Synthesis and Characterization of Block Copolymers of ε‐Caprolactone and DL‐Lactide Initiated by Ethylene Glycol or Poly(ethylene glycol)

Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL ‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by ¹H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

Block copolymerization of ε‐caprolactone and DL ‐lactide initiated by dihydroxyl PEG.     

Controlled Ring‐Opening Polymerization of Lactones and Lactide Initiated by Lanthanum Isopropoxide, 2. Mechanistic Studies

The mechanism of ring‐opening polymerization of some lactones and lactide initiated by lanthanum isopropoxide has been comprehensively investigated. NMR and viscosity analyses demonstrated that three active polymer chains grow per lanthanum atom and that, depending on the coordinating ability of the monomer, propagation proceeds on aggregated or unaggregated active species. It is also demonstrated that inter‐ and intramolecular side reactions are limited and that lanthanum‐based initiator selectivity in ring‐opening polymerization is analogous to that of aluminum ones.

Part of the proposed mechanism for the lanthanum alkoxide initiated polymerization of ε‐caprolactone.     

Synthesis of Poly(lauryl acrylate) by Single-Electron Transfer/Degenerative Chain Transfer Living Radical Polymerization Catalyzed by Na2S2O4 in Water

Living radical polymerization of lauryl acrylate was achieved by SET/DTLRP in water catalyzed by sodium dithionite. The work describes the synthesis of a highly hydrophobic and polar monomer in aqueous medium. The plots of versus conversion and ln[M]₀/[M] versus time are linear, indicating a controlled polymerization. This method leads to α,ω-diiodopoly(lauryl acrylate)s that can be further functionalized. The MWDs were determined using a combination of three detectors: RALLS, DV, and RI. The method studied in this work represents a possible route to prepare well-tailored macromolecules made of LA in environment friendly reaction medium. The syndiotactic content is 75%.

     

Electroactive Polymer with Oligoanilines in the Main Chain: Synthesis,Characterization and Dielectric Properties

An electroactive polymer with well‐defined oligoanilines in the main chain has been prepared by oxidative coupling polymerization. The detailed characteristics of the polymer were systematically studied by FT‐IR, ¹H NMR, GPC and XRD. Its electrochemical behavior was explored with UV‐Vis spectra and cyclic voltammetry showing that its intrinsic electroactivity was maintained and exhibited three distinct reversible oxidative states. Its electrical conductivity is about 3.91 × 10^?4 S · cm^?1 in HCl‐doped form at room temperature. Furthermore, the polymer displays a dielectric constant of 48.4 and 13.6 at 10 and 1 MHz, respectively, mainly attributed to the improvement in delocalized charge density and electrical conductivity of the polymer by introduction of the conjugated oligoaniline segments.

     

gem‐Dimethyl Effects in the Thermal Behavior of Polyethylene

Summary: Polyethylene can be rendered completely amorphous via the insertion of regularly spaced gem‐dimethyl defects along its backbone, where frequency of insertion is the deciding factor in achieving the totally amorphous state. Accurate placement of defects along polyethylene's backbone is achieved with step polymerization metathesis chemistry (the ADMET reaction) rather than using chain polymerization techniques. These gem‐dimethyl polyethylene macromolecules, when compared with other ADMET ethylene‐based model materials, demonstrate that steric bulk, frequency, and distribution of the defect along the polymer backbone are more important than stereoregularity (tacticity) when the defects are spaced nine carbon atoms or more apart. Comparisons are made between gem‐dimethyl “defects” and methyl “defects” in a polyethylene backbone.

Synthetic scheme for the ADMET‐made gem‐dimethyl model polyethylenes ( 14a–c ).     

Syndiotactic Poly(propylene) from [Me2Si(3,6‐di‐tert‐butyl‐9‐fluorenyl)(N‐tert‐butyl)]TiCl2–Based Catalysts: Chain‐End or Enantiotopic‐Sites Stereocontrol?

