophene was synthesized from benzyl chloride and sulfur and through the subsequent bromination of the intermediate 2,3,4,5‐tetraphenylthiophene. It was condensed with 2,7‐dibromo‐9,9‐dihexylfluorene via a nickel‐mediated Yamamoto coupling reaction to afford a new series of statistical copolymers with various compositions. In addition, poly(9,9‐dihexylfluorene) (PF) was synthesized under the same conditions for comparison. All the polymers were soluble in common organic solvents such as tetrahydrofuran (THF), chloroform, and dichloromethane. Their glass‐transition temperatures increased with an increase in the tetraphenylthiophene (TPT) content in the polymers, and they were 63–149 °C. The solutions of the polymers in THF emitted intense blue light with a photoluminescence maximum at 418–440 nm and quantum yields of 0.32–0.62. Thin films of the polymers with TPT fractions lower than 20 mol % emitted blue‐green light with two well‐resolved peaks at 445 and 520 nm and an optical band gap of about 2.85 eV. A thin film of the polymer with aTPT fraction of 50 mol % emitted pure blue light with a maximum at 419 nm and an optical band gap of 3.28 eV. An enhancement of the light‐emitting‐diode brightness by a factor of ∼8 with respect to that of PF was achieved in apolymer containing 5 mol % TPT. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4015–4026, 2006

Characterization of Random Copolymers by Liquid Chromatography under Limiting Conditions of Adsorption

Liquid chromatography under limiting conditions of adsorption (LC LCA) can be considered as a new tool to perform, simultaneously, molar mass and composition determination of statistical copolymers and to separate copolymer blends as a function of copolymer composition or microstructure. The suitability of LC LCA has been demonstrated for two series of statistical copolymers, poly(styrene-co-methyl methacrylate), poly(S/MMA), varying either in molar mass or composition. The binary eluent consisted of a mixture of n-hexane and THF, containing either 26 or 30 vol.% of n-hexane, respectively, for low and high LC LCA. Two parameters, the ratio of peak area over height and the ratio of peak area of chromatograms obtained by LC LCA and size exclusion chromatography (SEC), were employed to evaluate statistical copolymer elution through LC LCA. From the peak analyses, macromolecule recovery was complete over the entire composition and the molar mass ranges. LC LCA and SEC were further compared, and off-line fractionation was subsequently performed using a combination of both methods, with different statistical copolymer solutions having narrow, broad, and bimodal composition distributions.

Electroluminescent copolymers based on dihexylfluorene and 2‐{2,6‐bis[2‐(4‐diphenylaminophenyl)vinyl]pyran‐4‐ylidene}malononitrile units

AbstractA series of statistical copolymers (poly[(9,9‐di‐n‐hexylfluorene)‐co‐2‐{2,6‐bis‐[2‐(4‐diphenylaminophenyl)vinyl]pyran‐4‐ylidene}malononitrile) were synthesized by the Suzuki coupling reaction. The copolymers showed absorption bands at 379 and 483–489 nm, which were attributed to the oligofluorene segments and the segments containing 2‐[2,6‐bis(2‐{4‐[(4‐bromophenyl)phenylamino]phenyl}vinyl)pyran‐4‐ylidene]malononitrile (3), respectively. The absorption band around 483–489 nm increased with the feed ratio of 3. The photoluminescence (PL) spectra of the copolymers showed emission bands at 420 and 573–620 nm. As the feed ratio of 3 increased, the PL emission in the longer wavelength region redshifted, and the intensity increased as well. The electroluminescence (EL) spectrum of the copolymers showed a very weak emission at 420 nm. The PL and EL emission colors redshifted dramatically with the increase in the feed ratio of 3. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the model compound (2‐{2,6‐bis[2‐(4‐diphenylaminophenyl)vinyl]pyran‐4‐ylidene}malononitrile) were determined to be −5.34 and −3.14 eV, respectively. It was concluded that energy transfer took place from the oligofluorene blocks to the segments containing 3 and that direct charge trapping occurred in the segments containing 3 during the EL operation. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the copolymer (x = 0.63, y = 0.37) containing 10 mol % 3 were very close to those (x = 0.67, y = 0.33) for National Television System Committee (NTSC) red with a maximum photometric power efficiency of 0.27 cd/A. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3729–3737, 2006

Dilute Solution Characterization of Statistical Poly(styrene-co-methylmethacrylate) Polymers as Applied to Liquid Chromatography under Limiting Conditions of Adsorption

This study evaluates the dilute solution properties of model poly(styrene-co-methyl methacrylate) polymers with correlation established between the refractive index increment, second virial coefficient, and the radius of gyration to the molar mass and composition. Specifically, with the aim of studying the elution mechanism of liquid chromatography under limiting conditions of adsorption, two series of statistical copolymers, varying either in molar mass or in composition, have been employed. After having synthesized such models by radical solution polymerization, the experimental copolymer composition was determined by 1H NMR and was found to be in good agreement with the theoretical composition, calculated with the monomer reactivity ratios. Static light scattering measurements were then performed in various solvents, such as dichloroethane, dimethylformamide, tetrahydrofuran, and toluene. By fitting the apparent molar mass versus reduced dn/dc with a squared equation, the weight-average molar mass of the copolymers and the copolymer heterogeneity were determined in the aforementioned conditions. The second virial coefficient was also examined as a function of copolymer molar mass and composition. Finally, the radius of gyration was evaluated in solvents corresponding to the eluents used for liquid chromatography under limiting conditions of adsorption.