pubs.acs.org/Macromolecules Published on Web 03/30/2010 r 2010 American Chemical Society 3672 Macromolecules 2010, 43, 3672–3681 DOI: 10.1021/ma1004056 Synthesis of 1,4-Polybutadiene Dendrimer-Arborescent Polymer Hybrids Mario Gauthier* and Abdul Munam Institute for Polymer Research, Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada Received February 19, 2010; Revised Manuscript Received March 18, 2010 ABSTRACT: A divergent synthetic scheme was developed for the preparation of high branching function- ality hybrid polymers from carbosilane dendrimer substrates and polybutadiene side chains. Carbosilane dendrimers with 32, 64, or 128 peripheral Si-Cl functional groups were first coupled with 1,2-polybutadie- nyllithium chains having a number-average molecular weight M n ≈ 1000. The polybutadiene-grafted substrates were then hydrosilylated with dichloromethylsilane and reacted with high 1,4-microstructure content polybutadienyllithium chains to generate high branching functionality dendrimer-arborescent hybrids. Three series of hybrid polymers were synthesized containing 1,4-polybutadiene side chains with M n ≈ 1500, 5000, or 30 000. Size exclusion chromatography analysis of the polymers confirmed that a narrow molecular weight distribution was maintained (M w /M n e 1.14). The branching functionality of the arborescent hybrids varied from 140-335, 160-1110, and 360-2830 for the 32-, 64-, and 128-site coupling precursors, respectively. The experimental branching functionalities attained were lower than the theoretical values due to decreased coupling efficiency within each series, in particular for polymers with longer polybutadiene side chains, apparently due to steric limitations in the grafting reaction. Introduction Branched polymers are of interest, among others, because of their peculiar physical properties 1-3 and their potential useful- ness as rheological modifiers for other polymers. 4 Star-branched and arborescent (dendrigraft) polymers are two families of bran- ched polymers of particular importance because their controlla- ble architecture enables fundamental investigations of structure- property relations. The physical properties of these materials can be fine-tuned through variations in parameters such as their side-chain molecular weight and composition, branching func- tionality, the presence of functional end groups, etc. The mole- cular weight distribution (MWD) of star-branched and arbore- scent polymers is often very narrow because their synthesis typically relies on living polymerization techniques. As a result, these polymers are ideal to elucidate the influence of structural para- meters on the molecular (polymeric) and intermolecular (colloidal) properties of branched polymers. Different methods have been developed for the synthesis of star-branched polybutadiene, but the coupling reaction of living polybutadienyllithium chains with chlorosilane substrates is clearly most successful. 5 This technique was extended to the synthesis of 32-, 64-, and 128-arm regular star polybutadiene from carbo- silane dendrimers with chlorosilane functionalities as coupling agents. 6,7 The dendrimer path is versatile, but grafting on these substrates eventually restricts further growth of the molecules due to steric limitations. Grafting of 1,4-polybutadiene chains onto linear and 18-arm star-branched 1,2-polybutadiene substrates hydrosilylated with dichloromethylsilane has also been explored for the synthesis of high branching functionality stars, 8 leading to branching functionalities of up to 270. An alternate core-first strategy using dendrimeric initiators carrying hydroxyl end groups was reported for the synthesis of 4-, 8-, and 16-arm star polymers with poly(ethylene oxide) arms. 9 The synthesis of dendrimer- linear polymer hybrids was also achieved with lithiated carbosi- lane substrates to initiate the anionic polymerization of styrene, ethylene oxide, or hexamethylcyclotrisiloxane. 10 Unfortunately, the core-first methods are only practical if rapid exchange exists between the active and dormant anionic propagating centers, and characterization of the side chains cannot be carried out unless they are linked to the core through selectively cleavable bonds. The synthesis of arborescent polymers (a subclass of dendri- graft polymers) relies on a generation-based scheme to obtain a dendritic (multilevel) graft polymer architecture. 11 A linear poly- mer substrate is modified to introduce coupling sites serving in a grafting reaction for linear chains. The comb polymer (also called the generation zero or G0 arborescent polymer) thus obtained is then subjected to additional cycles of functionalization and grafting reactions to yield higher generation (G1, G2, ...) arbore- scent polymers, with a branching functionality and molecular weight increasing geometrically for each cycle. The random distri- bution of coupling sites on the substrates makes these molecules less sensitive to incomplete reactions because all the molecules are affected to the same extent when a large number of coupling sites are present. Consequently, a narrow MWD (M w /M n ∼ 1.1) is achieved even for very high branching functionalities. A strategy combining carbosilane dendrimer substrates and arborescent polymer chemistry is now presented for the synthesis of high branching functionality dendrimer-arborescent polybu- tadiene hybrids, characterized by a much denser core structure than analogous arborescent systems derived from linear polymer substrates. 12 Carbosilane dendrimer substrates bearing 32, 64, or 128 peripheral Si-Cl functional groups are first coupled with short 1,2-polybutadienyllithium segments and hydrosilylated with dichloromethylsilane. The polyfunctional substrates are then reacted with polybutadienyllithium chains of different number-average molecular weights (M n = 1500, 5000, and 30 000) with a high 1,4-microstructure content. This approach yields polymers with up to 2830 side chains, much higher than for the star-branched polymer structures reported previously. *Corresponding author: e-mail gauthier@uwaterloo.ca; Ph þ1-519- 888-4567; Fax þ1-519-746-0435.