The origin of the syndiotactic selectivity in propene polymerization of a typical bridged fluorenyl‐amido catalyst, namely [Me₂Si(3,6‐di‐tert‐butyl‐9‐fluorenyl)(N‐tert‐butyl)]TiCl₂ ( 1 ), activated with methylaluminoxane or [HMe₂N(C₆H₅)][B(C₆F₅)₄]/Al(i‐butyl)₃, was investigated by means of 150 MHz ¹³C NMR spectroscopic microstructural polymer analysis. The asymmetric induction was traced unambiguously to enantiotopic‐sites control. Compared with the better‐known cyclopentadienyl‐fluorenyl ansa‐zirconocenes, 1 turned out to be almost identically enantioselective (in agreement with computer modeling), but less stereoselective because of a higher propensity to undergo site epimerization. This results in a chain microstructure with large predominance of m over mm stereodefects, deceptively similar to that of syndiotactic poly(propylene) produced under chain‐end control.

Methyl region of the 150 MHz ¹³C NMR spectrum of a poly(propylene) sample prepared with catalyst system 1 /borate/TIBAl.     

Modification of Polyethylene by VUV Irradiation in the Presence of Trimethylsilane and Oxygen

The photoassisted modification of LDPE by VUV irradiation in a reactive trimethylsilane/oxygen atmosphere was investigated. The modified LDPE surface was investigated by FT‐IR, XPS, contact‐angle measurements and electron microscopy. Attachment of alkylsilyl and Si–O units to LDPE was observed. Best results were obtained using an equimolar trimethylsilane/oxygen mixture. Low‐energy surfaces were obtained after a few minutes of irradiation at an intensity of 1.7 mW · cm^?2. After prolonged irradiation, the thickness of the modified polyethylene layer is approximately 700 nm. Surface modifications of this type may be of interest to obtain protective and barrier layers on polyolefins.

     

Synchrotron X‐ray and DSC Studies of the Phase Behaviour of Poly(diethylene glycol p,p′‐bibenzoate)

Time‐resolved synchrotron X‐ray and DSC experiments were applied to investigate the phase behaviour of poly(diethylene glycol p,p′‐bibenzoate), PDEB. The DSC results are indicative of the formation of a smectic mesophase, previously identified as a SmCA type, which can be easily quenched down to room temperature. However, the synchrotron results show that the SmCA phase undergoes some kind of ordering or transformation at temperatures below 110 °C. Moreover, the annealing of PDEB at temperatures above T_g for sufficiently long times leads to the formation of a highly ordered structure, although very thin crystals and low crystallinities are obtained.

Scattering profiles corresponding to sample PDEB85 in a melting experiment.     

Synthesis of Defined Poly(silsesquioxane)s: Fast Polycondensation of Trialkoxysilanes in a Continuous‐Flow Microreactor

The polycondensation of trialkoxysilanes to PSSQs in a microreactor setup is demonstrated. While continuous‐flow processes involving microreactors found various applications in chain growth polymerization, their influence on step‐growth polymerization is less explored, and the polycondensation of multifunctional monomers has not been studied in detail. We found significantly increased yields and a decreased polydispersity of the obtained polymers in comparison to the batch process. By variation of the residence time molecular weights could be adjusted reproducibly ranging from = 1 900 to 11 000 g · mol^?1. Thus, the microreactor setup offers for the first time the possibility to synthesize PSSQ with adjustable properties.

     

Novel Polymerization of Fluorinated 2‐Vinylcyclopropane in Aqueous Solution via Cyclodextrin Complexes

We report on the first examples of the free radical polymerization of a fluorinated 2‐vinylcyclopropane 1 and its copolymerization with an alkyl 2‐vinylcyclopropane 2 in aqueous solution via their host‐guest complexation with a random methylated β‐cyclodextrin (RAMEB) using a water‐soluble initiator (VA50). Upon polymerization, the dethreaded cyclodextrin remained in water solution, whereas the water‐insoluble polymer precipitated out and was isolated. The polymers prepared exhibited mesophase behavior over a wide range of temperature. Both T_g and T_i of the copolymers increased linearly with the amount of fluorinated co‐units.

Complexation of fluorinated vinylcyclopropane 1 with RAMEB and polymerization in water solution.     

