Hyperbranched Polymers by Type II Photoinitiated Self‐Condensing Vinyl Polymerization

Type II photoinitiated self‐condensing vinyl polymerization for the preparation of hyperbranched polymers is explored using 2‐hydroxyethyl methacrylate (HEMA) or 2‐(dimethylamino)ethyl methacrylate (DMAEMA), and methyl methacrylate as hydrogen donating inimers and comonomer, respectively, in the presence of benzophenone and camphorquinone under UV and visible light. Upon irradiation at the corresponding wavelength, the excited photoinitiator abstracts hydrogen from HEMA or DMAEMA leading to the formation of initiating radicals. Depending on the concentration of inimers, type of the photoinitiator, and irradiation time, hyperbranched polymers with different branching densities and cross‐linked polymers are formed.

     

Glass‐transition temperature: Conversion relationship in the polycyclotrimerization of 4,4′‐thiodiphenylcyanate

Polycyclotrimerization of 4,4′‐thiodiphenylcyanate was adopted as a model system for general thermosetting polymers for studying the relationship between the glass‐transition temperature (T_g) and conversion (α) during network formation. Existing expressions for T_g‐α relationship were used and compared. The experimental T_g‐α data were well fitted to several one‐parameter equations although the physical significance of parametric values thus obtained could not be unambiguously identified. Among the two‐parameter models, both the Hale–Macosko–Bair equation and the so‐called “original” DiBenedetto equation were well fitted by experimental data (when the mean‐field crosslink density was used), yielding parametric values consistent with the original designated physical meanings within the corresponding theoretical frames. Relationships between the parameters in different theories were also discussed. Incidentally, a discontinuity of ΔC_pT_g at the gel point was observed (i.e., ΔC_pT_g is of different values in the pregel and postgel regimes, respectively). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 726–738, 2000     

Branched Azobenzene Side‐Chain Liquid‐Crystalline Copolymers Obtained by Self‐Condensing ATR Copolymerization

Summary: The one step synthesis of a series of branched azobenzene side‐chain liquid‐crystalline copolymers by the self‐condensing vinyl copolymerization (SCVCP) of a methyl acrylic AB* inimer, 2‐(2‐bromoisobutyryloxy)ethyl methacrylate (BIEM), with the monomer 6‐(4‐methoxy‐azobenzene‐4′‐oxy)hexyl methacrylate (M), by atom transfer radical polymerization (ATRP) in the presence of CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine as a catalyst system, and in chlorobenzene solvent, is reported. The degree of branching (DB), and the molecular weights and polydispersities of the resultant polymers were determined by NMR spectroscopy and size exclusion chromatography, respectively. The phase behaviors of the branched copolymers were characterized by differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). The degree of branching of the branched copolymers could be controlled by the comonomer ratio in the feed and influenced their liquid‐crystal properties. Liquid‐crystal properties were not exhibited when the comonomer ratio was low. Comonomer ratios greater than 8 gave polymers with a lower number of branches, which exhibited both a smectic and a nematic phase.

A polarized optical micrograph of the smectic phase texture of a polymer synthesized here with a higher comonomer feed ratio (magnification × 400).     

High Glass Transition Temperature Renewable Polymers via Biginelli Multicomponent Polymerization

A novel and straightforward one‐pot multicomponent polycondensation method was established in this work. The Biginelli reaction is a versatile multicomponent reaction of an aldehyde, a β‐ketoester (acetoacetate) and urea, which can all be obtained from renewable resources, yielding diversely substituted 3,4‐dihydropyrimidin‐2(1H)‐ones (DHMPs). In this study, renewable diacetoacetate monomers with different spacer chain lengths (C3, C6, C10, C20) were prepared via simple transesterification of renewable diols and commercial acetoacetates. The diacetoacetate monomers were then reacted with renewable dialdehydes, i.e., terephthalaldehyde and divanillin in a Biginelli type step‐growth polymerization. The obtained DHMP polymers (polyDHMPs) displayed high molar masses, high glass transition temperatures (T_g) up to 203 °C and good thermal stability (T_d5%) of 280 °C. The T_g of the polyDHMPs could be tuned by variation of the structure of the dialdehyde or the diacetoacetate component.

