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Papers by Ramesh Subramanian

Research paper thumbnail of Higher-molecular-weight hyperbranched polyethylenes containing crosslinking structures as lubricant viscosity-index improvers

Polymer Engineering and Science, 2009

As a new grade of polyethylene materials with unique chain architectures, hyperbranched polyethyl... more As a new grade of polyethylene materials with unique chain architectures, hyperbranched polyethylenes synthesized by chain walking ethylene polymerization have great potential for industrial application as novel viscosity index (VI) improver in lubricant formulation. Although high-molecular-weight hyperbranched polyethylenes (weight-average molecular weight of about 105 g/mol) possess high shear stability, their viscosity thickening properties are compromised due to their compact chain architectures. In this work, we aim at improving their viscosity thickening property by increasing polymer molecular weight. A range of hyperbranched polymers of various enhanced molecular weights were synthesized by chain walking ethylene polymerization in the presence of small amounts of 1,4-butanediol diacrylate as a difunctional crosslinker. The molecular weight dependences of viscosity thickening power and shear stability of these polymers containing crosslinking structures were evaluated. It is found that, with the increase of molecular weight via crosslinking, these polymers showed consistently enhanced viscosity thickening power, but with the reduced shear stability. However, their shear stability was still significantly better compared to linear polymers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Research paper thumbnail of Chain walking copolymerization of ethylene with cyclopentene – Effect of ring incorporation on polymer chain topology

Polymer, 2010

Chain walking ethylene copolymerizations with cyclopentene (CPE) as the ring-forming comonomer we... more Chain walking ethylene copolymerizations with cyclopentene (CPE) as the ring-forming comonomer were carried out in this study to investigate the tuning of polyethylene chain topology via the unique strategy of ring incorporation. Four sets of polymers containing five-membered rings on the polymer backbone at various low contents (in the range of 0–7.5 mol%) were synthesized by controlling CPE feed concentration at four different ethylene pressure/temperature combinations (1 atm/15 °C, 1 atm/25 °C, 1 atm/35 °C, and 6 atm/25 °C, respectively) using a Pd-diimine catalyst, [(ArN

Research paper thumbnail of Chain-topology-controlled hyperbranched polyethylene as effective polymer processing aid (PPA) for extrusion of a metallocene linear-low-density polyethylene (mLLDPE)

Journal of Rheology, 2008

... Jianli Wang, Marianna Kontopoulou, Zhibin Ye, Ramesh Subramanian, Shiping Zhu. ... noted that... more ... Jianli Wang, Marianna Kontopoulou, Zhibin Ye, Ramesh Subramanian, Shiping Zhu. ... noted that the NMR technique is ineffective in differentiating the chain topologies of the polymers due to the fact that this technique only determines the number of groups, the branch end groups ...

Research paper thumbnail of Long-Chain Branching and Rheological Properties of Ethylene1Hexene Copolymers Synthesized from Ethylene Stock by Concurrent Tandem Catalysis

Macromolecular Chemistry and Physics, 2005

Research paper thumbnail of One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with Pendant Acryloyl Functionalities by Chain Walking Copolymerizations

Macromolecular Chemistry and Physics, 2008

Research paper thumbnail of Synthesis of Block Copolymers of Ethylene with Styrene andn-Butyl Acrylate via a Tandem Strategy Combining Ethylene ``Living'' Polymerization Catalyzed by a Functionalized Pd-Diimine Catalyst with Atom Transfer Radical Polymerization

Macromolecules, 2008

We report a new two-step tandem strategy combining two versatile “living” polymerization techniqu... more We report a new two-step tandem strategy combining two versatile “living” polymerization techniques, Pd−diimine catalyzed ethylene “living” polymerization and atom transfer radical polymerization (ATRP), for the synthesis of functionalized polyethylene (PE) diblock copolymers ...

