The POM−POM architecture is the simplest yet defined branched architecture, showing both strain hardening in elongation and strain softening in shear. The molecular structure consists of q side chains at each end of a backbone segment. To study the rheological and mechanical properties, we synthesized low-disperse POM−POM-shaped polystyrenes (PS) with welldefined molecular properties via anionic polymerization and grafting-onto method. All samples had a backbone with a weightaverage molecular weight of M w,b ≅ 100 kg mol −1 and approximately similar numbers of side chains per star q = 11−14. We varied the side chain length systematically from unentangled up to highly entangled side chains (M w,a = 9−300 kg mol −1 , 0.5−18 entanglements). The POM−POMs having M w,a ≈ 3M e ≈ M c have a maximum decrease in zero-shear viscosity η 0 of over 3 decades compared to linear PS with the same molecular weight, together with the highest strain hardening factor of SHF = 43. Moreover, POM−POMs having M w,a > 5M e displayed enhanced mechanical fatigue resistance beyond those of linear, ultrahigh-molecularweight PS, by up to a factor of 10.

Vitrimers can combine the advantageous properties of cross-linked materials with thermoplastic processability. For the prominent case of polyethylene, established post-polymerization introduction of cross-linkable moieties results in extremely heterogeneous compositions of the chains. Here, we report the generation of functionalized polyethylenes directly by catalytic insertion polymerization, with incorporated cross-linkable aryl boronic esters or alternatively acetal-protected groups suited for cross-linking with difunctional boronic esters. In addition to the desired homogeneous in-chain distribution, the reactive cross-linkable groups are enriched at the chain ends. This enables the incorporation of all chains in the network, as also supported by simulations of all chains’ compositions. The uniform molecular composition of the chains reflects in resulting vitrimers’ material properties, particularly lack of leaching with solvents. At the same time, cross-linking is indeed fully reversible and the vitrimers can be recycled.

The shear and elongational rheology of linear and pom-pom shaped polystyrene (PS) blends was investigated experimentally and modeled using constitutive models such as the Doi–Edwards and the molecular stress function (MSF) model. The pom-pom molecule is the simplest topology to combine shear thinning with strain hardening in elongational flow. A PS pom-pom with a self-entangled backbone (Mw,bb = 280 kg mol−1) and 22 entangled sidearms (Mw,a = 22 kg mol−1) at each star was blended with two linear PS with weight average molecular weights of Mw = 43 and 90 kg mol−1 and low polydispersities (Ð < 1.05). A semilogarithmic relationship between the weight content of the pom-pom, ϕpom-pom, and the zero-shear viscosity was found. Whereas the pure pom-pom has in uniaxial elongational flow at T = 160 °C strain hardening factors (SHFs) of SHF ≈100, similar values can be found in blends with up to ϕpom-pom = 50 wt. % in linear PS43k and PS90k. By blending only 2 wt. % pom-pom with linear PS43k, SHF = 10 can still be observed. Furthermore, above ϕpom-pom = 5–10 wt. %, the uniaxial extensional behavior can be well-described with the MSF model with a single parameter set for each linear PS matrix. The results show that the relationship between shear and elongational melt behavior, i.e., zero-shear viscosity and SHF, can be uncoupled and customized tuned by blending linear and pom-pom shaped polymers and very straightforwardly predicted theoretically. This underlines also the possible application of well-designed branched polymers as additives in recycling.

Rheological properties highly impact the foaming behavior of polymers, as a high melt strength and low shear viscosity support cell expansion by reducing cell coalescence and rupture. Since rheological properties originate from molecular topology, topology is consequently a governing parameter for foaming. We synthesized well-defined polystyrene (PS) POM-POM-shaped model systems to study the impact of the topologyoriginated melt rheological parameters on the physical foaming behavior within purely amorphous polymers without any crystallization effects. Therefore, we systematically varied the topological parameters of the POM-POM topology, i.e., the number of arms q, the arm length M w,a , and the backbone length M w,b . We investigated the melt rheological properties of the samples concerning their zero-shear viscosity and elongational strain hardening and correlated their resulting foam structure. Within the sample series, η 0 can be reduced by up to 10 4 , compared to linear PS with the same total molecular weight, whereas the maximum achieved strain hardening factor is SHF = 141 at T = 160 °C. The foaming experiments revealed that the mean cell size D and the volume expansion VER can be precisely controlled via molecular topology. By controlling the topological parameters, it was possible to increase VER from VER = 2.0 to 15.6, whereas D varied between D = 1.3 and 17.2 μm within 100−150 °C under a pressure of 500 bar. In comparison, the VER of linear PS increased from VER = 2.6 to 5.4, while D increased from D = 2.3 to 4.8 μm. Moreover, the influence of topology is more pronounced at higher temperatures, indicating that the molecular relaxation times and the applied shear rates need to be related. Hence, it is possible to tailor the cellular structure to a specific application by considering and controlling the polymer topology.

New phosphine-functionalised tris(pyrazolyl)methane ligands, their coordination flexibility and their ability to generate heterobimetallic complexes are presented.