Solid‐State Olefin Metathesis: ADMET of Rigid‐Rod Polymers and Ring‐Closing Metathesis

Summary: Solid‐state olefin metathesis of rigid‐rod acyclic diene metathesis (ADMET) polymers and ring‐closing metathesis (RCM) have been investigated. 1,4‐Dipropoxy‐2,5‐divinylbenzene ( 4 ) was synthesized and used in a bulk ADMET polymerization to produce oligomers of dialkoxy poly(phenylene vinylene). The reaction was continued in the solid state, effectively doubling the molecular weight. Solid‐state RCM was investigated with a variety of solid dienes and metathesis catalysts, and demonstrated in low conversions using amide diene 5 with catalysts 9 , 13 , and 14 .

     

An Easy Strategy for the Synthesis of Well‐Defined Aliphatic‐Aromatic Hyperbranched Polyesters

A one‐pot solution polymerization under mild conditions was adapted for the synthesis of well‐defined aliphatic‐aromatic polyesters with different degrees of branching. The esterification was performed at room temperature using 4,4‐bis(4′‐hydroxyphenyl)valeric acid (AB₂) and 3‐(4‐hydroxyphenyl)propionic acid (AB) as monomers. DPTS was used as a catalyst and DCC as a coupling agent. Polyesters with statistical, dendritic topology, controlled degree of branching and > 35 000 g · mol^?1 were obtained. The polymers were characterized by ¹H and ¹³C NMR, SEC, DSC, and TGA. A strong dependence of the degree of branching and the thermal properties of the polymers depending on the AB/AB₂ monomer ratio was observed.

     

Photocurable Shape‐Memory Copolymers of ε‐Caprolactone and L‐Lactide

Biodegradable and photocurable block copolymers of ε‐caprolactone and L ‐lactide were synthesized by polycondensation of PLLA diol ( = 10 000 g · mol^?1), PCL diol ( = 10 000 g · mol^?1), and a chain extender bearing a coumarin group. The effect of copolymer composition on the thermal and mechanical properties of the photocured copolymers was studied by means of DSC and cyclic tensile tests. An increase in Young's modulus and a decrease in the tensile strain with increasing PLLA content was observed for the block copolymers. Block copolymers with high PCL content showed good to excellent shape‐memory properties. Random copolymers exhibited R_f and R_r values above 90% at 45 °C for an extremely large tensile strain of 1 000%.

     

Quantitative Analysis of Dielectric Constants from EFM Images of Multicomponent Polymer Blends

Summary: A simple quantitative model for the analysis of EFM images of three‐ or more‐component polymer blends is applied to determine the dielectric constants of the blend constituents. The value of the dielectric constant of PIPA calculated from the EFM images of POMA‐PIPA‐APP blend is determined in good agreement with the literature value.

     

Novel High and Ultrahigh Molecular Weight Poly(propylene) Plastomers by Asymmetric Hafnocene Catalysts

Summary: The novel asymmetric ansa‐complexes [1‐(9‐η⁵‐fluorenyl)‐2‐(2,5,7‐trimethyl‐indenyl)ethane]hafnium dichloride ( 7a ) and [1‐(9‐η⁵‐fluorenyl)‐2‐(2,4,6‐trimethyl‐indenyl)ethane]hafnium dichloride ( 7b ) were prepared and used as catalysts for propylene homopolymerization reactions after in situ activation. The synthetic route allows to separate the 4,6‐ and 5,7‐substituted ligand isomers before the complexation step. The orientation of the methyl groups to the “front” (4,6) or to the “back” (5,7) of the tetrahedral hafnocene dichloride species influences their performances in polymerization reactions. Whereas hafnocene ( 7b ) which bears trimethyl substitution at 2,4,6‐positions of the indenyl moiety exhibits only moderate activity, the 2,5,7‐trimethyl substituted structure ( 7a ) produces isotactic poly(propylene)s with high molecular weights (up to = 9.0 × 10⁵ g · mol^?1) and high activities [up to 3.2 × 10⁵ kg of PP (mol Hf × h)^?1]. A comparative analysis of polymerization data and mechanical behavior of 7a , and previously reported 6,7‐indenyl substituted complex 6b are reported.

Typical stress‐strain curves of different types of poly(propylene)s.     