Research paper thumbnail of A Trinuclear Pd-Diimine Catalyst for ``Core-First'' Synthesis of Three-Arm Star Polyethylenes via Ethylene ``Living'' Polymerization

Research paper thumbnail of Tuning structural parameters of polyethylene brushes on silica nanoparticles in surface-initiated ethylene “living” polymerization and effects on silica dispersion in a polyolefin matrix

Polymer Chemistry, 2013

ABSTRACT Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine c... more ABSTRACT Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine catalysts represents a novel technique for covalent surface functionalization of silica nanoparticles with polyethylene (PE) brushes. In this paper, we report on the successful tuning of various structural parameters of PE brushes in this surface-initiated polymerization technique, including brush length, density, and topology. To control/reduce the brush density, the density of the surface-tethered acryloyl groups for catalyst immobilization is adjusted by using mixed silane agents comprised of effective 3-acryloxypropyltrichlorosilane and inert ethyltrichlorosilane at different compositions in the surface functionalization step, which in turn adjusts the density of immobilized catalysts for rendering PE brushes. This approach gives rise to low-polydispersity PE brushes of controllable densities at the polymerization condition of 27 atm/5 °C: 0.022–0.055 chains per nm2 on a precipitated silica (Silica-I) and 0.07–0.17 chains per nm2 on a fumed silica (Silica-II). The length of PE brushes is controlled by adjusting the polymerization time, with the highest brush length of about 45 kg mol−1 achieved at 6 h of polymerization at 27 atm/5 °C. Unlike the linear brushes with short branch structures obtained at 27 atm/5 °C, hyperbranched PE brushes with compact topology are obtained at 1 atm/25 °C, benefiting from the chain walking mechanism of the Pd–diimine catalyst. The PE-grafted silicas of varying brush density and length are subsequently used as nanofillers to construct polymer nanocomposites with an ethylene–α-olefin copolymer as the matrix polymer. The effects of brush length and density on the nanofiller dispersion and physical properties of the composites are examined. This represents the first study on polyolefin composites containing silica nanoparticles grafted with polyolefin brushes as nanofillers.

Research paper thumbnail of Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd–diimine catalysts

Research paper thumbnail of Chain Walking Ethylene Copolymerization with an ATRP Inimer for One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with ATRP Initiating Sites

Psychology & Marketing, 2007

Research paper thumbnail of Higher-molecular-weight hyperbranched polyethylenes containing crosslinking structures as lubricant viscosity-index improvers

Polymer Engineering and Science, 2009

As a new grade of polyethylene materials with unique chain architectures, hyperbranched polyethyl... more As a new grade of polyethylene materials with unique chain architectures, hyperbranched polyethylenes synthesized by chain walking ethylene polymerization have great potential for industrial application as novel viscosity index (VI) improver in lubricant formulation. Although high-molecular-weight hyperbranched polyethylenes (weight-average molecular weight of about 105 g/mol) possess high shear stability, their viscosity thickening properties are compromised due to their compact chain architectures. In this work, we aim at improving their viscosity thickening property by increasing polymer molecular weight. A range of hyperbranched polymers of various enhanced molecular weights were synthesized by chain walking ethylene polymerization in the presence of small amounts of 1,4-butanediol diacrylate as a difunctional crosslinker. The molecular weight dependences of viscosity thickening power and shear stability of these polymers containing crosslinking structures were evaluated. It is found that, with the increase of molecular weight via crosslinking, these polymers showed consistently enhanced viscosity thickening power, but with the reduced shear stability. However, their shear stability was still significantly better compared to linear polymers. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Research paper thumbnail of Chain walking copolymerization of ethylene with cyclopentene – Effect of ring incorporation on polymer chain topology

Polymer, 2010

Chain walking ethylene copolymerizations with cyclopentene (CPE) as the ring-forming comonomer we... more Chain walking ethylene copolymerizations with cyclopentene (CPE) as the ring-forming comonomer were carried out in this study to investigate the tuning of polyethylene chain topology via the unique strategy of ring incorporation. Four sets of polymers containing five-membered rings on the polymer backbone at various low contents (in the range of 0–7.5 mol%) were synthesized by controlling CPE feed concentration at four different ethylene pressure/temperature combinations (1 atm/15 °C, 1 atm/25 °C, 1 atm/35 °C, and 6 atm/25 °C, respectively) using a Pd-diimine catalyst, [(ArN

