Initiator Transfer Constants

Different initiators have varying transfer constants (Table 3-5). Further, the value of C) for a particular initiator also varies with the reactivity of the propagating radical. Thus there is a fivefold difference in C) for cumyl hydroperoxide toward poly(methyl methacrylate) radical compared to polystyryl radical. The latter is the less reactive radical see Sec. 6-3b. 

Azonitriles have generally been considered to be cleaner initiators in the sense of being devoid of transfer, but this is not true [Braks and Huang, 1978]. The transfer with azonitriles probably occurs by the displacement reaction 

Many peroxides have significant chain-transfer constants. Dialkyl and diacyl peroxides undergo transfer by 

The typical effect of initiator chain transfer [Baysal and Tobolsky, 1952] can be seen graphically in Fig. 3-6. The decrease of polymer size due to chain transfer to initiator is much less than indicated from the Ci values because it is the quantity Ci[I]/[M], which affects Xn (Eq. 3-109b). The initiator concentrations are quite low (10 4 I0 2 M) in polymerization, and the ratio [I]/[M] is typically in the range 10 3-10 s. 

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A plot of the experimental data as the left side of Eq. 3-110 versus Rp yields a straight line whose slope is (C kt/ /Trf M 3). The initiator transfer constant can be determined from the slope because the various other quantities are known or can be related to known quantities through Eq. 3-32. When chain transfer to monomer is negligible, one can rearrange Eq. 3-109b to yield... 

A plot of the left side of Eq. (6.155) versus [I]/[M] yields a straight line whose slope is Cj. Some values of initiator transfer constants are listed in Table 6.10. 

Values of Cm for some common monomers are listed in Table 6.5. These being representative values, Cm is seen to be small (typically, in the range of 10 to 10 ). Since chain transfer to monomer cannot be avoided, the highest molecular weight that can be obtained for a polymer, in the absence of all other transfer reactions, will be limited by the value of Cm (see Problem 6.19). Table 6.6 lists some values of initiator transfer constants (Ci). 

At high initiator concentrations, chains can also be transferred to the initiator. In analogy to the other transfer constants, the initiator transfer constant Cj is given as... 

Except in the rare case of self-initiated polymerization, transfer to initiator is also unavoidable, but as long as initiator concentration is kept low, its impact on overall reaction kinetics is small. For some common initiators, transfer constants Q are given in Table 1.3. 

This suggests that polymerizations should be conducted at different ratios of [SX]/[M] and the molecular weight measured for each. Equation (6.89) shows that a plot of l/E j. versus [SX]/[M] should be a straight line of slope sx Figure 6.8 shows this type of plot for the polymerization of styrene at 100°C in the presence of four different solvents. The fact that all show a common intercept as required by Eq. (6.89) shows that the rate of initiation is unaffected by the nature of the solvent. The following example examines chain transfer constants evaluated in this situation. 

Chain transfer to initiator or monomer cannot always be ignored. It may be possible, however, to evaluate the transfer constants to these substances by investigating a polymerization without added solvent or in the presence of a solvent for which Cgj is known to be negligibly small. In this case the transfer constants Cjj and Cj determined from experiments in which (via... 

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

The thiol ( -dodecyl mercaptan) used ia this recipe played a prominent role ia the quaUty control of the product. Such thiols are known as chain-transfer agents and help control the molecular weight of the SBR by means of the foUowiag reaction where M = monomer, eg, butadiene or styrene R(M) = growing free-radical chain k = propagation-rate constant = transfer-rate constant and k = initiation-rate constant. 

Thus the thiol 0 2 25511 is capable of terminating a growiug chain and also initiating a new chain. If the initiation-rate constant, k is not much slower than the propagation-rate constant, the net result is the growth of a new chain without any effect on the overall polymerization rate (retardation). That represents a tme chain transfer, ie, no effect on the rate but a substantial decrease iu molecular weight (12). 

Cumene hydroperoxide [95], benzoyl peroxide, or tert-h iiy peroxide [96]. can be used as accelerators with alkylboron initiators. The chain transfer constant for MMA to tributylborane has been estimated to be 0.647, which is comparable to tripropylamine [97]. 

