Precise synthesis of polymers containing functional end groups by living ring-opening metathesis polymerization (ROMP): Efficient tools for synthesis of block/graft copolymers

This article summarizes recent examples for precise synthesis of (co)polymers containing functional end groups prepared by living ring-opening metathesis polymerization (ROMP) using molybdenum, ruthenium complex catalysts. In particular, this article reviews recent examples for synthesis of amphiphilic block/graft copolymers by adopting transition metal-catalyzed living ROMP technique. Unique characteristics of the living ROMP initiated by the molybdenum alkylidene complexes (so-called Schrock type catalyst), which accomplish precise control of the block segment (hydrophilic and hydrophobic) as well as exclusive introduction of functionalities at the polymer chain end, enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or the “grafting from” approach as well as “soluble” star shape polymers with controlled manner. The “grafting through” approach (polymerization of macromonomers) by the repetitive ROMP technique offers precise control of the amphiphilic block segments.     

Synthesis and characterization of polybenzoxazine networks nanocomposites containing multifunctional polyhedral oligomeric silsesquioxane (POSS)

A new class of polybenzoxazine/POSS nanocomposites with network structure is prepared by reacting multifunctional benzoxazine POSS (MBZ-POSS) with benzoxazine monomers (Pa and Ba) at various compositional ratios. Octafunctional cubic silsesquioxane (MBZ-POSS) is used as a curing agent, which is synthesized from eight organic benzoxazine tethers through hydrosilylation of vinyl-terminated benzoxazine monomer (VP-a) with octakis(dimethylsiloxy)silsesquioxane () using a platinum complex catalyst (Pt-dvs). Incorporation of the silsesquioxane core into polybenzoxazine matrix could significantly hinder the mobility of polymer chains and enhance the thermal stability of these hybrid materials. For these nanocomposites, increasing the POSS content in the hybrids is expected to improve its thermal properties with respect to the neat polybenzoxazine. The morphology feature is useful to explain the thermal property changes (T_g and T_d) and AFM images show that the presence of POSS aggregation in larger scales occurs at higher POSS contents. The reason of the heterogeneous phase separation may be from the less compatibility of the inorganic silsesquioxane core with organic benzoxazine species and the homopolymerization of MBZ-POSS. In the course of the formation of the polybenzoxazine/POSS hybrids, POSS particles were separated from the polybenzoxazine rich region, leading to POSS rich domains in the range of 50-1000 nm.     

Physical gelation in ethylene-propylene copolymer melts induced by polyhedral oligomeric silsesquioxane (POSS) molecules

The rheological behavior of ethylene-propylene (EP) copolymers containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of wide-angle X-ray diffraction (WAXD), oscillatory shear, stress and strain controlled rheology in the molten state and dynamic mechanical analysis (DMA) in the solid state. WAXD results showed that the majority of POSS molecules in the EP melt were present in the crystal form. Oscillatory shear results showed that the EP/POSS nanocomposites exhibited a solid-like rheological behavior compared with the liquid-like rheological behavior in the neat resin, i.e. POSS caused physical gelation in EP. While POSS exhibited only a minimum effect on the flow activation energy of EP, the high POSS concentration samples were found to induce higher yield stress than the neat resin. This behavior was similar to the Bingham rheology, indicative of a structured fluid. DMA results indicated that the presence of POSS increased the Young's modulus as well as the T_g of the EP copolymer. These results suggested that two types of interactions contributed to the physical gelation in EP/POSS melts were present: the strong particle-to-particle interactions between the POSS crystals and the weak particle-to-matrix interactions between the POSS crystals and the EP matrix.     

