For developing high performance graphene-based nanocomposites, dispersal of graphene nanosheets in polymer hosts and precise interface control are challenging due to their strong interlayer cohesive energy and surface inertia. Here we report an efficient method to functionalize graphene nanosheets. The initiator molecules were covalently bonded to the graphene surface via a diazonium addition and the succeeding atom transfer radical polymerization linked polystyrene chains (82 wt% grafting efficiency) to the graphene nanosheets. The prominent confinement effect arising from nanosheets resulted in a 15 °C increase in the glass transition temperature of polystyrene compared to the pure polymer. The resulting polystyrene nanocomposites with 0.9 wt% graphene nanosheets revealed around 70% and 57% increases in tensile strength and Young's modulus. The protocol is believed to offer possibilities for optimizing the processing properties and interface structure of graphene-polymer nanocomposites.

Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites

2.1.6. Tacticity and Sequence: Advanced Control 4967 2.2. Transition Metal Catalysts 4967 2.2.1. Overviews of Catalysts 4967 2.2.2. Ruthenium 4967 2.2.3. Copper 4971 2.2.4. Iron 4971 2.2.5. Nickel 4975 2.2.6. Molybdenum 4975 2.2.7. Manganese 4976 2.2.8. Osmium 4976 2.2.9. Cobalt 4976 2.2.10. Other Metals 4976 2.3. Cocatalysts (Additives) 4977 2.3.1. Overview of Cocatalysts 4977 2.3.2. Reducing Agents 4977 2.3.3. Free Radical Initiators 4977 2.3.4. Metal Alkoxides 4977 2.3.5. Amines 4978 2.3.6. Halogen Source 4978 2.4. Initiators 4978 2.4.1. Overview of Initiators: Scope and Design 4978 2.4.2. Alkyl Halides 4978 2.4.3. Arenesulfonyl Halides 4979 2.4.4. N-Chloro Compounds 4979 2.4.5. Halogen-Free Initiators 4979 2.5. Solvents 4980 2.5.1. Overview of Solvents 4980 2.5.2. Catalyst Solubility and Coordination of Solvent 4981 2.5.3. Environmentally Friendly Solvents 4981 2.5.4. Water 4981 2.5.5. Catalytic Solvents: Catalyst Disproportionation 4981

Transition metal-catalyzed living radical polymerization : toward perfection in catalysis and precision polymer synthesis

Of significant interest is the development of synthetic methodologies to prepare complexmacromolecular structures that contain functionalities, allowing for stimuli-responsivecharacteristics and further chemical modifications.1–6 The cylindrical molecular brush,which is composed of many side chain polymers distributed densely along a backbone, hasattracted much attention because its chemical composition, size and morphology can becontrolled precisely by choosing appropriate monomers and tuning the lengths of thebackbone and side chains.7–9 To prepare such structures, three synthetic strategies are oftenemployed: “grafting onto”8,9 (grafting side chains onto a pre-established multifunctionalbackbone by coupling reactions), “grafting from”10–13 (growth of side chains from amultifunctionalized initiating backbone) and “grafting through”14–18 (polymerization ofpreviously synthesized side chains through their terminal groups). Previously, our laboratoryhas demonstrated the compatibility of the three primary living radical polymerizations(nitroxide mediated radical polymerization (NMRP), atom transfer radical polymerization(ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization)together with ring-opening metathesis polymerization (ROMP) in the “grafting from”method.12,19,20 Our interest has focused recently on the “grafting through” method,because it provides exceptional control over the grafting density, the length of the backbone,and the length of the side chains, each independently.Although the “grafting through” approach offers versatility in the construction of complexmacromolecular systems, it is likely to encounter steric hindrance during the polymerizationof high molecular weight or sterically-bulky macromonomers.21,22 To overcome this issue,ROMP has often been used, which is driven by the release of enthalpy from cyclic monomerstructures and affords a polymer backbone with a relatively loose grafting density. Forexample, the syntheses of molecular brushes have been reported using ROMP fromnorbornene-terminated poly(ethylene oxide),16 polystyrene,15 polyphosphazene,14 poly(e-caprolactone),18 and polylactide,17 macromonomers synthesized mainly from anionic

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Facile syntheses of cylindrical molecular brushes by a sequential RAFT and ROMP “grafting‐through” methodology

Bottlebrush polymers: From controlled synthesis, self-assembly, properties to applications

A combination of reversible addition fragmentation chain transfer (RAFT) polymerization and hetero Diels-Alder (HDA) chemistry has been utilized to successfully generate functional core-shell microspheres. Initially, precipitation polymerization in conjunction with the RAFT technique has been employed to synthesize divinylbenzene (DVB) microspheres with surface expressed RAFT groups. Subsequently, HDA cycloaddition has been performed under mild reaction conditions (50 degrees C, 24 h) with a diene-functionalized poly(epsilon-caprolactone) (PCL). While the successful grafting is immediately evident by optical inspection of the microspheres (color change from purple to white), X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance spectroscopy (ATR) were additionally employed to characterize the chemical composition and surface functionalization of the microspheres. Further, confocal microscopy was used to con-firm the presence of grafted PCL chains after labeling them with rhodamine B.

Efficient Surface Modification of Divinylbenzene Microspheres via a Combination of RAFT and Hetero Diels-Alder Chemistry

Well defined graft copolymers are prepared by "grafting from" atom transfer radical polymerization (ATRP) at room temperature (30 °C). The experiments were aimed at grafting methacrylates and styrene at latent initiating sites of polystyrene. For this purpose, the benzylic hydrogen in polystyrene was subjected to allylic bromination with N-bromosuccinimide and azobisisobutrylnitirle to generate tertiary bromide ATRP initiating sites (Br-C-PS). The use of Br-C-PS with lesser mol % of bromide initiating groups results in better control and successful graft copolymerization. This was used to synthesize a series of new graft copolymers such as PS-g-PBnMA, PS-g-PBMA, PS-g-GMA, and PS-g-(PMMA-b-PtBA) catalyzed by CuBr/ PMDETA system, in bulk, at room temperature. The polymers are characterized by GPC, NMR, FTIR, TEM, and TGA. Graft copolymerization followed by block polymerization enabled the synthesis of highly branched polymer brush, in which the grafting density can be adjusted by appropriate choice of bromide concentration in the polystyrene.

A. V. Vivek

Papers

Grafting of methacrylates and styrene on to polystyrene backbone via a “grafting from” ATRP process at ambient temperature