The analytic expression for the weight‐average molecular weight development proposed in the first part of this series[1] is extended to account for the substitution effects of the functional groups in a polyfunctional chain‐transfer agent. The obtained functional form of the matrix formula, Pw = W { D w + ( I + T ) SPE ( I – TSPE )–1 D f } is essentially the same as for equal reactivity model, but the NxN square matrixes, S and P in the equal reactivity model is now expanded to NxfN and fNxfN matrixes, respectively, to account for the different reactivities of f functional groups. The number of chain types, N is extrapolated to infinity to obtain the Pw‐value for free‐radical polymerization. Practically, such extrapolation can be conducted with calculated Pw‐values for only three different N‐values. Illustrative calculations show that the obtained results agree with those from the Monte Carlo method. The present approach provides newer insight into the branched structure formation during complex polymerization reactions. 相似文献
Branched polymers like LDPE are known to possess a wide range of architectures. In this paper a modelling approach is developed, describing the relation between architectures, chemistry and reactor conditions with the general objective of improving characterisation and controlling visco‐elastic properties. More specifically, the particular scission kinetics of branched molecules as strongly contrasting with linear scission is described. A new method to synthesise branched architectures is developed as an alternative to full Monte Carlo (MC) sampling. It employs MC sampling for coupling of primary polymers only. Graph theory is used as an efficient storage method containing all topological information of individual molecules. The algorithm synthesises molecules for any given combination of chain length (n) and number of branches (N). The explicit and detailed knowledge of branched architectures allows finding the correct topological scission kinetics. Distributions of fragment lengths and numbers of branches on fragments after scission are obtained, showing a preference for short and long fragments. Approximate functions describing this have been implemented in another model, predicting molecular weight (MWD) and degree of branching (DBD) distributions using a Galerkin finite element method. Topological scission is seen to give MWD broadening and a higher branching density for long chains. Distributions of longest end‐to‐end distances could be computed for all architectural alternatives for given n, N. In conclusion, it is demonstrated that this method yields distributions of architectures consistent with MWD/DBD for radical polymerisation with long‐chain branching and random scission. 相似文献
The analytic expression for the weight‐average molecular weight development in free‐radical polymerization that involves a polyfunctional chain‐transfer agent is proposed. Free‐radical polymerization is kinetically controlled; therefore, the probability of chain connection with a polyfunctional chain‐transfer agent as well as the primary chain‐length distribution changes during the course of polymerization. We consider the primary chains formed at different times as different types of chains, and the heterochain branching model is used to obtain the weight‐average chain length at a given conversion level in a matrix formula, described as Pw = W { D w + ( I + T ) SP ( I – TSP )–1 Df }. Because the primary chains are formed consecutively, the number of chain types N is extrapolated to infinity, but such extrapolation can be conducted with the calculated values for only three different N values. The criterion for the onset of gelation is simply described as a point at which the largest eigenvalue of the product of matrixes, TSP reaches unity, i. e., det ( I – TSP ) = 0. The present model can readily be extended for the star‐shaped polyfunctional initiators, and the relationships between the model parameters and kinetic rate expression for such reaction systems are also shown. 相似文献
The formation of long‐chain branches (LCBs) during ethylene polymerization with a combination of catalysts was studied by Monte Carlo simulation. The model describes polymerization with a non‐branching catalyst that produces linear macromonomers, and a branching catalyst that produces linear and branched macromonomers. The LCBs are formed when the branching catalyst incorporates a macromonomer. The discussion is based on the three types of chain topology obtained during the synthesis: linear, comb‐branched, or hyperbranched. Simulation results show how the chain length distribution and the number of LCBs change according to the ratio between the two catalysts present in the reactor. The ratio hyperbranched/comb‐branched is defined to evaluate the system composition and the contribution of each catalyst. 相似文献
