Recently developed chain walking
(CW) catalysis is an elegant approach
to produce materials with controllable structure and properties. However,
there is still a lack in understanding of how the reaction mechanism
influences the macromolecular structures. In this study, a series
of dendritic polyethylenes (PE) synthesized by Pd-α-diimine-complex
through CW catalysis (CWPE) is investigated by means of theory and
experiment. Thereby, the exceptional ability of in situ tailoring
polymer structure by varying synthesis parameters was exploited to
tune the branching architecture, which allowed us to establish a precise
relationship between synthesis, structure, and solution properties.
The systematically produced polymers were characterized by state-of-the-art
multidetector separation and neutron scattering experiments as well
as atomic force microscopy to access molecular properties of CWPE.
On a global scale, the CWPE appear in a worm-like conformation independently
on the synthesis conditions. However, severe differences in their
contraction factors suggested that CWPE differ substantially in topology.
These observations were verified by NMR studies that showed that CWPE
possess a constant total number of branches but varying branching
distribution. Small angle neutron scattering experiments gave access
to structural characteristics from global to segmental scale and revealed
the unique heterogeneity of CWPE, which is predominantly based on
differences in their dendritic side chains. The experimental data
were compared to theoretical CW structures modeled with different
reaction-to-walking probabilities. Simple theoretical arguments predict
a crossover from dendritic to linear topologies yielding a structural
range from purely linear to dendritic chain growth. Yet, comparison
of theoretical and empirical scattering curves gave the first evidence
that a transition state to worm-like topologies is actually experimentally
accessible. This crossover regime is characterized by linear global
features and dendritic local substructures contrary to randomly hyperbranched
systems. Instead, the obtained CWPE systems have characteristics of
disordered dendritic bottle brushes and can be adjusted by the walking
rate/reaction probability of the catalyst.
Modifying material properties in simple macromolecules such as polyethylene (PE) is achieved by different connection modes of ethylene monomers resulting in a plurality of possible topologies-from highly linear to dendritic species. However, the challenge still lies within the experimental identification of the topology and conformation of the isolated macromolecules because of their low solubility, which demands methods with specific solvents and high operating temperatures. Additionally, a separation technique has to be coupled to different detection methods to meet the specific demands of the respective characterization goal. In this work, we report a quadruple-detector high temperature size exclusion chromatography (HT-SEC) system which contains online multiangle laser light scattering, dynamic light scattering, differential viscometry, and differential refractometry detectors. Quadruple-detector HT-SEC was successfully applied to explore the full range of physical parameters of various PE samples with different branching topologies ranging from highly linear macromolecules, polymers with moderate level of branching, to highly branched PEs with hyperbranched structure. This method is a useful tool not only to investigate molecular weight, mass distribution, and size but also to enable access to important factors which describe the conformation in dilute solution and branching density.
Thermal field-flow fractionation
(ThFFF) was designed to investigate the retention behavior of a series
of dendritic
polyethylenes synthesized using a chain walking catalyst (cwPE) with
variations in the branching architecture. The retention behavior of
these macromolecules correlates with their branching. Based on differences
in the Soret coefficient, a new model has been developed for the application
of ThFFF as an alternative to the branching calculation approach based
on light scattering or viscosity for the branching analysis of novel
short-chain branched PEs.
A series of tin(IV) guanidinates were prepared by a (4+1) oxidative cycloaddition of four 1,2-diones (3,5-di-tertbutyl-o-benzoquinone, 3,4,5,6-tetrachloro-1,2-benzoquinone, 9,10-phenanthrenedione, 1,2-diphenylethanedione) or by an oxidative addition of a C−Br bond (from 2-bromo-1,3-diphenylpropane-1,3-dione followed by rearrangement) and a Cl−Cl bond (Cl 2 generated from (dichloro-λ 3 -iodanyl)benzene) with {pTol-NC[N(SiMe 3 ) 2 ]N-pTol} 2 Sn (1). The reactivity of five pentane-1,3-diones and dimethyl malonate with compound 1 was assessed on the basis of the effect of 1,3-diones on the reaction mechanism in comparison with 1,2-diones. In contrast with oxidation reactions observed for compounds containing conjugated CO bonds, the reactions of the tin(II) guanidinate with 1,3-diones revealed a high ability for ligand substitution. All the tin compounds prepared were characterized, and ligand substitution reactions were monitored using 1 H, 13 C, and 119 Sn NMR spectroscopy. The molecular structures of one tin(II) and five tin(IV) guanidinato complexes investigated were determined by Xray diffraction. All tin(IV) compounds display six-or seven-coordination. The UV−vis absorption spectra were recorded and simulated by TDDFT methods in order to get insight into the origin of the nontypical colors of the target tin(IV) diolatoguanidinates and their keto-functionalized precursors.
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