Formation of nanostructured epoxy networks containing polyhedral oligomeric silsesquioxane (POSS) blocks

Nanostructured epoxy networks, based on DGEBA and poly(oxypropylene)diamine (Jeffamine D), containing nano-sized inorganic blocks, polyhedral oligomeric silsesquioxanes (POSS), were investigated. The POSS were incorporated in the network as crosslinks or as pendant units by using octa- or monoepoxy-POSS monomers, respectively, as well as diepoxides with pendant POSS. The authors focused on investigating the relationship between the network formation process and the final product properties. The reactivity of the epoxy-functional POSS monomers, the hybrid systems' time of gelation, the gel fractions and the phase structure of the networks were determined using ¹H or ¹³C NMR spectroscopy, chemorheology experiments, sol-gel analysis and transmission electron microscopy (TEM).All the POSS epoxides tested show a reduced reactivity if compared to their respective model compounds due to sterical crowding in the neighborhood of their functional groups and due to reduced epoxy group mobility. The incorporation of pendant POSS into networks of the type DGEBA-Jeffamine D-monoepoxy-POSS hence took place only in the late reaction stage. Together with the high tendency of these POSS to aggregation, the kinetics favors the formation of small nano-phase-separated POSS domains, which act as physical crosslinks due to their covalent bonds to the organic matrix. At POSS loadings higher than 70%, topological constraint by POSS leads to a strongly reduced elastic chain mobility, thus additionally strongly reinforcing the networks. The network build-up and gelation of the octaepoxy-POSS-Jeffamine D system were slow compared to the reference DGEBA-Jeffamine D network due to a low octaepoxy-POSS reactivity and due to its strong tendency to cyclization reactions with primary amines. The topology of the amino groups is shown to be very important. In contrast to monoepoxy-POSS, the octaepoxy-POSS becomes dispersed as oligomeric junctions (purely chemical crosslinks) of the network in the cured product. The octaepoxide's reinforcing effect is small and is given only by its high functionality and not by its inorganic nature. The functionality effect is reduced by the mentioned cyclizations.     

Synthesis,characterization and self-assembly of hybrid pH-sensitive block copolymer containing polyhedral oligomeric silsesquioxane (POSS)

In this study, a series of novel hybrid pH-sensitive block copolymers containing POSS (HBCPs), poly(methacrylisobutyl-POSS)-b-poly(4-vinylpyridine) (PMAiBuPOSS-b-P4VP) and poly(methacrylisobutyl-POSS)-b-polystyrene-b-poly(4-vinylpyridine) (PMAiBuPOSS-b-PS-b-P4VP), were synthesized via reversible addition fragmentation chain-transfer (RAFT) polymerization. Their structures and molecular weight were characterized via ¹H NMR, GPC and TEM. Their self-assembly behaviors, including pH-sensitive behaviors and self-assembly morphologies in aqueous solution, were investigated via DLS and TEM. It was found that the size of aggregates in aqueous solution would initially decrease and later increase as the pH value increased. It is supposed that this behavior was caused by the pH sensitivity of the P4VP block of the HBCPs. Our hybrid triblock copolymers were found to assemble nanowires and nanospheres. Unique dot-like phase separation was also observed in the aggregates of the HBCPs at pH 1. Furthermore, we investigated the effects of block length and structure on the self-assembly morphologies of the HBCPs.     

Synthesis and characterization of polyhedral oligomeric silsesquioxane (POSS) with multifunctional benzoxazine groups through click chemistry

We prepared a new class of polybenzoxazine-POSS nanocomposites with network structures through thermal curing of multifunctional benzoxazine groups of POSS (OBZ-POSS), which was synthesized from octa-azido functionalized POSS (OVBN₃-POSS) with 3,4-dihydro-3-(prop-2-ynyl)-2H-benzoxazine (P-pa) via a click reaction. Incorporation of the silsesquioxane core into the polybenzoxazine matrix could significantly enhance the thermal stability of these hybrid materials. For these nanocomposites, the POSS nanoparticles in the hybrids were improved their thermal properties with 2,2-bis(3,4-dihydro-3-methyl-2H-1,3-benzoxazine)propane (BA-m) and P-pa polybenzoxazine, analyzed via TGA analyses. In addition, the incorporation of the POSS led to the formation of an inorganic protective layer on the nanocomposite’s surface. Contact angle data provided positive evidence to back up this hypothesis that the incorporation of the POSS units would decrease the surface energy property. In addition, the low glass transition temperature of poly(4-vinyl pyridine) and polycarbonate thin films, which lack liquid resistance, could possess low surface free energy after modification with OBZ-POSS due to low temperature curing of this new compound.     

