Dendritic Polymers (supported by a current NSF Career Award Grant, and Kraton Polymers)
The modification of macromolecular chain architectures leads to new materials with very useful properties. Dendritic polymers (i.e., hyperbranched polymers and dendrimers) are the most recent development in molecular architecture variation. The syntheses of these potentially useful materials have been difficult in the case of dendrimers or have resulted in imperfectly branched, polydisperse structures in the case of hyperbranched polymers. Current synthetic methods are also not generally applicable to the synthesis of highly branched structures of well-known polymers. Our work advances the idea that living polymerization techniques can be combined with a convergent method to synthesize hyperbranched polymers of low polydispersity and high degrees of branching through a greatly-simplified one-pot procedure. The synthesis allows for the formation of dendritic polystyrene, polybutadiene, poly(alkyl methacrylate)s, and other polymers while also allowing variations in end functionalization, copolymer structure, and molecular weight. Specifically, the synthesis is done by first initiating and polymerizing a low molecular weight chain through living methods; the living end is then reacted with a compound having both a polymerizable group and a moiety capable of quantitatively coupling with the growing chain end. Examples of compounds that fit this requirement for polymerization by alkyl lithium initiated vinyl polymerization are 4-vinylbenzyl chloride, 4-(chlorodimethylsilyl)styrene, and 4-vinylstyrene oxide (Figure 1)
Figure 2. Proposed coupling agents for convergent living polymerization
The slow addition of the dual functional reactant either alone or with a comonomer results in a convergence of chains and ultimately in a dendritic polymer with a living chain end at the core. This chain end can then be functionalized with a variety of groups or can be used to initiate an added comonomer. A representation of the first step of the synthesis is depicted in Scheme 3 using 4-(chlorodimethylsilyl)styrene as coupling reactant.
Scheme 3. Technique of convergent living polymerization.
The synthetic method allows for a large number of architectural variations resulting in many different dendritically branched as well as more complex branched polymers. Main aspects that are being studied include the progression of the reaction, the synthesis of different dendritic polymer backbones, the initial chain length and its effect on entanglement, the morphology of architectural copolymers, and the solution, rheological, and other physical properties of these materials and their blends with linear polymers. Characterization of these novel materials will improve the understanding of the relationship between branching structure and physical properties, since direct comparison with the linear version of the polymer can be accomplished. Because this method can easily produce considerable quantities of material, the utility of such dendritic polymers can be determined. Some of the characterization of rheological and mechanical properties has been initiated in collaboration with Dow Chemical, Kraton Polymers, and various academic collaborators.
Some of the types of structures that we have been able to produce by variations of this technique are depicted in Figure 3, and include star shaped polymers, dendritic polymers with controlled length of polymer between branch points, controlled multi-arm star on star structures, and star-linear-star triblock copolymers. We have some examples of varied polymer compositions to make copolymers that are useful as novel thermoplastic elastomers with unique polymer morphologies studied by AFM and electron microscopy and materials with different functional groups at the core of the structure.
Figure 3. Examples of some structures produced by convergent anionic polymerization (FG = functional group)
Our most recent efforts have been focused on investigating 4-vinylstyrene oxide as a coupling agent to produce dendritically branched polystyrenes with hydroxyl functional groups at the branch points (Figure 4). The 4-vinylstyrene oxide reacts in a similar fashion as that depicted in Scheme 3, except the epoxide is opened through anionic addition to form an unreactive lithium alkoxide which effectively terminates the chain. Upon completion of the reaction and acidification, hydroxyl groups are produced. The hydroxyl groups have then been used to initiate the polymerization of other monomers such as ethylene oxide and lactide to produce copolymer structures unlike any produced by other techniques. The morphologies are being studied by AFM. Other copolymers and varied structures can be produced taking advantage of this technique.
Figure 4. Hydroxyl functional dendritic polymer.
Aspects of this synthetic technique and the properties and utility of the novel materials need to be studied further and a NSF proposal to DMR –Polymers is being prepared to continue the work initiated in the CAREER grant.