Chain-Transfer to Monomer

According to the Mayo procedure [36], for polymerizations at low initiation and thus also at low termination levels, the inverse (average) degree of poly- 

60) as compared to the situation were no chain transfer is present. 

Based upon the observations discussed above, it seems advantageous to conduct these single-pulse experiments at lower temperatures as transfer usually exhibits a higher activation energy than propagation (although experimental investigations were not able to reveal this [85]). 

In general terms, chain transfer to monomer can be written 

Chain transfer to vinyl monomer involves transfer of a 8-proton from the carbo-cation to a monomer molecule. Considering, for example, the polymerization of isobutylene, the chain transfer to monomer can be represented by the equation 

As a new propagating species is generated each time a growing chain is terminated by transfer to monomer, many polymer molecules can result for each molecule of catalyst-cocatalyst complex initially formed. 

The ratio ktr,ulkp defines the monomer chain transfer constant Cm and, as in the case of free-radical polymerization (Section 6.8.1), its value determines the polymer molecular weight, in the absence of other chain termination processes. 

Another type of chain transfer to monomer involves hydride ion transfer from monomer to the propagating center (Kennedy and Squires, 1965, 1967 Odian, 1991)  

The first theoretical papers on anionic chain transfer to monomer were published long ago [2]. Later on, the effect of chain transfer to monomer on the molecular characteristics of the polymers formed in nonterminating polymerization was considered in a number of publications [28-33]. Usually, chain transfer is described according to a scheme that is similar to the free radical polymerization [21] 

According to Eq. (3.17), the evolution of growing R(l) and dead P(l) macromolecules in time is described by differential equations  

Since the rate constant (reactivity) of transfer fetm is much less than the propagation rate kp, the consumption of monomer due to the chain transfer may be neglected. Then, the dependence of monomer conversion on time is given by the same 

The effect of simultaneous chain transfer to monomer and solvent was considered in Refs [28, 29]. Since the number of polymeric chains due to the two reactions increases additively, it is easy to calculate P  

The effect of chain transfer to monomer for multifunctional initiators was considered in Ref. [34]. Because of the complexity of the original differential equations, the author managed to derive analytical expressions without approximations only for P However, taking into account that km/fep is much less than unity and using methods of statistical moments, it was rather easy to derive the following analytical expressions for the evolution of average DPs with conversion [35] 

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The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

Chain transfer to monomer is the main reaction controlling molecular weight and molecular weight distribution. The chain-transfer constant to monomer, C, is the ratio of the rate coefficient for transfer to monomer to that of chain propagation. This constant has a value of 6.25 x lO " at 30°C and 2.38 x 10 at 70°C and a general expression of 5.78 30°C, chain transfer to monomer happens once in every 1600 monomer... 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small dial entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of n will usually be less than 0.4. In these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates or particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical termination, when it occurs in the particle phase, will usually be between a short species (one that lias just entered) and a long species. 

The key requirements for using Si-Cl functional initiators to produce polymers carrying Si Cl termini by carbenium ion polymerization are i) Si-Cl should be inert toward aUcylaluminum coinitiators, ii) Si-Cl should not react with propagating carbenium ions, in) chain transfer to monomer should be negligible so as to end up with one Si-Cl head-group per polymer chain. 

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

Assuming that the number average degree of polymerization (DP ) is determined by chain transfer to monomer and assuming unimolecular termination relative to propagation (i.e., chain breaking due to solvent, polymer, impurities are absent), the simple Mayo equation55 ... 

HSi(CH3)2CH2CH29)CH2Cl/Me3Al system is strong evidence for molecular weight control by termination, i.e., for polymerization without chain transfer to monomer. The proposition is further substantiated by results of model experiments of Kennedy et al.26 and H1 NMR analysis of HSi-PEB to be discussed in Sect. IH.B.4.C. 

Evidently, with the DCE/TiCl4 and TMPCl/TiCl4 systems chain transfer is operational under the experimental conditions examined. For example in Exp. 4, which represents the first point in a series of AMI experiments, chain transfer is absent (Ieff = 95%), however, in Exp. 5. (the 6th point in the same AMI series) Ieff = 253%, which is due to significant chain transfer to monomer. 

In sum, a relation of counteranion nucleophilicity and the molecular weight in isobutylene polymerization is discovered, according to which an increase in G nucleophility leads to an increase in the rate of termination but a decrease in the rate of chain transfer to monomer. Thus, an increase in G6 nucleophilicity leads to increased termination and hence decreased molecular weight for systems in which termination is molecular weight governing. Similarly, it leads to a decrease in rate of transfer and hence to an increase in molecular weights for systems in which chain transfer controls molecular weight. The nucleophilicity of G is determined by the... 

There is less information available in the scientific literature on the influence of forced oscillations in the control variables in polymerization reactions. A decade ago two independent theoretical studies appeared which considered the effect of periodic operation on a free radically initiated chain reaction in a well mixed isothermal reactor. Ray (11) examined a reaction mechanism with and without chain transfer to monomer. 

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