Branched polyolefin microstructures resulting from so‐called “chain walking” are a fascinating feature of late transition metal catalysts; however, to date it has not been demonstrated how desirable branched polyolefin microstructures can be generated thereby. We demonstrate how highly branched polyethylenes with methyl branches (220 Me/1000 C) exclusively and very high molecular weights (ca. 106 g mol?1), reaching the branch density and microstructure of commercial ethylene–propylene elastomers, can be generated from ethylene alone. At the same time, polar groups on the main chain can be generated by in‐chain incorporation of methyl acrylate. Key to this strategy is a novel rigid environment in an α‐diimine PdII catalyst with a steric constraint that allows for excessive chain walking and branching, but restricts branch formation to methyl branches, hinders chain transfer to afford a living polymerization, and inverts the regioselectivity of acrylate insertion to a 1,2‐mode. 相似文献
New families of highly branched polyethylenes containing alkyl short chain branches as well as polar and non‐polar long‐chain branches were prepared by combining migratory insertion copolymerization with controlled radical graft copolymerization. Key intermediate was a novel alkoxyamine‐functionalized 1‐alkene which was copolymerized with ethylene using a palladium catalyst. The resulting highly branched polyethylene with alkoxyamine‐functionalized short chain branches was used as macroinitiator to initiate controlled radical graft copolymerization of styrene and styrene/acrylonitrile. Novel polyethylene graft copolymers with molecular masses of Mw >100 000 g/mol and narrow polydispersities were obtained. Transmission electron microscopic studies (TEM) and the presence of two glass transition temperatures at –67 and +100°C indicated microphase separation. 相似文献
Upon activation with diethylaluminium chloride (Et2AlCl), a series of phenyl‐substituted α‐diimine nickel precatalysts conducted 4‐methyl‐1‐pentene (4MP) and ethylene (E) (co)polymerizations via controlled chain‐walking to generate branched amorphous polymers with high molecular weight and narrow molecular weight distribution (Mw/Mn < 1.6). The obtained poly(4MP)s were amorphous elastomers with glass transition temperature (Tg) of ?10 ~ ?24 °C, which are higher than that of E‐4MP copolymer ( ? 63.0 °C). At room temperature (25 °C), 4MP polymerization proceeds in a living manner. The microstructures of the produced poly(4MP)s indicated the 2,1‐ and 1,2‐insertion followed by chain‐walking, the latter being predominant. The NMR analyses of the polymers showed that the obtained poly(4MP) possessed methyl, isobutyl, 2,4‐dimethylpentyl and 2‐methylhexyl groups, while the isobutyl and 2,4‐dimethylalkyl branches derived from 4MP were observed in the E‐4MP copolymer. The branch structures and the insertion‐type of monomer were depended on the polymerization temperature, and the content of methyl branch increased with an increase in the polymerization temperature. 相似文献
Summary: The one step synthesis of a series of branched azobenzene side‐chain liquid‐crystalline copolymers by the self‐condensing vinyl copolymerization (SCVCP) of a methyl acrylic AB* inimer, 2‐(2‐bromoisobutyryloxy)ethyl methacrylate (BIEM), with the monomer 6‐(4‐methoxy‐azobenzene‐4′‐oxy)hexyl methacrylate (M), by atom transfer radical polymerization (ATRP) in the presence of CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine as a catalyst system, and in chlorobenzene solvent, is reported. The degree of branching (DB), and the molecular weights and polydispersities of the resultant polymers were determined by NMR spectroscopy and size exclusion chromatography, respectively. The phase behaviors of the branched copolymers were characterized by differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). The degree of branching of the branched copolymers could be controlled by the comonomer ratio in the feed and influenced their liquid‐crystal properties. Liquid‐crystal properties were not exhibited when the comonomer ratio was low. Comonomer ratios greater than 8 gave polymers with a lower number of branches, which exhibited both a smectic and a nematic phase.