Synthesis and characterization of star-shaped block copolymers with polyhedral oligomeric silsesquioxane (POSS)core via ATRP

Star-shaped PMMA-b-PS block copolymers with POSS core were prepared by atom transfer radical polymerization of St using star-shaped POSS/PMMA-Cl as a macroinitiator in presence of CuCl, 2,2,-bipyridine, toluene at 110 °C. The core-first method, which used an active multifunctional core to initiate the growth of polymer chains, was applicable to making star-shaped block copolymers with POSS core. The structure of hybrid star-shaped PMMA-b-PS block copolymers was characterized by GPC and ¹H NMR, respectively.     

ABA triblock copolymers containing polyhedral oligomeric silsesquioxane pendant groups: synthesis and unique properties

The synthesis and characterization of POSS containing ABA triblock copolymers is reported. The use of atom transfer radical polymerization (ATRP) enabled the preparation of well-defined model copolymers possessing a rubbery poly(n-butyl acrylate)(pBA) middle segment and glassy poly(3-(3,5,7,9,11,13,15-heptaisobutyl-pentacyclo[9.5.1.1^3,9.1^5,15.1^7,13]-octasiloxane-1-yl)propyl methacrylate(p(MA-POSS)) outer segments. By tuning the relative composition and degree of polymerization (DP) of the two segments, phase separated microstructures were formed in thin films of the copolymer. Specifically, dynamic mechanical analysis and transmission electron microscopy (TEM) observations reveal that for a small molar ratio of p(MA-POSS)/pBA (DP=6/481/6) no evidence of microphase separation is evident while a large ratio (10/201/10) reveals strong microphase separation. Surprisingly, the microphase-separated material exhibits a tensile modulus larger than expected (ca. 2×10⁸ Pa) for a continuous rubber phase for temperatures between a pBA-related T_g and a softening point for the p(MA-POSS)-rich phase. Transmission electron microscopy (TEM) images with selective staining for POSS revealed the formation of a morphology consisting of pBA cylinders in a continuous p(MA-POSS) phase. Thermal studies have revealed the existence of two clear glass transitions in the microphase-separated system with strong physical aging evident for annealing temperatures near the T_g of the higher T_g phase (p(MA-POSS). The observed aging is reflected in wide-angle X-ray scattering as the strengthening of a low-angle POSS-dominated scattering peak, suggesting some level of ordering during physical aging. The T_g of the POSS-rich phase observed in the microphase separated triblock copolymer was nearly 25 °C higher than that of a POSS-homopolymer of the same molecular weight, suggesting a strong confinement-based enhancement of T_g in this system.     

Nanotailoring of polyurethane adhesive by polyhedral oligomeric silsesquioxane (POSS)

The development and commercialization of nanoparticles such as nanoclays (NCs), carbon nanotubes (CNTs) and polyhedral oligomeric silsesquioxanes (POSS) offers new possibilities to tailor adhesives at the nanoscale. Four types of POSS, with reactive mono-functional groups of isocyanatopropyl, glycidoxypropyl, aminoethyl and non-reactive octaphenyl, were incorporated in concentrations of 1, 3 and 5 wt% into a polyurethane (PU)-based adhesive. Thermo-mechanical bulk properties were studied using dynamic mechanical analysis (DMA). Adhesive properties were characterized in shear and peel modes. Atomic force microscopy (AFM) was used to study the nanoscale morphology. DMA measurements indicated that the neat PU possessed a glass transition temperature (T _g) of ≈ 30°C. The T _g of PU/POSS-glycidoxypropyl nanocomposite adhesive increased gradually with POSS concentration to 50°C for 5 wt%. PU/POSS-octaphenyl nanocomposite adhesive exhibited an increased T _g by 10°C for 5 wt%. The incorporation of POSS-isocyanatopropyl in the PU had no effect on the T _g. With respect to shear properties of POSS-octaphenyl-, POSS-isocyanatopropyl- and POSS-glycidoxypropyl-based PU nanocomposite adhesives, shear strength improved by 230, 178 and 137%, respectively, compared to neat PU. POSS-aminoethyl exhibited lower shear and peel strengths, while POSS-isocyanatopropyl provided the best balance of both higher shear and peel strengths compared to neat PU. It was concluded that the grafted functional group on the POSS and its reactivity with the PU network components were the decisive factors with respect to the thermo-mechanical, morphological and adhesive properties of the resulting nanocomposite adhesives. Consequently, the POSS/polyurethane based nanocomposite adhesives could be tailored for a large range of applications.     

