Alternating Ring-Opening Metathesis Polymerization (AROMP) of Hydrophobic and Hydrophilic Monomers Provides Oligomers with Side-Chain Sequence Control

Abstract

We report the formation of oligomers with side-chain sequence control using ruthenium-catalyzed alternating ring-opening metathesis polymerization (AROMP). These oligomers are prepared through sequential, stoichiometric addition of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide (monomer A) at 85 °C and cyclohexene (monomer B) at 45 °C to generate sequences up to 24 monomeric units composed of (A-alt- B) _n and (A'-alt-B) _n microblocks, where n ranges from 1 to 6. Herein, monomer A has an alkyl side chain, and monomer A' has a glycine methyl ester side chain. Increasing microblock size from one to six results in an increasing water contact angle on spin-coated thin films, despite the constant ratio of hydrophilic and hydrophobic moieties. However, a disproportionately high contact angle was observed when n equals 2. Thus, the unique all-carbon backbone formed in the AROMP of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamides and cyclohexene provides a platform for the nontemplated preparation of materials with specific sequences of side chains.

Scheme 1. Oligomers of Controlled Sequences Are Prepared by Sequential Monomer Addition: (A) General Synthetic Approach To Prepare Oligomers with Controlled Side-Chain Sequence; (B) Cartoon Illustration of Oligomers Prepared.

Sequence-controlled oligomers are synthesized iteratively as outlined in (A). Control oligomers are synthesized by AROMP of the indicated mixture of monomers in a single cycle of polymerization.

Figure 1

Monomers 1a and 1b AROMP most efficiently. Monomer 1, monomer 2, and catalyst 3 were mixed in a 10:20:1 ratio in CDCl₃, [3] = 0.01 M. Percent conversion was determined by ¹H NMR spectroscopy and integration of shifted side-chain peaks. Each experiment was performed at least twice, and data from a representative experiment are shown.

Figure 2

Rapid (5 min) ring-opening metathesis of 1a requires high temperature. Monomer 1a (1 equiv) was treated with catalyst 3 (1 equiv) for 5 min in the indicated solvent at four different temperatures. The extent of monomer ring-opening metathesis was estimated based on the disappearance of the 1a·H⁺ ion at 194.1 in the ESI-MS spectrum. Each experiment was performed at least twice, and data from a representative experiment are shown.

Figure 3

Composition of oligomers is independent of number of monomer addition cycles. ¹H NMR spectra (CD₂Cl₂) of oligomers P1–P6 and Pr demonstrate the oligomers have the same bulk composition. (a) Stacked ¹H NMR spectra in the region of backbone olefin and terminal phenyl group. (b) Example integration of a oligomer spectrum. In P1, the ratio of 1a:1b:Ph is 6:6:5 and is consistent with the feed ratio of 1a and 1b used to synthesize P1.

Figure 4

Overlay of GPC traces of representative oligomers. The oligomers were analyzed by GPC on a 0–500K MW mixed bed column with THF as the eluent at a flow rate of 0.7 mL/min at 30 °C. P1, the synthesis of which requires the most steps, and Pr have identical bulk compositions. H1 and H2 only contain a single monomer type. Full traces for all oligomers can be found in the Supporting Information (Figure S1).

Figure 5

Surface hydrophobicity is dependent on microblock size, not bulk composition. Pure water droplet contact angles on oligomer thin films prepared by spin-coating were measured. Measurements were conducted with five different batches of each oligomer, and two measurements were taken for a single thin film for each synthetic batch of oligomer film. The error bars are the standard error of measurement based on a sample size of 10. Homocopolymers, H1: poly(1a-alt-2)₁₂; H2: poly(1b-alt-2)₁₂, are shown in red for reference. Random oligomer, Pr: poly[(1a-alt-2)-ran-(1b-alt-2)]₆, is shown in purple.