Termination kinetics molecular weights

In the next sections the quantitative effect of reactions (1), (3), (4), and (5) which can be considered as various deviations from ideal systems, on kinetics, molecular weights, and polydispersities, will be presented. The magnitude of only one variable will be changed each time to demonstrate clearly the effect of slow initiation, termination, transfer, and slow exchange on the polymerization rates and properties of the resulting polymers. 

More recently it has been shown that in the polymerization with TT-crotylnickel iodide the order in monomer falls from a value close to unity at [M] below 0.5 mole 1" to below 0.5 at [M] > 4 mole 1 . These observations have been interpreted in terms of scheme (c) on p. 162, namely coordination of two monomer molecules with the catalyst and with most of the catalyst existing in the complex (inactive) state. The molecular weights of the polymers are double those calculated from the kinetic scheme put forward [61] and this is attributed to coupling of live polymer chains on termination [251]. Molecular weight distributions are binodal consistent with slow propagation and transfer. 

Due to the low reaction temperature and the use of chain terminator, the molecular weight distribution in interfacial synthesis is kinetically controlled and may be far from thermodynamic equilibrium. In two parametric studies Mills [179] and Munjal [180] have tried to model the full molecular weight distribution of polycarbonate. Varying the ratio of mass transfer/kinetic rates, they show how mass transfer limitations can lead to a higher polydispersity or a higher oligomer content. 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

It is possible to balance all of these thermodynamic, kinetic, and mechanistic considerations and to prepare well-defined PTHF. Living oxonium ion polymerizations, ie, polymerizations that are free from transfer and termination reactions, are possible. PTHF of any desired molecular weight and with controlled end groups can be prepared. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

GPC-derived weight average molecular weights are often less prone to error than number average molecular weights. When termination is wholly hy disproportionation or chain transfer and chains are long (>10 units), classical kinetics predicts Xn = XJ2 (Section 5.2.1.3). It follows that Cit can be obtained from the slope of a plot of 21 Xw vs [T]0/[M]t>."4 "5 The errors introduced even when the dominant process for radical-radical termination is combination (e.g. S polymerization) are small as long as X n is small in relation to... 

Synthesis of hydrolytically stable siloxane-urethanes by the melt reaction of organo-hydroxy terminated siloxane oligomers with various diisocyanates have been reported i97,i98) -yhg polymers obtained by this route are reported to be soluble in cresol and displayed rubber-like properties. However the molecular weights obtained were not very high. A later report56) described the use of hydroxybutyl terminated disiloxanes in the synthesis of poly(urethane-siloxanes). No data on the characterization of the copolymers have been given. However, from our independent kinetic and synthetic studies on the same system 199), unfortunately, it is clear that these types of materials do not result in well defined multiphase copolymers. The use of low molecular weight hydroxypropyl-terminated siloxanes in the synthesis of siloxane-urethane type structures has also been reported 198). 

For purposes of simulation and illustration we have chosen a batch reactor, solution polymerization of methylmethacrylate (MMA). Kinetic data were taken from Schmidt and Ray (1981) and thermodynamic data from Bywater (1955). We do not here consider the influence of diffusion control on the termination or other rate processes because such effects may be small when in a solution which is siifHciently dilute or when the polymer is of low molecular weight. 

A series of theoretical studies of the SCV(C)P have been reported [38,40,70-74], which give valuable information on the kinetics, the molecular weights, the MWD, and the DB of the polymers obtained. Table 2 summarizes the calculated MWD and DB of hyperbranched polymers obtained by SCVP and SCVCP under various conditions. All calculations were conducted, assuming an ideal case, no cyclization (i.e., intramolecular reaction of the vinyl group with an active center), no excluded volume effects (i.e., rate constants are independent of the location of the active center or vinyl group in the macromolecule), and no side reactions (e.g., transfer or termination). 

If the degree of polymerization is controlled principally by chain termination so that Xn is proportional to the kinetic chain length, the temperature coefficient of the average molecular weight will depend... 

Detailed studies on the lipase-catalyzed polymerization of divinyl adipate and 1,4-butanediol were performed [41-44]. Bulk polymerization increased the reaction rate and molecular weight of the polymer however, the hydrolysis of the terminal vinyl ester significantly limited the formation of the polyester with high molecular weight. A mathematical model describing the kinetics of this polymerization was proposed, which effectively predicts the composition (terminal structure) of the polyester. 

The reasons behind this accelerated rate behavior have been attributed to a decrease in chain transfer processes (28,29) and a decreased termination rate (24,25) indicated by molecular weight measurements (26). Recently, direct evidence of decreases in the termination rate have been shown (27) and in these studies both the termination and propagation kinetic constants were determined for polymerizations exhibiting enhanced rates in a smectic phase. The propagation constant, kp, decreases slightly in the ordered phase from the isotropic polymerization. Such a decrease would be expected because of the lower temperature in the smectic phase. The termination kinetic constant, kt, however, decreases almost two orders of magnitude for the ordered polymerization, indicating a dramatically suppressed termination rate. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

Anionic polymerisation of hydrocarbon monomers is initiated by lithium butyl to produce a living polymer the association number of which in solution is required to elucidate the kinetics. When the living polymer (for example polystyryl lithium) is terminated, the polystyrene can be isolated and a solution then made to determine its molecular weight, M. If the living polymer is associated in solution, the ratio of its... 

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