Blocked polyisocyanates containing monofunctional polyhedral oligomeric silsesquioxane (POSS) as crosslinking agents for polyurethane powder coatings

Blocked polyisocyanate crosslinkers for powder coatings were synthesized using alicyclic diisocyanates (TMDI and IPDI), formic acid, (methylaminopropyl)hepta(isobutyl)Si₈O₁₂ (POSS), ?-caprolactam, dibutyltin dilaurate as well as triethylamine as catalysts. The chemical structures of these compounds were characterized by means of IR, ¹H NMR and ¹³C NMR spectroscopy. The three-dimensional surface topography and surface chemical structure of the resulting powder coatings were investigated by using confocal microscope and ATR FT-IR. The values of surface roughness parameters were calculated. The surface topography was correlated with the chemical structure of the coatings and macroscopic surface behaviour: surface free energy, abrasion resistance, hardness, adhesion to the steel surface and impact resistance. Thermogravimetric analysis was employed to assess the hardening property of powder coatings and the thermal decomposition processes.     

Synthesis and characterizations of a vinyl-terminated benzoxazine monomer and its blending with polyhedral oligomeric silsesquioxane (POSS)

Benzoxazine was synthesized through the Mannich condensation of phenol, formaldehyde, and primary amines through ring-opening polymerization. Polybenzoxazines are phenolic-like materials that possess dimensional and thermal stability, and they release no toxic byproducts during their polymerization. To further improve the thermal stability of polybenzoxazines, a hydrosilane-functionalized polyhedral oligomeric silsesquioxane (H-POSS) was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) which we then subjected to ring-opening polymerization. In addition, we also prepared hybrids from a non-reactive POSS (IB-POSS) and VB-a. The glass transition temperature (T_g) of a regular polymerized VB-a (i.e. PVB-a) is 307 °C, while the hybrid containing 5 wt% of H-POSS is 333 °C. The IB-POSS modified PVB-a hybrids, in general, results in lower T_g than the pure PVB-a due to poor missibility.     

POSS基聚合物的热性能和阻燃性

本文介绍了POSS单体和POSS基聚合物的结构、性能和制备方法。重点阐述了各类POSS基聚合物以及它们的热性能。同时简单介绍了POSS对聚合物阻燃性的影响,分析了POSS提高聚合物热性能和阻燃性的原因,并展望了POSS基聚合物的发展方向。     

Surface-initiated atom transfer radical polymerization of polyhedral oligomeric silsesquioxane (POSS) methacrylate from flat silicon wafer

A polyhedral oligomeric silsesquioxane (POSS) methacrylate monomer, i.e. 3-(3,5,7,9,11,13,15-heptacyclopentyl-pentacyclo [9.5.1.1.^3,91.^5,151^7,13]-octasiloxane-1-yl) propyl methacrylate (POSS-MA), was directly grafted from flat silicon wafers using surface-initiated atom transfer radical polymerization (ATRP). Two methods were used to improve the system livingness and control of polymer molecular weights. By ‘adding free initiator’ method, a linear relationship between the grafted poly(POSS-MA) layer thickness and monomer conversion was observed. By ‘adding deactivator’ method, the poly(POSS-MA) thickness increased linearly with reaction time. Poly(POSS-MA) layers up to 40 nm were obtained. The chemical compositions measured by X-ray photoelectron spectroscopy (XPS) agreed well with their theoretical values. Water contact angle measurements revealed that the grafted poly(POSS-MA) was extremely hydrophobic. The surface morphologies of the grafted polymer layers were studied by an atom force microscopy (AFM).     

