Determination of the Crystal Structure of Isotactic cis‐1,4‐Poly(1,3‐hexadiene) by X‐Ray Diffraction and Molecular Mechanics

The crystal structure of isotactic cis‐1,4‐poly(1,3‐hexadiene) has been determined through a combination of X‐ray diffraction analysis and molecular mechanics. Two reasonable values for unit cell parameters were obtained from the fiber diffraction pattern of a well‐oriented sample. Energy maps performed on a molecular model of the polymer chain yielded an absolute minimum corresponding to a conformation having the internal parameters similar to those of an s(2/1) helical polymer chain. Packing energy minimizations performed for all the orthorhombic and monoclinic space groups having crystallographic s(2/1) chain symmetry indicate unequivocally that the best mode of packing is obtained for a crystal structure characterized by a unit cell with parameters a = 8.98 Å, b = 7.82 Å, and c = 8.10 Å in the P2₁2₁2₁ space group. The calculated powder and fiber diffraction patterns of this structural model are in very good agreement with the experimental diffraction patterns. Similarities and differences with the crystal structures of isotactic cis‐1,4‐poly(1,3‐pentadiene) and isotactic cis‐1,4‐poly(2‐methyl‐1,3‐pentadiene) are discussed.

     

Low Conversion 4‐Acetoxystyrene Free‐Radical Polymerization Kinetics Determined by Pulsed‐Laser and Thermal Polymerization

Summary: The free‐radical polymerization kinetics of 4‐acetoxystyrene (4‐AcOS) is studied over a wide temperature range. Pulsed‐laser polymerization, in combination with dual detector size‐exclusion chromatography, is used to measure k_p, the propagation rate coefficient, between 20 and 110 °C. Values are roughly 50% higher than those of styrene, while the activation energy of 28.7 kJ · mol⁻¹ is lower than that of styrene by 3–4 kJ · mol⁻¹. With known k_p, conversion and molecular weight data from 4‐AcOS thermal polymerizations conducted at 100, 140, and 170 °C are used to estimate termination and thermal initiation kinetics. The behavior is similar to that previously observed for styrene, with an activation energy of 90.4 kJ · mol⁻¹ estimated for the third‐order thermal initiation mechanism.

Joint confidence (95%) ellipsoids for the frequency factor A and the activation energy E_a from non‐linear fitting of k_p data for 4‐AcOS (black) and styrene (grey).     

Syntheses and Properties of Novel Copolymers of Poly(1,4‐dioxane‐2‐one) and Aliphatic Polycarbonate Based on Ketal‐Protected Dihydroxyacetone

Novel random copolymers of 1,4‐dioxane‐2‐one (DON) and 2,2‐ethylenedioxy‐1,3‐propanediol carbonate (EOPDC) are synthesized in bulk at 120 °C using Sn(Oct)₂ as a catalyst. The effects of different molar feed ratios of EOPDC/DON on the yield and molecular weight of the copolymers are investigated. The copolymers are obtained with a yield of 55.4–98%. The number‐average molecular weight of the copolymer is 0.49–4.18 × 10⁴ g mol^?1 with a polydispersity of 1.52–1.68. The poly(DON‐co‐EOPDC)s obtained are characterized by FTIR, ¹H NMR, and ¹³C NMR spectroscopy, gel‐permeation chromatography (GPC), and DSC. The hydrolytic degradation of the copolymer in phosphate buffered saline (PBS) is also investigated. The results show that both the hydrophilicity and the degradation rate of the copolymers increase with increasing copolymer DON content.     

Initiating the Photopolymerization Behaviors of Water‐Soluble Polymerizable Polysiloxane–Polyether Block Imidazolium Ionic Liquids with Antibacterial Capability

