Synthesis and Polymerization of Heterobifunctional Amphiphiles to Cross-Link Supramolecular Assemblies Sanchao Liu, Thomas M. Sisson, and David F. O’Brien* Department of Chemistry, C. S. Marvel Laboratories, University of Arizona, Tucson, Arizona 85721 Received June 13, 2000 ABSTRACT: The cross-linking of supramolecular assemblies of hydrated amphiphiles is an effective method to stabilize the assembly. A well-known strategy for cross-linking of lipids in lyotropic phases is the inclusion of identical reactive groups in each hydrophobic tail of the lipids. An alternative approach is to incorporate two similar but distinct groups into different locations within a single hydrophobic tail of the amphiphile. In principle, these heterobifunctional amphiphiles could react to form ladderlike polymers or cross-linked polymers. This report describes the synthesis, characterization, and polymer- ization in lipid vesicles of two series of heterobifunctional phosphatidylcholine (PC) lipids with different separation distances between the two reactive groups. One of the groups, i.e., dienoyl (Den), is attached to the secondary oxygen of the glycerol backbone of the lipid, and the other is either an acryloyl (Acryl) or sorbyl (Sorb) functional group located at the sn-2 chain terminus. Polymerization of both reactive groups in the Acryl/DenPC or the Sorb/DenPC was achieved by either redox polymerization or direct photoirradiation. The degree of polymerization depends on the initiation chemistry. Photoirradiation yields oligomers that are insufficient to cross-link the vesicles, whereas redox-initiated radical polymerization affords cross-linked polymeric vesicles. Under radical polymerization conditions a spacer length of seven or more atoms between the two reactive groups was long enough to ensure that each reactive group can follow an independent reaction path. Introduction The polymerization of supramolecular assemblies of hydrated amphiphiles is an effective way to modify the chemical and physical properties of the assembly. 1 The polymerization in a lipid assembly can proceed in a linear or cross-linked manner depending on the number of polymerizable groups in the monomeric lipid. It is known that polymerization of hydrated lipids with a single reactive moiety in either the hydrophobic tails or the lipid headgroup yields linear polymers, whereas polymerization of lipids with reactive groups in both tails yields cross-linked polymers. 2-6 The formation of cross-linked polymeric assemblies can be ascertained by the physical properties of the resulting assembly, in- cluding the chemical stability of the polymerized as- sembly toward added surfactant, general insolubility in organic solvents including hexafluoro-2-propanol (HFIP), and the rate of lateral diffusion of the lipids in the assembly. 7,8 Heterobifunctional lipid monomers have two reactive moieties with distinct reactivities, which could result in differences in the rate and degree of polymerization. Moreover, if the two reactive groups are located in the same tail of the lipid, then there is the additional possibility that the groups could form cross-linked or linear-ladder polymers. Cross-linking was observed in the polymerization of rodlike mesogens with acryloyl and cyano groups separated by 13 atoms. 9,10 On the other hand, linear-ladder polymers appear to be formed in the polymerization of hydrated bilayers of 1-palmi- toyl-2-(2,4,12,14-tetraenehexadecanoyl) phosphatidyl- choline, where the two diene groups were separated by only six methylenes. 11,12 Several factors could influence the preferred course of polymerizations in organized media, such as thermotropic and lyotropic liquid crys- tals. These include the relative reactivity of the mono- mers, the percent conversion of the monomers, the distance of separation between the reactive groups, and the flexibility of the spacer group that separates the monomers, among others. In this study, we have examined the nature of the polymerization of hydrated bilayers, at high conversion, of two types of heterobi- functional lipids, each bearing a pair of reactive groups in the sn-2 tail that are separated by either 7, 9, or 11 atoms. The properties of the resulting polymers were determined by polymer solubility and bilayer vesicle lysis techniques reported previously. Results and Discussion The key considerations in the design of a heterobi- functional lipid is the choice and location of the poly- merizable groups in the lipid molecule. Reactive lipids may have polymerizable moieties located in the hydro- philic headgroup, near the lipid backbone, or in the hydrophobic lipid tail. The heterobifunctional lipids described here contain two polymerizable groups in the sn-2 acyl chain of the lipid. One of the groups, i.e., dienoyl, is attached to the secondary oxygen of the glycerol backbone of the lipid, and the other is an acryloyl or sorbyl functional group located at the sn-2 chain terminus. These lipids can be polymerized in a bilayer by either radical initiation or direct photoacti- vation. Since the two reactive groups are in regions of different polarity, selective polymerization of one reac- tive group in the presence of a second is possible if the initiation chemistry is only effective in regions of low polarity or in regions of moderate to high polarity. The dienoyl (Den) group located near the lipid backbone can be polymerized by polar initiators, whereas the acryloyl (Acryl) or sorbyl (Sorb) groups located in the interior of the bilayer can be selectively polymerized by hydropho- bic initiators. 12 The reactive groups were selected to yield one series of monomeric lipids with groups having similar reactivity, i.e., Sorb/Den, and a second series where the reactivity of the groups differ significantly, 465 Macromolecules 2001, 34, 465-473 10.1021/ma001029q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/21/2000