Monte Carlo Simulation of the Topology and Conformational Behavior of Hyperbranched Molecules: Pd–Diimine‐Catalyzed Polyethylene

A kinetic Monte Carlo model was developed to simulate the polymerization of ethylene with palladium–α‐diimine catalyst wherein hyperbranched molecules are formed through a chain‐walking mechanism. The total degree of branching and the distribution of short branches obtained with the model agree well with reported ¹³C NMR experimental results. Different chain topologies were generated by varying the probability of chain walking, P_w , which controls the competition between chain‐walking and monomer insertion. Molecular Monte Carlo simulations were subsequently conducted to study the conformations of isolated molecules (created by the kinetic Monte Carlo scheme) to relate molecular shape and topology. Our results provide evidence that the topology varies from predominantly linear with many short branches at low P_w to a densely branched, globular structure at high P_w . In contrast to experimental observations, our results for the molecular weight (N) dependence of the radius of gyration (R_g ∝ N^v) indicate that the branching topology has an effect on this relation, i. e., high‐P_w molecules have a smaller scaling exponent v. The simulated N‐dependence of the second virial coefficient exhibits a similar behavior. We also discuss the unusual conformational behavior of highly branched polymers obtained when P_w → 1.     

Phase behavior of low molecular weight polyethylenes from homogeneous and heterogeneous solutions

The pseudophase diagrams of solutions of low molecular weight polyethylene (PE) (number‐average molecular weight < 1500 g/mol) in octamethyl cyclotetrasiloxane (OCTS) and isododecane (IS) were determined by direct observation of cloud points and optical microscopy. In addition, melting temperatures were also determined by differential scanning calorimetry. In the range of single liquid–solid transitions, the data conformed to the classical melting temperature composition relation as a result of the formation of extended crystallites. The melting data were used to determine the interaction parameter of the PE in OCTS (1.4 ± 0.1) and IS (0.22 ± 0.05). The structural and thermal properties of the gels formed by a competing liquid–liquid and liquid–solid phase separation, under nonequilibrium conditions, contrast with the properties of the crystals formed from a single liquid–solid transition. Coarsening within the liquid phases was evidenced by optical microscopy, and insights about the mechanism of the kinetics of the coarsening process are given. The temporal changes of the melting temperature of crystallites formed from the heterogeneous phase (OCTS) reveal dynamics within a nonequilibrium state. In contrast, the crystallites formed from a homogeneous solution (IS) showed negligible melting‐temperature changes with time. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 878–889, 2002     

2-亚胺吡啶镍配合物的合成、晶体结构及催化乙烯聚合

经过一系列化学反应,在吡啶环上引入特丁基及甲醛基,再与2,6-二异丙基苯胺反应生成2-（N-2,6-二异丙基苯亚氨基）亚甲基-6-特丁基吡啶（g）配体,并合成了相应的配合物[NiBr₂（g）]（h）。配合物h的单晶结构表明其为单配体单金属结构,属于正交晶系,a=1.52776（14）nm,b=1.60846（15）nm,c=1.83777（17）nm,晶胞体积4.5160（7）nm³,Z=8。在助催化剂MAO的作用下,考察了催化乙烯聚合特性,在0℃时,所得聚合产物中包括支化聚合物和齐聚物,其中支化聚合物的分子量M_w=1.09×10⁴,其支链大部分为甲基、丁基和长支链,支链密度为58.5。     

2-亚胺吡啶镍配合物的合成、晶体结构及催化乙烯聚合

经过一系列化学反应,在吡啶环上引入特丁基及甲醛基,再与2,6-二异丙基苯胺反应生成2-(N-2,6-二异丙基苯亚氨基)亚甲基-6-特丁基吡啶(g)配体,并合成了相应的配合物[NiBr2(g)](h)。配合物h的单晶结构表明其为单配体单金属结构,属于正交晶系,a=1.527 76(14)nm,b=1.608 46(15)nm,c=1.837 77(17)nm,晶胞体积4.516 0(7)nm3,Z=8。在助催化剂MAO的作用下,考察了催化乙烯聚合特性,在0℃时,所得聚合产物中包括支化聚合物和齐聚物,其中支化聚合物的分子量Mw=1.09×104,其支链大部分为甲基、丁基和长支链,支链密度为58.5。     

NMR Microscopy of an “Anisotropic” Polymer

Two different samples of high-density polyethylene (HDPE) have been studied. One (isotropic) is extracted from the material core whereas the other (anisotropic) involves two sides which have been in contact with the injection mold. It is observed by NMR microscopy (using radiofrequency field gradients) that these two sides favor toluene penetration into the material. The distribution of toluene nuclear spin relaxation times (extracted from proton T₁ and T₂ images) exhibits likewise important differences between the two samples. These differences can be accounted for by partial molecular ordering at the vicinity of the “mold sides”. Finally, in investigating the anisotropic sample (without solvent), three different phases (two amorphous and one crystalline) are revealed by ¹³C chemical shift imaging experiments (performed with radiofrequency field gradients under CP/MAS conditions). Each amorphous component is preferentially present at one of the two “mold sides”.