Three water‐soluble polymerizable polysiloxane–polyether block imidazolium ionic liquids containing α‐hydroxy alkylphenone groups ([Si‐E_n‐2959Im‐A][TsO] (n = 1, 3, 5) are designed and synthesized as novel green photoinitiators. The photochemical properties and photoinitiation mechanism of the [Si‐E_n‐2959Im‐A][TsO] are investigated by ultraviolet absorption spectroscopy, real‐time infrared spectroscopy and electron spin resonance. [Si‐E_n‐2959Im‐A][TsO] exhibit good water solubility that reaches up to 12%. [Si‐E_n‐2959Im‐A][TsO] not only show desirable thermal properties and low saturated vapor pressure, but also effectively initiate photopolymerization. The initial weight loss of [Si‐E_n‐2959Im‐A][TsO] (n = 1, 3, 5) is at 256 °C and their saturated vapor pressures are less than 3.0 Pa. The migration of the photolysis fragments of [Si‐E_n‐2959Im‐A][TsO] from the cured material is mitigated significantly by their polymerizability and high molecular weights. Furthermore, biological tests involving Escherichia coli and Staphylococcus aureus strains in contact with the UV‐cured films initiated by [Si‐E_n‐2959Im‐A][TsO] demonstrate that [Si‐E_n‐2959Im‐A][TsO] are effective antibacterial agents. Therefore, [Si‐E_n‐2959Im‐A][TsO] are promising components for the preparation of environmentally friendly materials and antibacterial coatings.     

Stereocomplex‐ and Homo‐Crystallization and Phase‐Transition Behavior of Relatively High‐Molecular‐Weight Linear One‐ and Two‐Armed and Star‐Shaped Four‐Armed Poly(l‐lactide)/Poly(d‐lactide) Blends

Relatively high‐molecular‐weight linear one‐ and two‐armed and star‐shaped four‐armed poly(l ‐lactide) and poly(d ‐lactide) are synthesized and the multiplicate effects of arm‐number (branching architecture, coinitiator moiety), crystallization temperature (T _c), and number‐average molecular weight (M _n) on stereocomplex (SC)‐ and homo‐crystallization and phase‐transition behavior are investigated. For nonisothermal and isothermal crystallization, in addition to SC crystallites, homo‐crystallites are formed in the blends with higher M _n values, irrespective of arm number. For isothermal crystallization, the transition T _c ranges below which in addition to SC crystallites, homo‐crystallites are formed depended on M _n per one arm‐determining melting temperature or thickness of homo‐crystallites. The transition M _n ranges above which in addition to SC crystallites, homo‐crystallites are formed are not affected by arm number. The high molecular weight disturbs the change of crystalline growth mechanism of one‐ and two‐armed blends, whereas the branching architecture inhibits the change of crystalline growth mechanism of four‐armed blends.     

CO2‐Based Non‐ionic Surfactants: Solvent‐Free Synthesis of Poly(ethylene glycol)‐block‐Poly(propylene carbonate) Block Copolymers

Copolymerization of carbon dioxide (CO₂) and propylene oxide (PO) is employed to generate amphiphilic polycarbonate block copolymers with a hydrophilic poly(ethylene glycol) (PEG) block and a nonpolar poly(propylene carbonate) (PPC) block. A series of poly(propylene carbonate) (PPC) di‐ and triblock copolymers, PPC‐b‐PEG and PPC‐b‐PEG‐b‐PPC, respectively, with narrow molecular weight distributions (PDIs in the range of 1.05–1.12) and tailored molecular weights (1500–4500 g mol^?1) is synthesized via an alternating CO₂/propylene oxide copolymerization, using PEG or mPEG as an initiator. Critical micelle concentrations (CMCs) are determined, ranging from 3 to 30 mg L^?1. Non‐ionic poly(propylene carbonate)‐based surfactants represent an alternative to established surfactants based on polyether structures.

     

Stereoblock Copolymerization of Propylene and Methyl Methacrylate with Single‐Site Metallocene Catalysts