     

Hyperbranched Polyethylenes and Functionalized Polymers by Chain Walking Polymerization with Pd‐Diimine Catalysis

Chain walking polymerization (CWP) with Pd‐diimine catalysis represents a novel concept for the synthesis of hyperbranched polyethylenes (HBPEs) and functionalized polymers from ethylene stocks. This article summarizes recent developments in this research area. The properties of HBPEs have recently been studied and their application as lubricant viscosity additives, polymer processing aids, and polymers for the functionalization and solubilization of carbon nanotubes in organic solvents have been explored, with some outstanding features having been discovered. Using the CWP strategy, we have also synthesized a range of functionalized HBPEs covalently tethered with a variety of functional groups, including POSS nanoparticles, ATRP‐initiating sites, methacryloyl and acryloyl double bonds, and backbone‐incorporated functionalized ring units.

     

Crosslinkable Vesicles Self‐Assembled by Amphiphilic Hyperbranched Polyester

Summary: Amphiphilic hyperbranched polyester (H20‐AM) with methacrylate end groups was synthesized based on hyperbranched aliphatic polyester (Boltorn™ H20). Narrow‐dispersed crosslinkable vesicles were obtained by dissolving H20‐AM in water, and characterized by laser light scattering and TEM. The hollow structural vesicle is composed of around 350 H20‐AM molecules, having a radius of around 40 nm and of 1.9 × 10⁶ g · mL⁻¹. The vesicles were fixed by crosslinking of methacrylate groups to form shape‐persistent structures.

TEM images of the crosslinked vesicles at lower magnification.     

Entanglement Transition in Hyperbranched Polyether‐Polyols

Are hyperbranched polymers capable of forming entanglements? This is the central issue of this contribution. Hyperbranched polyglycerol (hbPG) samples with different molecular weights (600–106 000 g · mol⁻¹), narrow polydispersities (1.2–1.8) and high degrees of branching (≈0.6) were prepared by anionic ring‐opening polymerization. The viscoelastic properties of these polymers with respect to molecular architecture and molar mass were investigated. At low molecular weights “classical” scaling behavior between zero shear viscosity and molecular weight can be observed, whereas between 3 000 and 10 000 g · mol⁻¹ a plateau‐like area is found. The results indicate entanglement dynamics when exceeding a critical molar mass ( ≈ 20 000 g · mol⁻¹) due to entangled hyperbranched polyglycerols.

     

Self‐Assembled Monolayer Formed by a Rhenium Complex Containing Hyperbranched Polymer

Summary: The synthesis of a hyperbranched polymer containing a rhenium bipyridine complex is reported. The polymer was synthesized from a monomer that contains two chlorotricarbonyl rhenium(I ) bipyridine moieties and a stilbazole ligand, and the polymer was formed by the coordination reaction in one single step. Gel permeation chromatography results showed that the resulting polymer had a strong interaction with the column packing material, which was reduced when the eluent was added with an electrolyte. Both atomic force microscopy and laser light scattering showed that the size of the polymer molecules was in the range between 25–30 nm. A monolayer of polymer molecules could form on a pretreated substrate by the self‐assembly process, which can serve as the building block for multilayer ultrathin film devices.

The metal‐containing hyperbranched polymer synthesized here.     

