Synthesis and Dilute Solution Properties of Well-Defined H-Shaped Polybutadienes M. Shahinur Rahman, † Ravi Aggarwal, † Ronald G. Larson, ‡ John M. Dealy, § and Jimmy Mays* ,† Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37966; Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109; and Department of Chemical Engineering, McGill UniVersity, Montreal, Quebec H3A 2B2, Canada ReceiVed July 21, 2008; ReVised Manuscript ReceiVed September 14, 2008 ABSTRACT: A novel and advantageous approach to synthesis of H-shaped polybutadienes (H-PBd) is reported. The synthetic strategy employs classical anionic polymerization using high-vacuum techniques and utilizes a difunctional linking agent 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE). The synthesis involves (a) growing a living PBd chain using s-BuLi as initiator in benzene at room temperature, (b) titration of DCMSDPE with living PBdLi, (c) addition of s-BuLi to activate the double bond of DPE, (d) subsequent addition of butadiene to generate a living “ 1 / 2 H”, which has two arms and half of the final cross-bar, and (e) finally coupling the two “ 1 / 2 H” molecules with dichlorodimethylsilane to produce an H-PBd, which has two arms attached to each end of the cross-bar. The weight-average molecular weight, number-average molecular weight, molecular weight distribution, intrinsic viscosity, and radius of gyration were characterized by multidetector size exclusion chromatography (SEC) coupled with a refractive index detector, a two-angle (15° and 90°) light scattering detector, and a Viscotek differential viscometer in tetrahydrofuran at 40 °C. The H-PBds showed narrow and symmetrical molecular weight distributions (polydispersity indices, PDI ) 1.03-1.06). Furthermore, the use of light scattering detectors showed that there were no detectable high molecular weight, more highly branched components present in these materials. This is an important advantage of this novel approach over previous synthetic routes to H-polymers. The values of the branching parameters g (0.58-0.77) and g′ (0.60-0.75) in the thermodynamically good solvent, tetrahydrofuran, are consistent with values reported previously by Roovers and Toporowski for H polystyrenes in the good solvent toluene. Effects of architecture on the branching parameters are elucidated. Introduction The importance of branched polymers is rapidly growing due to their interesting solution, rheological, and mechanical proper- ties. 1 The effects of long chain branching on the properties of polymers and their industrial applications make these materials highly fascinating. 2 However, branched polymers as produced commercially are generally extremely complex mixtures ex- hibiting polydispersity in molecular weight, extent of branching, and branching architectures. Thus, macromolecules of precisely controlled architectures and narrow molecular weight distribu- tions (MWD) are necessary to understand the effects of branching on polymer properties. Much attention has thus been given to the synthesis of regular star, comb, graft, and dendritic polymers 3-8 in the past, but relatively few studies have focused on H-shaped polymers due to their difficult synthesis. H-shaped polymers have been synthesized by anionic po- lymerization using a chlorosilane coupling agent and following a three-step strategy: (a) the difunctional cross-bar was synthe- sized by anionic polymerization, (b) the cross-bars were end- functionalized by reaction with methyltrichlorosilane, and (c) the monofunctional arms were produced and then coupled with the reactive cross-bars. 7-10 However, this procedure can result in a significant amount of high molecular weight side products with two cross-bars and up to five arms due to the rapid coupling reaction of difunctional cross-bars with trichloromethylsilane. 10 The molecular weights of these byproducts are not too different from that of the H polymer, and therefore it is difficult to separate them by fractionation and difficult to detect them by conventional concentration sensitive size exclusion chromatog- raphy (SEC) detectors. Furthermore, the use of difunctional initiators, especially in nonpolar solvents, generates broader MWDs for the connectors as compared to chains produced using monofunctional initiators such as s-BuLi. 7 As a result, the disparities between the MWDs of the arms and cross-bars cannot be avoided. Coupling of two side arms first prior to reaction with the cross-bars appears to offer a superior approach to the more common strategy described above. Roovers et al. reported the first H-shaped polystyrene, where the two living arms (prepared by s-BuLi in benzene) were coupled with trichloromethylsilane and then added to the difunctional living cross-bars (prepared by using a difunctional initiator in THF/benzene 50/50 v/v). 11 This procedure can significantly reduce the amount of high molecular weight undesirable products. However, the use of a difunctional initiator and mixed solvent system limits the application of this strategy to the synthesis of polybutadienes with high 1,4-microstructure as well as structural homogeneity of the side arms and cross-bars. The use of a monofunctional initiator and a special linking agent 4-(chlorodimethylsilane) (CDMSS) or 4-chloromethylsi- lane provides another alternative approach for the synthesis of H-shaped polymers. A key aspect of this synthesis is the faster coupling of living anions with the Si-Cl bonds of CDMSS to produce a macromonomer with two side arms as opposed to a mixture of macromonomer and initiated living polymers. 12 In the next step, addition of living polymers to the CDMSS results in an off-center living polymer, which then can be coupled with a dichloromethylsilane to obtain the desired H-shaped polymers. However, during the coupling reaction the styrenic double bond also reacts to some degree, which results in a small percentage of graft or comb-shaped polymers with nearly the same or higher molecular weight than the H-shaped polymers, and they are difficult to eliminate by fractionation. * Corresponding author. E-mail: mays@ion.chem.utk.edu. † University of Tennessee. ‡ University of Michigan. § McGill University. 8225 Macromolecules 2008, 41, 8225-8230 10.1021/ma801646u CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008