Research paper thumbnail of Chain-topology-controlled hyperbranched polyethylene as effective polymer processing aid (PPA) for extrusion of a metallocene linear-low-density polyethylene (mLLDPE)

Journal of Rheology, 2008

... Jianli Wang, Marianna Kontopoulou, Zhibin Ye, Ramesh Subramanian, Shiping Zhu. ... noted that... more ... Jianli Wang, Marianna Kontopoulou, Zhibin Ye, Ramesh Subramanian, Shiping Zhu. ... noted that the NMR technique is ineffective in differentiating the chain topologies of the polymers due to the fact that this technique only determines the number of groups, the branch end groups ...

Research paper thumbnail of Long-Chain Branching and Rheological Properties of Ethylene1Hexene Copolymers Synthesized from Ethylene Stock by Concurrent Tandem Catalysis

Macromolecular Chemistry and Physics, 2005

Research paper thumbnail of One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with Pendant Acryloyl Functionalities by Chain Walking Copolymerizations

Macromolecular Chemistry and Physics, 2008

Research paper thumbnail of Synthesis of Block Copolymers of Ethylene with Styrene andn-Butyl Acrylate via a Tandem Strategy Combining Ethylene ``Living'' Polymerization Catalyzed by a Functionalized Pd-Diimine Catalyst with Atom Transfer Radical Polymerization

Macromolecules, 2008

We report a new two-step tandem strategy combining two versatile “living” polymerization techniqu... more We report a new two-step tandem strategy combining two versatile “living” polymerization techniques, Pd−diimine catalyzed ethylene “living” polymerization and atom transfer radical polymerization (ATRP), for the synthesis of functionalized polyethylene (PE) diblock copolymers ...

Research paper thumbnail of A Trinuclear Pd-Diimine Catalyst for ``Core-First'' Synthesis of Three-Arm Star Polyethylenes via Ethylene ``Living'' Polymerization

Research paper thumbnail of Tuning structural parameters of polyethylene brushes on silica nanoparticles in surface-initiated ethylene “living” polymerization and effects on silica dispersion in a polyolefin matrix

Polymer Chemistry, 2013

ABSTRACT Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine c... more ABSTRACT Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine catalysts represents a novel technique for covalent surface functionalization of silica nanoparticles with polyethylene (PE) brushes. In this paper, we report on the successful tuning of various structural parameters of PE brushes in this surface-initiated polymerization technique, including brush length, density, and topology. To control/reduce the brush density, the density of the surface-tethered acryloyl groups for catalyst immobilization is adjusted by using mixed silane agents comprised of effective 3-acryloxypropyltrichlorosilane and inert ethyltrichlorosilane at different compositions in the surface functionalization step, which in turn adjusts the density of immobilized catalysts for rendering PE brushes. This approach gives rise to low-polydispersity PE brushes of controllable densities at the polymerization condition of 27 atm/5 °C: 0.022–0.055 chains per nm2 on a precipitated silica (Silica-I) and 0.07–0.17 chains per nm2 on a fumed silica (Silica-II). The length of PE brushes is controlled by adjusting the polymerization time, with the highest brush length of about 45 kg mol−1 achieved at 6 h of polymerization at 27 atm/5 °C. Unlike the linear brushes with short branch structures obtained at 27 atm/5 °C, hyperbranched PE brushes with compact topology are obtained at 1 atm/25 °C, benefiting from the chain walking mechanism of the Pd–diimine catalyst. The PE-grafted silicas of varying brush density and length are subsequently used as nanofillers to construct polymer nanocomposites with an ethylene–α-olefin copolymer as the matrix polymer. The effects of brush length and density on the nanofiller dispersion and physical properties of the composites are examined. This represents the first study on polyolefin composites containing silica nanoparticles grafted with polyolefin brushes as nanofillers.

Research paper thumbnail of Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd–diimine catalysts

Research paper thumbnail of Chain Walking Ethylene Copolymerization with an ATRP Inimer for One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with ATRP Initiating Sites

Psychology & Marketing, 2007