Extension of the chain‐end,free‐volume theory for predicting glass temperature as a function of conversion in hyperbranched polymers obtained through one‐pot approaches

Compared with linear polymers, more factors may affect the glass‐transition temperature (T_g) of a hyperbranched structure, for instance, the contents of end groups, the chemical properties of end groups, branching junctions, and the compactness of a hyperbranched structure. T_g's decrease with increasing content of end‐group free volumes, whereas they increase with increasing polarity of end groups, junction density, or compactness of a hyperbranched structure. However, end‐group free volumes are often a prevailing factor according to the literature. In this work, chain‐end, free‐volume theory was extended for predicting the relations of T_g to conversion (X) and molecular weight (M) in hyperbranched polymers obtained through one‐pot approaches of either polycondensation or self‐condensing vinyl polymerization. The theoretical relations of polymerization degrees to monomer conversions in developing processes of hyperbranched structures reported in the literature were applied in the extended model, and some interesting results were obtained. T_g's of hyperbranched polymers showed a nonlinear relation to reciprocal molecular weight, which differed from the linear relation observed in linear polymers. T_g values decreased with increasing molecular weight in the low‐molecular‐weight range; however, they increased with increasing molecular weight in the high‐molecular‐weight range. T_g values decreased with increasing log M and then turned to a constant value in the high‐molecular‐weight range. The plot of T_g versus 1/M or log M for hyperbranched polymers may exhibit intersecting straight‐line behaviors. The intersection or transition does not result from entanglements that account for such intersections in linear polymers but from a nonlinear feature in hyperbranched polymers according to chain‐end, free‐volume theory. However, the conclusions obtained in this work cannot be extended to dendrimers because after the third generation, the end‐group extents of a dendrimer decrease with molecular weight. Thus, it is very possible for a dendrimer that T_g increases with 1/M before the third generation; however, it decreases with 1/M after the third generation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1235–1242, 2004     

通过自缩合乙烯基氧阴离子聚合制备超支化聚甲基丙烯酸羟丙酯

由甲基丙烯酸羟丙酯通过自缩合乙烯基氧阴离子聚合(self-condensing vinyl oxyanionic polymerization)制备了端羟基的超支化聚甲基丙烯酸酯. 以氢化钾(KH)和冠醚的复合物为引发剂时, 可以得到高分子量的聚合物. 用¹H NMR和¹³C NMR谱图证实了聚合物的超支化结构. 由于在聚合过程中存在质子转移反应, 引发剂与单体的摩尔比会影响所得聚合物的结构. 超支化聚合物的玻璃化转变温度在58.1～81.4 ℃之间, 且随着引发剂与单体的比例的减小而降低. 当引发剂与单体等摩尔比时, 所得聚合物的支化度为0.49.     

Chain‐Growth Click Polymerization of AB2 Monomers for the Formation of Hyperbranched Polymers with Low Polydispersities in a One‐Pot Process

Hyperbranched polymers are important soft nanomaterials but robust synthetic methods with which the polymer structures can be easily controlled have rarely been reported. For the first time, we present a one‐pot one‐batch synthesis of polytriazole‐based hyperbranched polymers with both low polydispersity and a high degree of branching (DB) using a copper‐catalyzed azide–alkyne cycloaddition (CuAAC) polymerization. The use of a trifunctional AB₂ monomer that contains one alkyne and two azide groups ensures that all Cu catalysts are bound to polytriazole polymers at low monomer conversion. Subsequent CuAAC polymerization displayed the features of a “living” chain‐growth mechanism with a linear increase in molecular weight with conversion and clean chain extension for repeated monomer additions. Furthermore, the triazole group in a linear (L) monomer unit complexed Cu^I, which catalyzed a faster reaction of the second azide group to quickly convert the L unit into a dendritic unit, producing hyperbranched polymers with DB=0.83.     

Nanocomposites of Size‐Tunable ZnO‐Nanoparticles and Amphiphilic Hyperbranched Polymers

Highly dispersed ZnO nanoparticles with variable particle sizes were successfully prepared within an amphiphilic hyperbranched polyetherpolyol matrix via decomposition of an organometallic precursor in the presence of air leading to stable nanocomposites. The high degree of stabilization during and after the synthesis by the polymer permits control over the nanoparticle size and therefore, due to the quantum‐size‐effect, the particle properties. Furthermore, these polymer‐inorganic nanocomposites can easily be dispersed in apolar solvents to yield highly transparent, stable solutions.