A polarized optical micrograph of the smectic phase texture of a polymer synthesized here with a higher comonomer feed ratio (magnification × 400). 相似文献
Given some lattice, the number ZHP of Hamiltonian paths and also the number ZN of N‐step shorter self‐avoiding walks on the surface of cylinders, cones, tori, and spheres has been Monte Carlo estimated. The procedure is an extension of the technique used in a previous paper for plane squares and rectangles, which is based on the Rosenbluth‐Rosenbluth chain‐generation procedure. Starting from a rectangle having m lines and n columns, and thus m×n lattice sites, one may obtain cylindrical, conical, toroidal and spherical surfaces through continuous deformations, which respect the topology. Then a correspondence is established between a plane figure of the ‘polar’ coordinates kind and the topology of the above surfaces. Using this topological equivalence, and thus operating exclusively on the plane ‘polar’ figure, Monte Carlo simulations show that for given m and n, ZHP and ZN increase when going from the plane rectangle to the cylinder and then to the cone and the torus. The number ZNC of N‐step cycles (closed configurations) has also been Monte Carlo estimated. The Monte Carlo results for the surfaces studied here have been condensed in fifth degree polynomials in Φ, where Φ is the fraction of available lattice sites on the surface which are occupied by the N‐step self‐avoiding walk. The variation of the ratio ZNC /ZN with m and n has been estimated for cylindrical and conical surfaces. Finally, an effective coordination number qeff has been introduced for finite surfaces, and its variation with Φ studied. 相似文献
The thermal decomposition of polystyrene clusters prepared by using 4‐methacryloyloxy‐2,2,6,6‐tetramethylpiperidine‐N‐oxyl as a branching agent was studied. The weight‐average molar mass (M̄w) and the radius of gyration (Rg) of the resultant clusters can be scaled as M̄w ∝︁ Rg3.0 ± 0.1, revealing that these clusters have a uniform chain density. Moreover, the dynamic properties of such clusters converge unexpectedly faster than the static properties as the molar mass decreases. 相似文献
The thermodynamical properties of the star-branched polymers on the tetrahedral lattice are studied taking into account nearest-neighbor interactions. The excess free energy and energy and heat capacities are computed for wide ranges of chain lengths, reduced potential ?/kT, and number of branches. A significant influence of the degree of branching on long-range interactions in the polymer random coil is observed. The possibilities of phase transitions in both linear and branched systems are discussed on the basis of the Monte Carlo data. 相似文献
Ten unsymmetrical N,N'‐bis (imino) acenaphthene‐nickel (II) halide complexes, [1‐[2,6‐{(4‐MeOC6H4)2CH}2–4‐MeC6H2N]‐2‐(ArN)C2C10H6]NiX2, each appended with one N‐2,6‐bis(4,4'‐dimethoxybenzhydryl)‐4‐methylphenyl group, have been synthesized and characterized. The molecular structures of Ni1 , Ni3 , Ni5 and Ni6 highlight the variation in steric protection afforded by the inequivalent N‐aryl groups; a distorted tetrahedral geometry is conferred about each nickel center. On activation with diethylaluminum chloride (Et2AlCl) or methylaluminoxane (MAO), all complexes showed high activity at 30°C for the polymerization of ethylene with the least bulky bromide precatalysts ( Ni1 and Ni4 ), generally the most productive, forming polyethylenes with narrow dispersities [Mw/Mn: < 3.4 (Et2AlCl), < 4.1 (MAO)] and various levels of branching. Significantly, this level of branching can be influenced by the type of co‐catalyst employed, with Et2AlCl having a predilection towards polymers displaying significantly higher branching contents than with MAO [Tm: 33.0–82.5°C (Et2AlCl) vs. 117.9–119.4°C (MAO)]. On the other hand, the molecular weights of the materials obtained with each co‐catalyst were high and, in some cases, entering the ultra‐high molecular weight range [Mw range: 6.8–12.2 × 105 g mol?1 (Et2AlCl), 7.2–10.9 × 105 g mol?1 (MAO)]. Furthermore, good tensile strength (εb up to 553.5%) and elastic recovery (up to 84%) have been displayed by selected more branched polymers highlighting their elastomeric properties. 相似文献