The synthesis and characterization of polystyrene/magnetic polyhedral oligomeric silsesquioxane (POSS) nanocomposites

Fc-CHCH-C₆H₆-(C₅H₉)₇Si₈O₁₂ (POSS1, Fc: ferrocene) which contain both metal and CC double bond was firstly synthesized by Wittig reaction. The chemical structure of POSS1 was characterized by FTIR, ¹H, ¹³C and ²⁹Si NMR, mass spectrometry and elemental analysis, and the magnetic property of POSS1 have also been studied. Polystyrene composites containing inorganic-organic hybrid polyhedral oligomeric silsesquioxane (POSS1) were prepared by bulk free radical polymerization. XRD and TEM studies indicate that POSS1 is completely dispersed at molecular level in PS matrix when 1 wt% POSS1 is introduced, while some POSS1-rich nanoparticals are present when content of POSS1 is beyond 3 wt%. GPC results show that molecular weight of the PS/POSS1 nanocomposites are increased with addition of POSS1. TGA and TMA data show the thermal stabilities of PS/POSS1 nanocomposites have been improved compared to neat PS. The PS/POSS1 nanocomposites also display higher glass transition temperatures (T_g) in comparison with neat PS. Viscoelastic properties of PS/POSS1 nanocomposites were investigated by DMTA. The results show the storage modulus (E′) values (temperature>T_g) and the loss factor peak values of the PS/POSS1 nanocomposites are higher than that of neat PS. Mechanical properties of the PS/POSS1 nanocomposites are improved compared to the neat PS.     

ε-Caprolactam polymerization in presence of polyhedral oligomeric silsesquioxanes (POSS)

Polyhedral oligomeric silsesquioxanes (POSS) have been covalently linked to polyamide 6 (PA6) chains with the aim of synthesizing hybrid organic/inorganic polymer materials. The synthesis has been achieved by in situ polymerization of ε-caprolactam (CL) in presence of increasing amounts of POSS molecules, using two polymerization mechanisms (hydrolytic and anionic). The latter method has been carried out by three different approaches, in order to get PA6 samples characterized by various morphologies and content of structural defects: (i) quasi-adiabatic bulk polymerization; (ii) isothermal bulk polymerization; (iii) quasi-isothermal suspension polymerization. The products obtained have been characterized in term of structure, morphology, thermal properties and molecular mass.     

Synthesis of POSS-containing fluorosilicone block copolymers via RAFT polymerization for application as non-wetting coating materials

By using a polydimethylsiloxane (PDMS) macro-chain transfer agent with trithiocarbonate groups at both ends, fluorosilicone block copolymers containing polyhedral oligomeric silsesquioxane (POSS) were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. Acryloisobutyl POSS (APOSS) and 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) were sequentially introduced into the copolymers. The obtained triblock copolymers PDMS-b-(PAPOSS)₂ exhibited a low polydispersity index (PDI) of less than 1.42 in the first 6 h of polymerization, but the PDI value became broader later because of the steric hindrance of the POSS macromer. The POSS-containing fluorosilicone pentablock copolymers with a PDI of about 2.0, which were prepared by the further RAFT polymerization of HFBA, showed clear microphase separation. The average roughness values of the copolymer films were enhanced by introducing POSS, and a certain POSS content led to a significant decrease of the receding contact angle. Measurements of water contact angles and ice shear strengths demonstrated that the non-wetting properties of the copolymer films were improved by the incorporation of both POSS and fluorine blocks. The block copolymers combine the advantages of POSS, PDMS and fluoropolymers, and can be potentially applied as non-wetting coating materials for anti-icing or anti-frosting.     

Phenolic resin-trisilanolphenyl polyhedral oligomeric silsesquioxane (POSS) hybrid nanocomposites: Structure and properties