     

Ferrocene‐Based Hyperbranched Polytriazoles: Synthesis by Click Polymerization and Application as Precursors to Nanostructured Magnetoceramics

Ferrocene‐based polymers have drawn much attention in the past decades due to their unique properties and promising applications. However, the synthesis of hyperbranched polymers is still a great challenge. Here, two ferrocene‐based hyperbranched polytriazoles with high molecular weights are facilely prepared by the click polymerization reactions of ferrocene‐containing diazides ( 1 ) and tris(4‐ethynylphenyl)amine ( 2 ) using Cu(PPh₃)₃Br as catalyst in dimethylformamide at 60 °C for 5 and 9 h in satisfactory yields of 54.0% and 52.3%. The resulting polytriazoles are soluble in common organic solvents and thermally stable, with 5% weight loss temperatures up to 307 °C. They can be used as precursors to produce nanostructured ceramics with good magnetizability by pyrolysis at elevated temperature.

     

Hyperbranched Polyamidoamines Containing β‐Cyclodextrin for Controlled Release of Chlorambucil

This work is focused on the controlled drug release behavior of hyperbranched HPMA in the presence of β‐CD. Hence, three HPMA‐β‐CDs and a pure HPMA were synthesized by Michael addition polymerization. As a model drug, CLB (an anti‐cancer drug) was loaded into them via a solution method for in vitro release studies. The DSC results indicate that the CLB/polymer interactions are at the molecular level. Loading CLB into these polymers results in an evident increase in their glass transition temperatures, and ΔT_g depends on the β‐CD content. The controlled‐release experiments show that the presence of β‐CD can appropriately slow the release of CLB from HPMA‐β‐CDs and adjust the ratio of CLB released in total drug loading.

     

Hyperbranched Polymers Formed through Irreversible Step Polymerization of AB2‐Type Monomer in a Continuous Flow Stirred‐Tank Reactor (CSTR)

Hyperbranched polymers formed through step polymerization of AB₂‐type monomer with equal reactivity for both B groups in a continuous flow stirred‐tank reactor (CSTR) are investigated theoretically. The weight fraction distribution at high molecular weight tail follows a power law, W (P ) ∝ P ^−1/ξ for ξ ≤ 0.5 with , where is the mean residence time. The degree of branching (DB) at the large degree of polymerization (P ) limit is DB _{P →∞} = 0.6 irrespective of the ξ‐value, which is larger than the case for the corresponding batch polymerization that gives DB _{P →∞} = 0.5. The relationship between the radius of gyration 〈s ²〉₀ and P shows that the hyperbranched polymers formed in a CSTR are very compact, and the 〈s ²〉₀‐values for large polymers are even smaller than the smallest possible case for a batch reactor with DB _{P →∞} = 1. For large polymers, the power law 〈s ²〉₀∝P ^1/3 holds, which is 〈s ²〉₀∝P ^1/2 for batch polymerization.

     

Hyperbranched Polymers Formed Through Irreversible Step Polymerization of AB2‐Type Monomer with Substitution Effect in a Continuous Flow Stirred‐Tank Reactor (CSTR)

A new Monte Carlo simulation method is proposed for the step polymerization of AB₂‐type monomer conducted in a continuous flow stirred‐tank reactor (CSTR). The effect of the second B group reactivity, represented by the reactivity ratio r is investigated. The degree of branching (DB) at large degree of polymerization (P ) limit, DB_{P →∞} does not change with the mean residence time . The value of DB_{P →∞} becomes larger by increasing r and is larger than the corresponding batch polymerization. The weight fraction distribution at high molecular weight tail follows a power law , and a simple formula to predict the power exponent α is proposed. The relationship between the radius of gyration 〈s ²〉₀ and P does not change with , and large polymers obtained in a CSTR are much more compact than those formed in batch polymerization. CSTR is advantageous to synthesize compact HB polymers, especially with a smaller r‐value.