Sequential stereoblock copolymerization of propylene (P) and methyl methacrylate (MMA) using Group IV single‐site metallocene catalysts efficiently produces PP‐b‐PMMA stereodiblock copolymers. When activated with B(C₆F₅)₃, C₂‐symmetric rac‐Et(Ind)₂ZrMe₂ yields isotactic‐PP‐b‐isotactic‐PMMA diblock copolymer, whereas C_s‐symmetric Me₂Si(C₅Me₄)(tBuN)TiMe₂ affords atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer. In the copolymerization catalyzed by the C₂‐symmetric catalyst, a very small amount of PMMA homopolymer formed can be removed from the copolymer by extracting the bulk polymer product with boiling methylene chloride. However, separation of isotactic PP formed if any from the copolymer product approves very difficult, due to very similar solubility between the diblock copolymer and isotactic PP homopolymer in various high‐boiling chlorinated solvents. On the other hand, in the copolymerization catalyzed by the C_s‐symmetric catalyst, both PMMA and atactic PP homopolymers formed in small weight fractions during the copolymerization can be successfully removed from the predominant copolymer product by solvent extraction using boiling heptane. After successful removal of both homopolymers, for example, an atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer has high molecular weight (M _n = 21 100), narrow molecular weight distribution (PDI = 1.08), high PMMA incorporation (33.8 mol‐% of PMMA), and moderate syndiotacticity for the PMMA block ([rr] ≈ 80%). Furthermore, the comonomer composition in the copolymer can be controlled by the time for propylene polymerization and the conversion of MMA. A pronounced activator effect is observed; when the same C_s‐symmetric catalyst is activated with Ph₃CB(C₆F₅)₄, formation of homopolymers is predominated.

     

Enhancement of Tg of Poly(l‐lactide) by Incorporation of Biobased Mandelic‐Acid‐Derived Phenyl Groups by Polymerization and Polymer Blending

A high‐molecular‐weight polyester of poly(mandelate‐co‐lactate) (PML) is prepared by ring‐opening polymerization of stereo‐configuration controlled cyclic diester monomers of methyl‐6‐phenyl‐1,4‐dioxane‐2,5‐dione (MPDD) and lactide. The attained PML shows excellent glassy properties, although the original stereo‐configuration of MPDD is not preserved. The intrinsic high glass transition temperature (T_g) of PML is promising, and it is able to be further enhanced by thermal treatment to as high as 90 °C. Interestingly, the enhanced high T_g is attained by only 15 mol% of mandelate content in the polymer chain which is far lower than the ones suggested by theoretical calculation. The enhancement in T_g is also attained by polymer blending of PML and poly(l ‐lactide) (PLLA). The T_g of the polymer blend also reaches 90 °C which is almost 20 °C higher than the ones suggested by theoretical calculations. These results indicate that the rigid mandelate unit consisting of phenyl groups in PML chain effectively interact with PLLA chains in amorphous domain to restrict their chain mobility. The thermal and glassy properties are sufficient to explore new applications in engineering fields.     

Synthesis of Alkyl‐Substituted Quinoxaline‐Based Copolymers Along with Photophysical Property Modulation for Polymer Solar Cells

A series of alkyl‐substituted quinoxaline‐based copolymers are prepared and their properties are studied. Stille copolymerization of 2,5‐bis(trimethylstannyl)thiophene (M3) with different ratios of 2,7‐dibromo‐9,9‐dioctyl‐9H‐fluorene (M4) and 5,8‐dibromo‐6,7‐difluoro‐2,3‐didodecylquinoxaline (M1) affords five new random copolymers, labeled P1–P5. Suzuki copolymerization of 4,5,5‐tetramethyl‐2‐[2‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)‐9,9‐dioctyl‐9H‐fluoren‐7‐yl]‐1,3,2‐dioxaborolane (M5) and 5,8‐bis(5‐bromothiophen‐2‐yl)‐2,3‐didodecyl‐6,7‐difluoroquinoxaline (M2) yielded a new alternating copolymer, labeled P6. All copolymers show high thermal stability, and the 5% weight loss temperature is above 400 °C. The estimated optical bandgap (E _g) values of random copolymers P1–P5 (E _g ≈ 1.93, 1.97, 1.97, 2.02, and 2.08 eV, respectively) are found to be relatively lower than that of alternating copolymer P6 (E _g ≈ 2.14 eV). All copolymers P1–P6 exhibit deep highest occupied molecular orbital (HOMO) energy levels, and the determined HOMO levels are ?5.65, ?5.67, ?5.67, ?5.60, ?5.59, and ?5.66 eV, respectively. The maximum power conversion efficiencies of polymer solar cells made with individual copolymers P1–P6 as a donor materials and PC₇₀BM as an acceptor are 3.98, 2.91, 3.33, 3.55, 3.07, and 2.78%, respectively.     

