Search results for "Kinetic chain length"
showing 10 items of 14 documents
Equilibria in the anionic polymerization of methyl methacrylate, 1. Chain-length dependence of the rate and equilibrium constants
1986
The rate constants of polymerization and cyclization and the equilibrium constants of polymerization are calculated from kinetic data of the anionic oligomerization of methyl methacrylate initiated by methyl α-lithioisobutyrate in THF at 25°C, and of the “disproportionation” of living oligomers. The numeric method reported earlier was improved. The rate constants of cyclization decrease with chain-length, due to increasing steric hindrance. The rate constants of polymerization depend on the concentration of active centres, due to the formation of associates. Rate and equilibrium constants of polymerization decrease with chain-length. This is discussed in terms of intramolecular coordination…
Carbanionic Polymerization: Kinetics and Thermodynamics
1989
For a kinetic analysis, the process of anionic polymerization has to be divided into at least three main reactions common to all types of polymerization (equations 1–3). I* denotes initiator, M monomer, Pi* and Pi′ an active or inactive polymer chain of degree of polymerization i, respectively, and X a terminating agent.
Solid-state polymerization of oxetanes. II. Investigation of the growth of the polymer phase as related to the mechanism of polymerization
1973
The radiation-induced solid-state polymerization of 3,3-bischloromethyloxetane (BCMO) was investigated by direct observation of the development of the morphology of the growing polymer phase in single crystals of the monomer. Electron microscopy shows that the polymerization gives rise to amorphous polymer in the first step. The polymer forms irregular platelets which aggregate into larger units without reflecting the crystalline order of the monomer. Subsequent to polymerization, the amorphous polymer crystallizes to the β-modification of poly-BCMO. If the partially polymerized crystals are extracted by solvents of the monomer, crystallization of the polymer is enhanced, and morphological …
Polymerization reactions and modifications of polymers by ionizing radiation
2020
International audience; Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons in…
Initiatoren für die polymerisation des trioxane. 20. Mitt. über polyoxymethylene1
1963
Die Polymerisation des Trioxans bei 60°C in Cyclohexan mit verschiedenen kationischen Initiatoren wurde untersucht. Die aktivsten Initiatoren sind Acetylperchlorat, Perchlorsaure, FeCl3 und SnCl4. Die Molekulargewichte der erhaltenen Polyoxymethylene sind bei den aktivsten Initiatoren am hochsten. Der Einflus der ubertragung durch Wasser und der kinetischen Kettenlange auf die Molekulargewichte wird diskutiert. Wahrend der Polymerisation kann durch den Initiator eine Spaltung bereits gebildeter Makromolekule erfolgen, welche die Molekulargewichte stark reduziert. Die Frage der Cokatalyse bei der kationischen Polymerisation des Trioxans mit Ansolvosauren wurde untersucht. Bei der Polymerisat…
What Limits the Molecular Weight and Controlled Synthesis of Poly(3-alkyltellurophene)s?
2016
Polytellurophenes are an emerging class of conjugated polymers; however, their controlled polymerization leading to high molecular weight materials has been a major challenge. Here we present a systematic investigation of the synthesis of poly(3-alkyltellurophene)s using the catalyst transfer polycondensation methodology. Learning that previous syntheses were limited by both polymerization reaction kinetics and polymer solubility, we design new tellurophene monomers to overcome these limitations. Controlled polymerization behavior up to Mn = 25 kDa, chain extension, block copolymerization, external initiation, and well-defined end groups are demonstrated for poly(3-alkyltellurophene)s with …
Branched and Functionalized Polybutadienes by a Facile Two-Step Synthesis
2008
Anionic polymerization was used to prepare silane-endfunctionalized polybutadiene macromonomers with different molecular weights ranging from 9 000 to 34000 g .mol- 1 . These were polymerized by a hydrosilylation reaction in bulk to obtain branched polymers, using Karstedt's catalyst. Surprisingly, the addition of monofunctional silanes during the polymerization showed only a minimal effect concerning the degree of polymerization. Furthermore, it was possible to introduce a variety of functional silanes without increasing the overall number of reaction steps by a convenient AB 2 + A type "pseudocopolymerization" method. All branched polymers were analyzed by SEC, SEC-MALLS, SEC-viscosimetry…
Kinetic Analysis of “Living” Polymerization Processes Exhibiting Slow Equilibria. 5. Effect of Monomer Transfer in Cationic Polymerization and Simila…
1996
This work deals with the kinetics of polymerization processes with chain transfer to monomer and reversible formation of dormant species. Such a mechanism is typical for cationic polymerization in the presence of Lewis acids as co-initiators. The expressions of number- and weight-average degrees of polymerization and polydispersity index are derived rigorously for a mechanism with free ions as the active species, but it is also applied to other mechanisms, e.g., ion pairs as active species. Plots of polydispersity index versus monomer conversion can be easily computed on a PC computer even though the expressions for the weight-average degree of polymerization and the concentration of residu…
Reaktionskinetik der Polymerisationshemmung durch molekularen Sauerstoff
1957
Die Polymerisationshemmung geschieht bei Styrol nach demselben Mechanismus wie er kurzlich fur Methylmethacrylat berichtet wurde1. Wahrend der Inhibierungsperiode findet streng alternierende Copolymerisation zwischen molekularem Sauerstoff und dem monomeren Styrol statt. Die fur die Anlagerung von O2 an das Radikal RM · masgebende Konstante kMO2 ist etwa von der Grosenordnung 106–107, d. h. die Reaktionsfahigkeit des Sauerstoffs in dieser Reaktion entspricht der eines Radikals. Bruttogeschwindigkeit und kinetische Kettenlange wahrend der Inhibierungsperiode werden auf Grund des Reaktionsmechanismus berechnet und experimentell bestatigt. The inhibition mechanism in the polymerization of styr…
Solid-state polymerization of oxetanes. I. Lattice parameters and packing properties of the monomers
1973
The crystallographic unit cells of melt-crystallized 3,3-bischloromethyloxetane and 3,3-bisbromomethyloxetane were determined by the Weissenberg method. The two isomorphous lattices are triclinic with two molecules in the unit cell. 3,3-Bisfluoromethyloxetane forms plastic crystals in the temperature range between −36°C and +22°C, as shown by differential calorimetry and NMR broad-line spectroscopy. The Debye-Scherrer diagram and the general physical properties indicate the formation of a face-centered cubic lattice. No correlation between the lattice parameters of the monomer and polymer can be found On the basis of these results, the question is raised as to whether a topochemical polymer…