Effect of Ethanol on the Rheological Properties of Water‐Rich Gels of Diblock Copolymer E43B11

Summary: The effect of added ethanol on the rheological behaviour of aqueous solutions of diblock copolymer E₄₃B₁₁ (E = oxyethylene unit, B = oxybutylene unit) has been investigated. Thermally reversible sol‐gel transitions were observed for concentrated solutions of the copolymer in solutions containing 0–30 wt.‐% ethanol. Storage (G′) and loss (G″) moduli and yield stress (σ_y) were used to define hard and soft gel phases. The introduction of 10 wt.‐% ethanol did not alter the phase behaviour greatly, whereas higher ethanol concentrations had a large effect, which differed in kind from that reported for triblock copolymers of ethylene oxide and propylene oxide, type EPE.

Temperature dependence of the logarithm of (?) storage and (○) loss modulus for 20 wt.‐% copolymer in 10 wt.‐% ethanol.     

Atom Transfer Radical Polymerization Using Multidentate Amine Ligands Supported on Soluble Hyperbranched Polyglycidol

Summary: Controlled polymerization of styrene in toluene was achieved by atom transfer radical polymerization (ATRP) using hyperbranched polyglycidol‐supported multidentate amine ligands/Cu^IBr catalyst systems. These catalyst systems with nanoscopic dimensions were more active than the corresponding low‐molecular‐weight ligands. The controlled/living nature of the polymerization is supported by linear first order plots (ln[M]_o/[M] versus time) and a linear increase of versus conversion as well as low polydispersity. Similar controlled polymerization of methyl methacrylate was possible in acetonitrile. Up to 97% of total copper used for polymerization could be removed from the polymer by simple precipitation in methanol and filtration.

Molecular weight characteristics for styrene polymerization using PG‐triamine as a ligand.     

A Study of the Use of Oscillatory Flow to Characterize Isotactic and Syndiotactic Poly(propylene)s

Summary: A rheological characterization of poly(propylene)s (PP) of different tacticity, monomodal and bimodal molecular distribution and long chain branching (LCB), is presented. Loss factor δ plotted as a function of the complex modulus G* allows to discriminate syndiotactic from isotactic PPs, as well as syndiotactic monomodal from bimodal samples. However, using such rheological plots is not effective to investigate the broadness of the molecular weight distribution and other rheological techniques are proposed. Isotactic and syndiotactic PPs submitted to electro beam irradiation give rise to thermorheological complexity, associated to LCB. The degree of LCB is estimated using complex viscosity results. Syndiotactic poly(propylene)s are more prone to give long branches than isotactic samples.

A symptom of long chain branching is detected in irradiated syndiotactic PPs.     

One‐Pot Synthesis of Micelles with a Cross‐Linked Poly(acrylic acid) Core

Summary: Stable micelles with polystyrene (PS) as a shell and cross‐linked poly[(acrylic acid)‐co‐(ethylene glycol diacrylate)] as a core have been successfully prepared by reversible addition fragmentation chain transfer (RAFT) copolymerization of acrylic acid and ethylene glycol diacrylate in a selective solvent with PS‐SC(S)Ph as a RAFT agent. For the preparation of stable micelles, the RAFT polymerizations are carried out in different solvents: benzene, cyclohexane, and mixtures of tetrahydrofuran and cyclohexane. The monomer/PS‐SC(S)Ph molar ratio and molecular weight of the macro‐RAFT agent, PS‐SC(S)Ph, influence the RAFT polymerization and the formation of micelles.

Block copolymerization in selective solvent with the RAFT agent.     

Synthesis and self‐assembly of polymers containing dicarboximide groups by living ring‐opening metathesis polymerization

Polymers with narrow molecular weight distributions and controlled architectures were generated by living ring‐opening metathesis polymerization of exo‐7‐oxabicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboximide. The dicarboximide units have been previously shown to exhibit biological activity, can selectively bind to the nucleic acid base adenine by hydrogen‐bonding, and are readily functionalizable. Block copolymers containing these moieties were generated, and underwent self‐assembly into nanoscale spherical aggregates, with surface localized molecular recognition motifs.