Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions

Abstract

Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP). One-pot copolymerization of an ω-norbornenyl macromonomer and a discrete norbornenyl co-monomer (diluent) provides opportunities to control the backbone sequence and therefore the side chain distribution. Toward sequence control, the homopolymerization kinetics of 23 diluents were studied, representing diverse variations in the stereochemistry, anchor groups, and substituents. These modifications tuned the homopolymerization rate constants over two orders of magnitude (0.36 M −1 s −1 < khomo < 82 M −1 s −1 ). Rate trends were identified and elucidated by complementary mechanistic and density functional theory (DFT) studies. Building on this foundation, complex architectures were achieved through copolymerizations of selected diluents with a poly(D,L-lactide) (PLA), polydimethylsiloxane (PDMS), or polystyrene (PS) macromonomer. The cross-propagation rate constants were obtained by non-linear least squares fitting of the instantaneous co-monomer concentrations according to the Mayo-Lewis terminal model. Indepth kinetic analyses indicate a wide range of accessible macromonomer/diluent reactivity ratios (0.08 < r1/r2 < 20), corresponding to blocky, gradient, or random backbone sequences. We further demonstrated the versatility of this copolymerization approach by synthesizing AB graft diblock polymers with tapered, uniform, and inverse-tapered molecular "shapes." Small-angle X-ray scattering analysis of the self-assembled structures illustrates effects of the graft distribution on the domain spacing and backbone conformation. Collectively, the insights provided herein into the ROMP mechanism, monomer design, and homo-and copolymerization rate trends offer a general strategy for the design and synthesis of graft polymers with arbitrary architectures. Controlled copolymerization therefore expands the parameter space for molecular and materials design.

Effect of side chain and backbone length on lamellar spacing in polystyrene‐block‐poly(dimethyl siloxane) brush block copolymers

“…Lattice spacings on the order of d 0 = 100 nm are consistently reported in the literature . Several intricate designs of BBCPs are reported with variable graft density, block composition, and side chain chemistries . BBCPs have found practical use as templates for the fabrication of functional materials such as coatings, photonic crystals, scaffolds for well‐ordered nanoparticle arrays, metallo‐dielectric composites, and novel elastomeric materials …”

“…The high grafting density introduces new physical considerations that strongly influence the phase behavior, specifically resulting from the significant excluded volume and steric repulsions of the side chains . Several studies have systematically investigated self‐assembly in A‐B di‐block BBCP systems of both symmetric and asymmetric block composition (f) in the bulk and thin film regimes .…”

Comb and Bottlebrush Polymers with Superior Rheological and Mechanical Properties

“…Synthesis of welldefined densely grafted bottlebrushes using ROMP and grafting-through method guarantees the presence of one side chain per repeating unit on the backbone. [15] It should be mentioned that the bottlebrushes synthesized with ROMP technique have different repeating units in the backbone than the side chains which might cause microphase separation. [13] These bottlebrushes are similar to that of the dendronized polymer with only one layer of grafting.…”

“…Bottlebrushes with more than one branch per backbone repeating unit can also be synthesized using ROMP and macromonomers with more bulky groups referred as wedge polymers. [15] It should be mentioned that the bottlebrushes synthesized with ROMP technique have different repeating units in the backbone than the side chains which might cause microphase separation. Loosely grafted bottlebrushes with longer distance between the branching points can be achieved using ROMP technique through copolymerization of macromonomer and a diluent molecule, e.g., racemic endo,exo-norbornenyl diesters, with different ratios.…”

Synthesis of Well‐Defined Poly(norbornene) Containing Carbon Nanodots by Controlled ROMP

“…The ROMP of norbornene (NB) derivatives has become a powerful tool to synthesize complex macromolecules including block copolymers, graft‐polymers, and dendronized polymers . Despite the versatility of the ROMP, its synthetic application for the production of polymers with PAH moieties has been rarely investigated.…”