Monodisperse comb polystyrenes (comb-PS) with loosely to densely grafted architectures up to loosely grafted bottlebrush structures were synthesized via anionic polymerization. This comb-PS series, named PS290-N br -44, had the same entangled backbone, M w,bb = 290 kg/mol, corresponding to a number of entanglements along the backbone Z bb ≅ 20, and similar branch length, M w,br ≅ 44 kg/ mol or Z br ≅ 3, but varied in the number of branches per molecule, N br , from 3 to 190 branches. Consequently, the average number of entanglements between two consecutive branch points along the backbone (branch point spacing), Z s , ranged from well entangled, Z s ≅ 5, to values that were far less than one entanglement, Z s ≅ 0.1. Linear viscoelastic data including the zero-shear rate viscosity, η 0 , diluted modulus, G N,s 0 , and a new diluted modulus extracted from the van Gurp−Palmen plot, |G*| at δ = 60°, were analyzed as a function of the M w of the combs. Scaling of η 0 versus M w revealed three different regions for increasing N br or decreasing Z s : (1) loosely grafted combs with Z br < Z s and η 0 ∼ exp(M w ), ( 2) densely grafted combs with 1 < Z s < Z br and η 0 ∼ M w −3.4 followed by η 0 ∼ M w −1 for 0.2 < Z s < 1, and (3) loosely grafted bottlebrushes with Z s < 0.2 and η 0 ∼ M w 5 . The relative maximum in η 0 corresponded to a comb-PS with Z s ≅ Z br , and the relative minimum resulted from a comb-PS with Z s ≅ 0.2, which displayed almost the same η 0 as the linear PS290. Strain hardening factors, SHF ≡ η E,max /η DE,max , measured in extensional experiments increased with increasing N br and reached SHF > 200 for Hencky strains below ε H = 4, which is tremendously high and has to the best of our knowledge not been observed yet. Such a high strain hardening is of great fundamental and technical importance in extensional processes, e.g., foaming, film blowing, or fiber spinning.

combined with the grafting-to method [1,10] results in comb polystyrenes (PSs) with well-entangled monodisperse backbone and side chains; however, branch points along the backbone are randomly distributed. This random distribution of branch points has no distinct effect on the rheological properties of dense comb topologies; however, it will broaden the backbone relaxation time of loose comb topologies even though the molecular weight distribution is narrow enough. [11] A controlled branching point spacing can be achieved through the coupling of living macroanions; [12] however, this method is prone to slightly high molecular weight distribution, Ð > 1.5. 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. 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. [13] These bottlebrushes are similar to that of the dendronized polymer with only one layer of grafting. 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. However, different reactivity of macromonomer and diluent leads to a gradient of branching point spacing along the backbone, [14] where the conformations of side chains along the backbone are affected by this gradient. [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. However this issue could be ignorable in dense bottlebrushes where the volume fraction of backbone is less than 1 wt%. In order to avoid any microphase separation, Sheiko and co-workers [16] synthesized a series of dense combs and loose bottlebrushes homo poly(n-butyl acrylates) (PBA) with similar side chains and systematically varied grafting density through copolymerization of nonfunctional n-butyl acrylates with trimethylsilyl-protected acrylates by ATRP. However, in order to achieve high degree of polymerization (DP) of backbone (entangled system) for dense bottlebrushes, they had to use slightly different backbones, i.e., poly(n-butyl methacrylate). An organometallic coordinative insertion polymerization of α-olefins results in entangled densely grafted bottlebrushes with rod-like side chains where both backbone and side chains have alkane groups. However the dispersity index of such homopolymer bottlebrushes would Comb and bottlebrush polymers present a wide range of rheological and mechanical properties that can be controlled through their molecular characteristics, such as the backbone and side chain lengths as well as the number of branches per molecule or the grafting density. This review investigates the impact of these characteristics specifically on the zero sh...

The combination of the Passerini reaction and olefin cross-metathesis is shown to be a very useful approach for the divergent synthesis of dendrimers. Castor oil-derived platform chemicals, such as 10-undecenoic acid and 10-undecenal, are reacted in a Passerini reaction with an unsaturated isocyanide to obtain a core unit having three terminal double bonds. Subsequent olefin cross-metathesis with tert-butyl acrylate, followed by hydrogenation of the double bonds and hydrolysis of the tert-butyl ester, leads to an active core unit bearing three carboxylic acid groups as reactive sites. Iterative steps of the Passerini reaction with 10-undecenal and 10-isocyanodec-1-ene for branching, and olefin cross-metathesis with tert-butyl acrylate, followed by hydrogenation and hydrolysis allow the synthesis of a third-generation dendrimer. All steps of the synthesis are carefully characterized by NMR, GPC, MS, and IR.

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