Thermally Cross‐Linkable Poly(p‐xylylene)s for Advanced Low‐Dielectric Applications

A novel series of poly(p‐xylylene) homopolymer and copolymers containing thermally cross‐linkable cyclohexenyl moiety are prepared via base‐catalyzed Gilch route to yield high‐molecular‐weight polymers. The resulting polymers are highly soluble in a wide range of organic solvents and could be solution cast into flexible and transparent films. The polymers are thermally stable up to 350 °C and the glass transition temperature (T_g) is in the range of 136 ? 250 °C. They undergo thermal cross‐linking via the cyclohexenyl moiety. The cross‐linked polymer exhibits a high T_g of 294 °C, a low coefficient of thermal expansion (CTE) of 45 ppm K^?1. A low dielectric constant of 2.5 and a very low dielectric loss tan δ of 0.0004 at 1 GHz are obtained, which are superior to conventional interconnect polymers.     

Metallocene Catalysts for 1‐Butene Polymerization

Summary: Isotactic polybutenes of variable isotacticity and melting points of form I in the range 100–125 °C have been prepared with both C₂‐ and C₁‐symmetric zirconocenes. The C₁‐symmetric zirconocenes bearing the bilateral symmetric 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand connected by a dimethylsilandiyl bridge to a substituted indenyl ligand produce iPB with higher molecular mass, up to 400 000 at polymerization temperature of 70 °C in liquid butene. The degree of isotacticity depends on the substitution pattern of the indenyl ligand. The correlations between microstructure and melting points of the crystalline forms I and II of iPB have been defined. Some relevant differences in catalyst selectivity between propylene and 1‐butene polymerizations have been identified.

Linear correlation of melting points of form I and form II in isotactic poly(1‐butene)s of different chain regularities.     

Propylene Polymerization Using ansa‐Zirconocenes with Water/Pentafluorophenol‐Modified Methylaluminoxane

Summary: We have confirmed that water‐ and pentafluorophenol‐modified isobutylmethylaluminoxane (W/PM‐MMAO) enhances the productivity and stereoregularity of the yielded polymer and increases its molecular weight when propylene is polymerized using C₂‐symmetric ansa‐zirconocenes. ¹³C NMR analysis shows that when W/PM‐MMAO is used as a cocatalyst with C₂‐symmetric ansa‐zirconocenes, although the pentad peaks of the stereoerrors derived from enantiofacial misinsertion have still existed; those derived from the epimerization of the growing chain end that occurred with β‐hydride elimination disappear. Therefore, we propose that W/PM‐MMAO interacts more strongly to the cationic metal center of metallocene than does MMAO owing to steric and electronic effect, and W/PM‐MMAO suppresses β‐hydride elimination, resulting in the production of polypropylene with higher molecular weight and isotacticity. To the contrary, there was no significant difference in the structure of the yielded polymer when W/PM‐MMAO was used with C_s‐symmetric ansa‐zirconocenes. In the case of C_s‐symmetric ansa‐zirconocene, the stereoerror is mostly the result of monomer insertion error, and small amount of stereoerror caused by “skipped” insertion, not from the epimerization of the polymer chain end that occurs after β‐hydride elimination; therefore, there might not be any difference of the microstructure of yielded polymers between MMAO and W/PM‐MMAO. It can be concluded that the both cases of C₂‐ and C_s‐symmetric ansa‐zirconocenes are used as a catalyst, and water and phenol modification of MMAO does not affect the monomer misinsertion which is one of the mechanisms of the stereoerrors.

¹³C NMR spectra of isotactic PP. (a) Sample 1. (b) Sample 2.     

Hydrolytic and Thermal Degradation of Random Copolyesters of ε‐Caprolactone and 2‐Oxepane‐1,5‐dione

The hydrolytic and thermal stability of random copolyesters of ε‐caprolactone (ε‐CL) and ca. 30 mol‐% 2‐oxepane‐1,5‐dione (OPD) have been investigated. Compared with poly(ε‐caprolactone) (PCL) of a comparable molecular weight, the hydrolytic degradation of the copolyester is faster in a phosphate buffer (pH = 7.4) at 37 °C as confirmed by the time dependence of water absorption, weight loss, melting temperature, and molecular weight. This difference is a result of the higher hydrophilicity imparted to the copolyester by the ketone of the OPD units. The thermal degradation has been studied by thermogravimetric analysis (TGA), ¹H NMR spectroscopy, and size exclusion chromatography (SEC). The activation energy of the thermal degradation under nitrogen has been found to be lower for the copolyesters than for PCL, which indicates that the OPD co‐units have a deleterious effect on the thermal stability of PCL. The thermal degradation primarily occurs by pyrolysis of the ester functions.

