Nanoparticles of Polybutadiene with Unconventional Microstructures and their Post-Polymerization Modification

Abstract

Butadiene is a readily available monomer. Due to the different modes of incorporation, 1,4 cis, 1,4 trans, and 1,2 (vinylic) in the polymer chain, polybutadienes cover a wide range of glass transition temperatures and crystallinities, and thus material properties. Also, the presence of a double bond in every repeat unit renders polybutadiene amenable to a variety of post polymerization reactions. These properties are of particular interest for polybutadienes in the form of aqueous dispersions. However, established free radical emulsion polymerization provides dispersions of polybutadiene with a given, invariable microstructure. Catalytic polymerizations are of interest to this end, as they can provide microstructure control. Careful selection of the catalyst is important, however, as water can decompose metal-alkyl complexes. Mini- or microemulsion polymerization techniques are necessary to employ with lipophilic catalysts in order to obtain polymer dispersions. Catalytic insertion polymerization in aqueous emulsion is limited mainly to apolar substrates because conjugated functionalized olefins substantially reduce the catalytic activity or even deactivate most catalysts. Another strategy for the synthesis of polar or functional groups containing nanoparticles with defined microstructure is post polymerization modification of preexisting polymer particles.This work presents a toolbox for the synthesis of polymer nanoparticles based on polybutadiene by catalytic polymerization in aqueous media and post polymerization reactions. An overview over accessible microstructures of the colloidally stable nanoparticles is given in Scheme 7 1.Cationic Ni(II) complexes of the type [(η3-C3H5)Ni(SbPh3)2][X], X¬ = BArF4- (ArF = 3,5 (F3C)2C6H3) or Al(OC(CF3)3)4-, were synthesized and studied in polymerizations in disperse aqueous systems. A subtle balance of the solubility of the catalyst precursor during the different stages of the polymerization is required in order to obtain colloidally stable polymer dispersions. For miniemulsion polymerization a predominant insolubility during the dispersing step is believed to prevent decomposition of the catalyst precursor and/or polymerization-active species, subsequent gradual complete dissolution by formation of the polymerization-active Ni-polymeryl species is thought to occur. Thus polybutadienes with number average molecular weights of typically 3×104 g mol-1 are obtained. By contrast to polymerization in the absence of water, or polymerization in aqueous suspension, in the highly disperse emulsion system incorporation of butadiene occurs in a 1,4-trans fashion predominantly. This is likely due to coordination of water as a ligand in the active species. Colloidally stable dispersions of semicrystalline polybutadiene particles of around 200 nm are obtained.The catalyst system [Co(C8H13)(C4H6)]/CS2 was known to give access to semicrystalline 1,2 polybutadiene nanoparticles when employed in aqueous microemulsion polymerization.23 Consecutive hydroformylation gives access to very small nanoparticles (< 20 nm) with adjustable carbonyl content in the form of aqueous dispersions. This represents a convenient route employing readily available starting materials (butadiene, CO and H2) to nanoparticles in this size range with a broadly adjustable and controllable polarity. The latter was demonstrated by fluorescence studies of the environment experienced by pyrene as a probe molecule.Copolymerization of butadiene and isoprene in microemulsion with [Co(C8H13)(C4H6)]/CS2 was shown to be an effective protocol to obtain nanoparticles (< 25 nm) of polydienes of adjustable crystallinity. The catalyst slightly prefers butadiene over isoprene. Butadiene is inserted almost exclusively in 1,2 fashion, and isoprene inserts accordingly in a vinyl (3,4) fashion, and probably in at least one other mode. The crystallinity of the polymer particles can be adjusted from semicrystalline with a high melting point of over 150 °C to amorphous with a Tg of -25 °C. A preliminary copolymerization of phenylbutadiene with butadiene resulted in a stable dispersion with 5 mol-% incorporated comonomer. Phenylbutadiene can be taken as a model substance for more complex compounds like dyes.A protocol was developed for the modification of 1,2-polybutadiene nanoparticles with polar groups via thiol-ene addition. Colloidally stable dispersions are obtained with the appropriate combinations of reagents. The largest portion of the vinyl groups of the starting material is available for substitution, as the amount of cyclic units formed is negligible despite the high concentration of vinyl groups given in particles. Employing comparatively less polar mercaptanes (esters or acids) complete conversion of the double bonds can be achieved, resulting in polar, functional group containing polymer nanoparticles. NMR studies provide detailed information of the polymer composition. With highly polar mercaptans in contrast, grafting of hydrophilic molecules to the surface of hydrophobic particles occurs. The resulting particles, stabilized by covalently bound mercaptan-based polar moieties (sulfate) bound to their surface, can be redispersed subsequent to complete drying. The approach pursued was also demonstrated for the tripeptide glutathione. Beyond the aspect of nanoparticle modification, the approach demonstrated allows for post-polymerization modification of syndiotactic 1,2-polybutadiene to otherwise inaccessible polymers. The crystallinity and low solubility of the starting material in organic solvents prohibits reactions in organic solutions. The findings reported underline that thiol-ene additions are a potentially useful method for polymer nanoparticle modification, also compatible with aqueous dispersions.