Using Cyclodextrin To Control The Branching Structure Of Polymerized Products

The structure and properties of polymers is one of the main topics in polymer science. It has been widely accepted that the performance of polymer mostly lies on its structure. However, how to efficiently design and control the molecular structure of polymerized products is still a challenge. Here, we have developed a supramolecular method to control the architecture of the polymer. By introduction of supramolecular host cyclodextrins (CDs) into multifunctional reaction system, CDs will form inclusion complexes with the appropriate monomers to decrease the degree of function of them. The branching structure of the polymerized samples can be easily controlled by merely adjusting the amount of CDs.1. Influence of molecular recognition on polymerizationAfter complexation with CDs, the degree of function of the guest monomers decreases. In the first part of our work, we discussed the influence of molecular recognition on polymerization by adding different kinds of CDs into a typical hyperbranched reaction system. It has been found that molecular recognition plays an important role. The complexation between CDs and guest 1-(2-aminoethyl)piperazine (AP) is a dynamic process. Among three CDs, the stability ofβ-CD/AP complex is the highest. Therefore, the control effect ofβ-CD is the best and the linear polymer can be obtained in the presence of a large amount ofβ-CDs.γ-CD, whose cavity is much larger than AP, can form loose complex with AP. So the influence of CD amount on the structure of the final product is low and the polymerized samples obtained in the presence of a large amount ofγ-CD are still hyperbranched. Because of its small cavity,α-CD is hard to form stable complex with AP. However, when adding a large amount ofα-CDs, the hydrogen bonds and the synergical effects in the system make great contribution to stablize the complex, which results in the formation of polymerized products with low degree of branching.2. Using CDs to control the cross-linking systemMultifunctional monomers are widely used to synthesize hyperbranched polymer, so the polymerization condition has to be controlled carefully to avoid cross-linking. In this part, we extend the supramolecular control method to A2+B4 cross-linking reaction system. Depending on the feed ratio, the polycondensation-addition of divinylsulfone (DV, an A2 monomer) and hexamethylenediamine (HDA, a B4 monomer) gives highly branched polymer (DV:HDA=1:1) or chemical cross-linking gel (DV:HDA=2:1). Introducingβ-cyclodextrin (β-CD) into this reaction system, HDA molecule selectively encapsulate into the cavity ofβ-CD. Interestingly, one hydrogen atom of each primary amino group in HDA molecule is physically protected by CD cavity, so the dendritic unit (HDA molecule) is transformed into linear unit through the inclusion complexation. Therefore, by merely adjusting the amount ofβ-CD, cross-linking gel, hyperbranched polymer, highly branched polymer, slightly branched polymer or linear polymer can be obtained respectively. In short, the branched topology of polymerized product from A2+B4 reaction system can be controlled easily by using this supramolecular approach. Besides, due to the molecular recognition of CDs, the control effect of three natural CDs is also different:β-CD is the best,α-CD is the next andγ-CD is the last.3. Using CDs to control the free radical polymerization of divinyl monomer Free radical polymerization is the most popular polymerization method in industry. Here, we extend successfully the supramolecular control method to the free radical polymerization. The divinyl monomer ethylene glycol dimethacrylate is reacted withβ-CD to form a 1:1 inclusion complex. The experimental results reveal that one methacrylate unit of the guest molecule loses its reactivity due to the encapsulation ofβ-CD cavity, and the other methacrylate unit outsideβ-CD cavity can be reacted. By adjusting the amount ofβ-CD, a series of hyperbranched polymers with different degree of branching are obtained uing the traditional free radical polymerzition.