Transfer to initiator introduces a new end group into the polymer, lowers the molecular weight of the polymer, reduces the initiator efficiency, and increases the rate of initiator disappearance. Methods of evaluating transfer constants are discussed in Section 6.2.1. 

The ratio kjkp is called the transfer constant (Ctr) and Ct, C, and CM are the transfer constants for transfer to transfer agent, initiator and monomer respectively. Appropriate substitution gives eq. 3 ... 

The cyclohcxadicnc 84 is a good H donor but the cyclohcxadicnyl radical 85 is slow to react and fragments to provide the silyl radical 86 which initiates polymerization. The reported transfer constant for 84 in styrene polymerization at 80 °C is very low (0.00045).ni>... 

It is possible to exercise control over this form of compositional heterogeneity (i. e. the functionality distribution) by careful selection of the functional monomer and/or the transfer agent taking into account the reactivities of the radical species, monomers, and transfer agents, and their functionality.11 250 Relative reactivities of initiator and transfer agent-derived radicals towards monomers are summarized in Section 3.4. Some values for transfer constants are provided in Chapter 6. 

Since the experiment is no longer reliant on the dithiocarbatnyl radical to both initiate and terminate chains (cf Section 9.3.2.1), lower reaction temperatures may be used (where the dithiocarbamyl radical is slower or unable to add monomer) and better control over the polymerization process can be obtained. The transfer constants for the benzyl dithioearbamates in polymerization of acrylic and styrcnic... 

The processes described in this section should be contrasted with RAFT polymerization (Section 9.5.3), which can involve the use of similar thioearbonylthio compounds. A. A -dialkyl dithiocarbamates have very low transfer constants in polymerizations of S and (mctb)acrylatcs and arc not effective in RAFT polymerization of these monomers. However, /V,A -dialkyl dithiocarbamates have been successfully used in RAFT polymerization of VAc. Certain O-alkyl xanthates have been successfully used to control RAFT polymerizations of VAc, acrylates and S. The failure of the earlier experiments using these reagents and monomers to provide narrow molecular weight distributions by a RAFT mechanism can he attributed to the use of non-ideal reaction conditions and reagent choice. A two part photo-initiator system comprising a mixture of a benzyl dithiocarhamate and a dithiuram disulfide has also been described and provides better control (narrower molecular weight distributions).43... 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

For very active RAFT agents, the RAFT agent derived radical (R ) may partition between adding to monomer and reacting with the transfer agent (polymeric or initial). In these circumstances, the transfer constant measured according to the Mayo or related methods will appear to be dependent on the transfer agent concentration and on the monomer conversion. A reverse transfer constant can be defined as follows (eq. 19)... 

As shown by the data in Fig. 31, the chain transfer constant of this initiator, Q = 1.0. In this context it is of interest to remember that the effect of initiator concentration on the molecular weight of HSi-PaMeSt was negligible, probably because of unfavorable thermodynamics (Sect. III.B.3.b.iv.). In contrast, with isobutylene chain transfer from the propagating carbenium ion to initiator is thermodynamically favorable (see Sect. IH.B.4.b.i.). Thus it is not surprising to find a large Q. The chain transfer mechanism has been illustrated in Scheme 5. 

Figure 39.4a represents schematically the intravenous administration of a dose D into a central compartment from which the amount of drug Xp is eliminated with a transfer constant kp. (The subscript p refers to plasma, which is most often used as the central compartment and which exchanges a substance with all other compartments.) We assume that mixing with blood of the dose D, which is rapidly injected into a vein, is almost instantaneous. By taking blood samples at regular time intervals one can determine the time course of the plasma concentration Cp in the central compartment. This is also illustrated in Fig. 39.4b. The initial concentration Cp(0) at the time of injection can be determined by extrapolation (as will be indicated below). The elimination pool is a hypothetical compartment in which the excreted drug is collected. At any time the amount excreted must be equal to the initial dose D minus the content of the plasma compartment Xp, hence ...

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