The structure and properties of organic-inorganic hybrid nanocomposites prepared from a resole phenolic resin and a POSS mixture containing >95 wt% trisilanolphenyl POSS was investigated by POM (polarized optical microscopy), SEM, TEM, WAXD, FT-IR, DSC, and TGA techniques. Composites with 1.0-10.4 wt% of POSS were prepared by dissolving the POSS and the phenolic resin into THF, followed by solvent removal and curing. Both nano- and micro-sized POSS filler aggregates and particles were shown to be heterogeneously dispersed in the cured matrix by POM, TEM, SEM, and X-EDS. POSS was found everywhere, including in both dispersed phase domains and in the matrix. The nanocomposite morphology appears to form by a multi-step POSS aggregation during the process of phase separation. Both the matrix and dispersed ‘particulate’ phase domains are mixtures of phenolic resin and POSS. POSS micro-crystals act as the core of the dispersed phase. The bigger dispersed domains consist of smaller particles or aggregates of POSS molecules that exhibit some order but regions of matrix resin are interspersed. A WAXD peak at 2θ∼7.3° indicates crystalline order in the POSS aggregates. This characteristic peak's intensity increases with an increase in POSS loading, suggesting that more POSS molecules have aggregated or crystallized. FT-IR spectra confirm that hydrogen bonding exists between the phenolic resin and POSS Si-OH groups. This increases their mutual compatibility, but H-bonding does not prevent POSS aggregation and phase separation during curing. TGA measurements in air confirmed the temperature for 5% mass loss in increases with increase of POSS loading and at T>550° the thermal stability increases more sharply with POSS loading. The nanocomposite glass transition temperatures (T_g) are only slightly be affected by the POSS filler.     

Castor oil-based thermosets with varied crosslink densities prepared by ring-opening metathesis polymerization (ROMP)

Two castor oil-based monomers, (1) norbornenyl-functionalized castor oil (NCO), which has ∼0.8 norbornene rings per fatty acid chain and (2) norbornenyl-functionalized castor oil alcohol (NCA), which has ∼1.8 norbornene rings per fatty acid chain, have been prepared. Ring-opening metathesis polymerization (ROMP) of different ratios of NCO/NCA using the 2nd generation Grubbs catalyst results in rubbery to rigid biorenewable-based plastics with crosslink densities ranging from 318 to 6028 mol/m³. Increased crosslink densities result in shorter gelation times, better incorporation of the monomers into the polymer network, and much less soluble materials in the bulk materials. The increased crosslink densities obtained by adding NCA enhance the thermal properties, including the glass transition temperature (T_g) and room temperature storage modulus, which increase from −17.1 °C to 65.4 °C and from 2.4 MPa to 831.9 MPa, respectively. The TGA results, where T₁₀ increased from 285 °C to 385 °C, illustrate that improved thermal stabilities can be obtained for thermosets with higher crosslink densities. Young’s modulus (11-407 MPa), tensile strength (1.6-18 MPa) and toughness (0.14-1.6 MPa) are also improved dramatically with higher crosslink densities.     

Polysiloxanes containing polyhedral oligomeric silsesquioxane groups in the side chains; synthesis and properties

Linear polysiloxanes having different amounts of polyhedral oligomeric silsequioxane (POSS) side groups were prepared from the hydrosilylation reaction of poly(ethylhydrosiloxane) with different amount of POSS and 1-octene using platinum(0)-divinyl tetramethyldisiloxane as a catayst. 1-Octene and platinum(0)-divinyl tetramethyldisiloxane were found to be very important for the preparation of the linear polymer without any cross-linked structures. The linear polysiloxanes with POSS side groups are soluble in various organic solvents. When the content of POSS-containing monomeric unit is larger than 10 mol%, free standing films can be prepared from a routine solution casting method, although this polysiloxane is not cross-linked.     

The effect of sulfonic acid groups within a polyhedral oligomeric silsesquioxane containing cross-linked proton exchange membrane

In this study, polyhedral oligomeric silsesquioxane (POSS) moieties were incorporated into sulfonated poly(ether ether ketone) (SPEEK) to form a new cross-linked proton exchange membrane (PEM). The distribution of the POSS containing cross-linkers with or without sulfonic acid groups dictates the water behavior and connectivity of hydrophilic domains of the PEM. A PEM formed by incorporating 17.5 wt% of the cross-linker (containing POSS macromer and sulfonic acid groups) into SPEEK exhibits high proton conductivity (0.0153 S/cm), low methanol permeability (1.34 × 10⁻⁷ cm²/s), and high selectivity (1.14 × 10⁵ Ss/cm³).