Pyrolysis of the ester function of the ε‐CL/OPD diad.     

Orientation of the Main Chain in Liquid‐Crystalline Side‐Chain Polyesters with Variation of Main‐Chain Spacer Length

Previous X‐ray investigations on liquid‐crystalline side chain polyesters with variable spacer length in the side chain as well as in the main chain led to the assumption that the structure will change qualitatively if the main chain spacer length exceeds a certain value. We obtained indications supporting this assumption by applying two‐dimensional ¹³C NMR. It was shown that for a sample with main chain spacer shorter than the critical length, the main chain segments align perpendicular to the side chains (〈P₂〉 = ?0.46), and for spacer length larger than critical length, the main chain segments preferentially orient themselves parallel to the side chain (〈P₂〉 = 0.28).

Visualization of the main‐chain order as revealed by the values of Table 2 . The opening angle of the cones corresponds to the average angle of deviation as defined in the text.     

Elastomeric Poly(propylene) from “Dual‐side” Metallocenes: Reversible Chain Transfer and its Influence on Polymer Microstructure

Summary: The influence of the cocatalyst nature on the distribution of the stereoerrors along the polymer chain has been studied using either MAO or [(C₆H₅)₃C⁺] [(C₆F₅)₄B^?] to activate a C₁‐symmetric (Flu‐Ind) complex in propene polymerization experiments. The in situ activation with borate indicated the chain back‐skip as the decisive mechanism responsible for stereoerror formation. When MAO is used for activation, additionally the reversible chain transfer to aluminum occurs, which can be called into account as a second mechanism for stereoerror formation. By the combination of ¹³C NMR, DSC, WAXS and SFM, it was shown that the differences in polymerization mechanisms result in variations of stereoerror formation. Due to this, the isotactic block length n_iso as well as their distribution along the chain changes. Using MAO activation, polypropenes with crystallizable blocks consisting of 23–32 monomers in isotactic sequences were generated, which co‐crystallized in α‐ and γ‐phase lamellae. When the reversible chain transfer was occluded (in situ borate activation) the bimodal distribution of crystalline lamellae strongly referred to a homogeneous random distribution of stereoerrors. In this case, two crystalline populations were present. The prevailing one, which crystallized in the orthorhombic γ‐modification, contained 23 consecutive isotactic blocks. Additionally, small amounts of α‐phase lamellae were present consisting of longer isotactic blocks (n_iso > 35). The different crystalline modifications resulted in different polymer morphologies. These changes caused in turn variations in the mechanical properties, such as elasticity and mechanical strength. This clearly shows that, by using different cocatalysts for activating C₁‐symmetric complexes, the properties of poly(propylenes) with statistically distributed stereoerrors can be tailored.

     

Synthesis and Thermoresponsive Properties of Biocompatible and Biodegradable Triblock Copolymers Bearing Linear or Y‐Shaped OEG Pendants

Biocompatible and biodegradable thermoresponsive polymers are of great interest in “smart” biomedical applications. Herein, triblock copolymers (6a and 6b) comprised of hydrophobic poly(propylene glycol) (PPG) and OEGylated polypeptides bearing Y‐shaped or linear pendants are prepared by ring‐opening polymerization of reactive N‐carboxyanhydride monomers and subsequent side chain modification of polypeptides. OEGylated triblock copolymer bearing Y‐shaped (6a) or linear pendants (6b) with similar main chain length and the same amount of oligo(ethylene glycol) (OEG) units are successfully prepared as verified by ¹H NMR, FTIR, and GPC. CD analysis reveals that 6a with Y‐shaped OEG pendants shows thermal‐ or salt‐induced destabilization of α‐helical conformation in water. Both 6a and 6b show reversible lower critical solution temperature (LCST)‐type phase transitions in DI H₂O or PBS. Incorporation of a PPG segment leads to a significant decrease of the LCST‐type cloud point temperature (T_cp). Consequently, 6a and 6b show T_cp to human body temperature at low polymer concentrations (≤0.5 mg mL^?1 in PBS). Moreover, 6a and 6b show low in vitro cytotoxicity and biodegradability in the presence of proteinase K. The results indicate that the thermoresponsive properties of LCST‐type polymers can be readily tuned by constructing Y‐shaped pendants as well as incorporation of hydrophobic polymer building blocks.     

