Molecular weight information on the (polynorbornene-g-polystyrene)-b-(polynorbornene-g-polylactide) polymer series. a) The molar ratios used in the synthesis of these brush block copolymers of the catalyst (C) and the MMs. SEM cross-sections reveal the morphology of the center of the brushed BCP films prepared from the controlled evaporation of DCM (B), THF before (C) and after (D) thermal annealing, as well as by direct thermal annealing under compression (E ). SEM images provided insight into the self-assembly of the films using different techniques, clearly revealing the polymer morphologies and domain sizes (Figure 2-4 B-E).
Top: Reflectance of films of the brush block copolymers made by controlled evaporation of DCM. From left to right are samples A-H as described in Table 2-2. Bottom: Transfer of films of the brush block copolymers made by controlled evaporation of DCM. Top: Reflectance of films of the brush block copolymers made by controlled evaporation of THF.
From left to right are samples C-H as described in Table 2-2. Bottom: Transfer of films of the brush block copolymers made by controlled evaporation of THF. Top: Reflectance of films of the brush block copolymers made by controlled evaporation of THF after heating. From left to right are samples A-G as described in Table 2-2. Bottom: Transfer of films of the brush block copolymers made by controlled evaporation of THF after heating.
From left to right are samples B-G as described in Table 2-2. Bottom: Transfer of films of the brush block copolymers made by thermal compression.
Wavelength (nm)!
Plot of λmax versus BCP MW for the glass side of films prepared from controlled evaporation from DCM, before (blue) and after (purple) heating, or THF, before (green) and after (red) heating, as well as by thermal compression (orange).
SEM image of the thickness of a cross section of F (Mn g/mol) prepared by A) controlled evaporation from DCM, B) controlled evaporation from THF before heating, and C) after heating as well as D) prepared by thermal compression. The domain size increased approximately linearly with backbone length, which indicated an extended conformation of the backbone in the ordered state. Profiles of the scattering intensity versus scattering vector were also generated from Figure 3-1 and Figure 3-2, and are shown in Figure 3-3.
A systematic decrease in q*, the first-order reflection scattering vector, was observed from low MW brush BCP to high MW brush BCP, indicating the expected increase in domain spacing, ranging from 19.2 to 235 nm ( Table 3-1). . In contrast to linear BCPs, the many chain ends of the side chains attached to the main chain would preferentially segregate toward the surface and substrate interfaces. The slight increase in the exponent as the side chain length increases, and the apparent saturation, with a further increase in the side chain molecular weight, arises from the entropic damage associated with the stacking of the side chains.
As discussed in the previous section, similar results were found in the case of the thermally annealed bulk samples of brush BCPs. Analysis of the surface topography shows that terraces were formed on the surface with a step height of only 9 nm for both brush BCPs. These values correspond roughly to the diameters of the brushes and suggest that there is a single layer of the brush BCP covering the surface of the film.
This would be consistent with the lower surface energy of the PS brush block, but also suggests that the brush BCP should adopt a rather unusual bent configuration at the surface to minimize the surface energy. The plots above SFM images are cross-sectional analysis of the corresponding height images below. Polymer entanglement causes a kinetic barrier for polymers to self-assemble, thus leading to a slow kinetic process of the self-assembly.
Nevertheless, these results indicate that well-ordered lamellar structures were formed within one hour for all samples shown. The size of the domain increases approximately linearly with the DP of the backbone, indicating that the backbone is in an extremely extended conformation. Schematic illustration of the expected self-assembly of brush BCPs with terminally deuterated PS side chains undergoing either interdigitated packing (left) or conventional packing (right).
The form of the sample codes, [g-Sx]p-b-[g-LAy]q, is the same as used in Chapter 3, where the subscripts x and y are the molecular weights of the side chains of each type (in units of one thousand), and the subscripts p and q represent the degree of polymerization of each brush block. To determine the morphologies of the above samples, more data such as small angle X-ray scattering (SAXS) were required.
PS Content
The SAXS profiles of the samples with the highest degree of symmetry were best indexed to a lamellar morphology. Brush copolymers were subsequently synthesized from each of the MMs via ring-opening metathesis polymerization (ROMP) using a Ruthenium-based third generation initiator (Scheme 5-1). We attribute this to the confinement of the side chains by the brush copolymer architecture, which inhibits their mobility and ability to crystallize.
General reaction scheme for the synthesis of the macromonomers (C) from a norbornene initiator (A) and lactide (B) (top) and for the synthesis of the brush copolymer (D) from the macromonomer (C) (bottom). Mixtures of the MMs were formed by pairing stereoisomers of similar molecular weight in equal weight ratios. This may allow some interaction between brush copolymers at the chain end of the side chains, allowing for some stereocomplex formation (Figure 5-1b).
This solution was allowed to evaporate in the vial at room temperature, and after air-drying, it was placed under high vacuum to ensure removal of the solvent. The synthetic strategy involves a tandem cross-metathesis/ring-closing metathesis of the bisdiarylbutene macrocycles as the key step. Since vinyl halides are less reactive than monosubstituted alkenes, it was expected that the cross metathesis of the monosubstituted alkenes would be observed first.
It was hypothesized that eventually some cross-metathesis of the vinyl halide with the monosubstituted alkene would be observed and that this step would be irreversible due to the lack of metathesis reactivity expected from the resulting product. Metathesis of the remaining alkenes in the nascent molecule would lead to ring closure of the macrocycles to give the desired product (Scheme A-2). These attempts were unsuccessful and either resulted in decomposition of the essential allylic group (possibly by chlorination of the alkene) or no apparent reaction, depending on the reaction conditions (Scheme A-6).
A brief study of the solvent effect on the regioselectivity of the Claisen rearrangement was performed. Due to the small conversion of the reaction, the NMR results may have been somewhat inaccurate, but they still gave a rough idea of the solvent effect (Table A-1). Thus, only one deprotection step would be required at the end of the synthesis instead of two.
Alternatively, one could reverse the order of the cross-metathesis reaction and the Ullmann coupling reaction of the biaryl ether to try to complete the synthesis. No desired product was isolated, but some 4-(2-chloroallyl)-2,6-dimethoxyphenyl acetate was observed by NMR.
