Efficient Synthesis of Narrowly Dispersed Brush Copolymers and Study of Their Assemblies: The Importance of Side Chain Arrangement

Abstract

Efficient, one-pot preparation of synthetically challenging, high molecular weight (MW), narrowly dispersed brush block copolymers and random copolymers in high conversions was achieved by ring-opening metathesis (co)polymerization (ROMP) of various macromonomers (MMs) using the highly active, fast-initiating ruthenium olefin metathesis catalyst (H2IMes)(pyr)2(Cl)2RuCHPh. A series of random and block copolymers were prepared from a pair of MMs containing polylactide (PLA) and poly(n-butyl acrylate) (PnBA) side chains at similar MWs. Their self-assembly in the melt state was studied by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). In brush random copolymers containing approximately equal volume fractions of PLA and PnBA, the side chains segregate into lamellae with domain spacing of 14 nm as measured by SAXS, which was in good agreement with the lamellar thickness measured by AFM. The domain spacings and order−disorder transition temperatures of brush random copolymers were insensitive to the backbone length. In contrast, brush block copolymers containing approximately equal volume fractions of these MMs self-assembled into highly ordered lamellae with domain spacing over 100 nm. Their assemblies suggested that the brush block copolymer backbone adopted an extended conformation in the ordered state.

Structure, function, self-assembly, and applications of bottlebrush copolymers

“…All samples prepared were self-assembled in the melt to form 14.3 nm lamellar domains, independent of backbone DP. These random bottlebrush copolymers also exhibited a T ODT dependent on the side-chain length and independent of backbone length 33 .…”

“…Subsequently, in 2008, Xia and Grubbs et al reported the use of a highly-active ruthenium catalyst for achieving improved control over bottlebrush molecular weight and molecular weight dispersity 22,33 . The reactivity of norbornenyl macromonomers in ROMP is sufficient for achieving high macromonomer conversions for a range of concentrations and macromonomer molecular weights, enabling the preparation of bottlebrush polymers with systematically varying backbone and side-chain lengths.…”

“…This is reflected in studies of the self-assembly of random bottlebrush polymers 33 and stimuli-responsive properties of bottlebrush polymers 2 with mixed side chains.…”

Effects of Grafting Density on Block Polymer Self-Assembly: From Linear to Bottlebrush

“…[49][50][51][52] We targeted poly(D,L-lactide)-b-polystyrene (PLA-b-PS) graft diblock polymers to permit comparisons with brush PLA-b-PS systems previously investigated in the context of self-assembly. [21][22][23][24][25] All polymerizations were carried out in CH2Cl2 at room temperature under an inert atmosphere. To vary the grafting density (Figure 1), the first block was synthesized by copolymerizing a PLA macromonomer (Mn = 3230 g/mol) with a discrete co-monomer (i.e., diluent), DME (endo,exonorbornenyl dimethyl ester, Mn = 210 g/mol).…”

“…[13][14][15][16][17][18][19][20] However, bottlebrush block polymers display much larger scaling exponents ( = 0.8-0.9), [21][22][23][24] consistent with extended backbone conformations. Steric repulsion between the densely grafted side chains imparts a certain bending rigidity to the backbone, which can be modeled as a wormlike chain.…”