Microwave‐Assisted Reversible Addition–Fragmentation Chain Transfer Polymerization of Cationic Monomers in Mixed Aqueous Solvents

Cationic polymers are an interesting class of macromolecules due to their versatility and emerging properties that can be used for various industrial and biomedical purposes. This report is focused on investigating the use of microwave heating in the reversible addition–fragmentation chain transfer polymerization of functional cationic monomers, N‐(3‐aminopropyl)methacrylamide hydrochloride (APMA) and N‐[3‐(dimethylamino)propyl]methacrylamide (DMAPMA). Under comparable polymerization reaction conditions, the microwave‐assisted reaction achieves up to 270% (APMA) and 375% (DMAPMA) rate enhancement over conventional oil‐bath mediated set‐up. Linear relationships are observed between number average molecular weight and monomer conversion for different target degrees of polymerization to give low‐ to high‐molecular‐weight cationic polymers. Chain extension experiments show increase in molecular weight of the cationic polymers with narrow dispersities (Ð < 1.2) indicating retention of the chain transfer agent with no observable aminolysis or hydrolysis during polymerization.     

Thermoplastic Poly(urethane urea)s From Novel,Bio‐based Amorphous Polyester Diols

In this study, two novel, bio‐based, amorphous polyester diols, namely poly(1,2‐dimethylethylene adipate) (PDMEA) and poly(1,2‐dimethylethylene succinate) (PDMES) are used to prepare thermoplastic poly(urethane urea)s (TPUUs). Interestingly, the TPUUs based on PDMEA show similar thermal and mechanical properties as their counterparts based on poly(1,2‐propylene glycol). By decreasing the hard segment length, the flow temperature (T_fl) of the TPUUs decreases. The T_fl values of the 3U series are around 170 °C, which is below their degradation temperatures. The methyl groups adjacent to the ester groups in PDMEA and PDMES may hinder the hydrolysis of the polyester soft segments in the TPUUs.     

Measurement of Transfer Coefficients to Monomer for n‐Butyl Methacrylate by Molecular Weight Distributions from Emulsion Polymerization

The average kinetic coefficient for chain transfer to monomer 〈k_tr,M〉 in the free‐radical polymerization of n‐butyl methacrylate (BMA) has been determined by the analysis of molecular weight distributions obtained by seeded emulsion polymerization under conditions such that chain transfer to monomer is the dominant chain‐stopping event. Measurements between 40 and 70 °C gave data fitting an Arrhenius‐type relationship with exponential factor E_A = 30 900 ± 4 500 J · mol^?1 and pre‐exponential factor log A = 3.45 ± 0.15. The value for E_A is comparable with published data for chain transfer to monomer from methyl methacrylate (MMA) and n‐butyl acrylate (BA). The A value, however, is 1–3 orders of magnitude smaller, suggesting that there is more hindrance for chain transfer to monomer for BMA than for either MMA or BA.

     

Electron‐Deficient Acridone Derivatives as a New Functional Core Towards Low‐Bandgap Metallopolyynes

The synthesis, thermal, and photoluminescence properties of novel platinum metallopolyynes of electron‐deficient acridone derivatives with electronically tunable bandgaps and their molecular model complexes are described. Polymer P2 has an optical bandgap (E_g) of 2.10 eV which is much lowered than that of P1 (2.60 eV). The incorporation of electron‐accepting dicyanomethylene units on the acridone ring in the main chain creates a strong π‐conjugated system that features unique donor‐acceptor interactions and intramolecular charge‐transfer states.