“…Orders higher than 2 were observed herein after temperature corrections (center column, Table ) and are a result of this effect. The concentration of protic impurities has been previously determined to be typically about 0.002 M; therefore, a correction was made to yield an effective TiCl 4 concentration, [TiCl 4 ] eff = [TiCl 4 ] − 2[HA], where [HA] = 0.002 M, which was used for subsequent reaction order analyses. For condition A, the TiCl 4 order after this correction was slightly less than two, i.e., 1.86 (right column, Table ).…”
Section: Resultsmentioning
confidence: 99%
“…Contributing to the controversy is the fact that conditions involving low [TiCl 4 ] are particularly ill-suited for careful kinetic/mechanistic investigation and necessitate corrections to [TiCl 4 ] and k app so that an accurate propagation reaction order in TiCl 4 can be measured. When [TiCl 4 ] is low and similar to the [LB] (∼concentration of protic impurities, HA, ∼2 × 10 -3 M), it is difficult to accurately assign the effective concentration of TiCl 4 due to its loss through the proton trapping function, as shown in Figure , and possible complexation with LB. The true [TiCl 4 ] is known with greater certainty when it is high relative to [LB] and [HA].…”
“…Orders higher than 2 were observed herein after temperature corrections (center column, Table ) and are a result of this effect. The concentration of protic impurities has been previously determined to be typically about 0.002 M; therefore, a correction was made to yield an effective TiCl 4 concentration, [TiCl 4 ] eff = [TiCl 4 ] − 2[HA], where [HA] = 0.002 M, which was used for subsequent reaction order analyses. For condition A, the TiCl 4 order after this correction was slightly less than two, i.e., 1.86 (right column, Table ).…”
Section: Resultsmentioning
confidence: 99%
“…Contributing to the controversy is the fact that conditions involving low [TiCl 4 ] are particularly ill-suited for careful kinetic/mechanistic investigation and necessitate corrections to [TiCl 4 ] and k app so that an accurate propagation reaction order in TiCl 4 can be measured. When [TiCl 4 ] is low and similar to the [LB] (∼concentration of protic impurities, HA, ∼2 × 10 -3 M), it is difficult to accurately assign the effective concentration of TiCl 4 due to its loss through the proton trapping function, as shown in Figure , and possible complexation with LB. The true [TiCl 4 ] is known with greater certainty when it is high relative to [LB] and [HA].…”
“…However, the rate at −40 °C is significantly higher in the absence of an additive, and it is this effect that causes the relative insensitivity toward temperature of the conventional polymerizations. According to literature reports, the dramatically higher rate in the absence of an Lewis base is due to uncontrolled initiation by protic impurities. ,,, 11 First-order kinetic plots for IB control polymerizations (absence of additives) at various temperatures. [IB] 0 = 1.0 M; [ t -Bu- m -DCC] 0 = 1.19 × 10 -2 M; [TiCl 4 ] nom = 0.12 M; 60/40 Hex/MeCl cosolvents (v/v).…”
Section: Resultsmentioning
confidence: 99%
“…Peak deconvolution of the RI GPC traces revealed that the high MW distributions comprised approximately 90, 64, and 23 wt % of the sample for polymerization temperatures of −40, −60, and −80 °C, respectively. On the basis of literature reports, ,,, we initially assumed that the high molar mass distribution resulted from fast, uncontrolled protic initiation; however, the results of two different analyses showed this to be incorrect. First, the overall number-average molecular weight (both modes of the distribution combined) was equal to the target (number-average DP = 84) for all three temperatures.…”
Section: Resultsmentioning
confidence: 99%
“…Assuming IB polymerizations mediated by n -Bu 4 NCl are converted from “conventional” to quasiliving strictly due to the suppression of free ions via the production of the common anion Ti 2 Cl 9 - , it stands to reason that Lewis bases must operate in a similar fashion. It is well established that externally added Lewis bases scavenge protic impurities; , the product(s) formed from such processes are onium salts, which in the presence of excess TiCl 4 , possess the counteranion, Ti 2 Cl 9 - . Formation of common anions is shown below for the case of water (B = a Lewis base such as DMP)…”
Isobutylene was initiated using 5-tert-butyl-1,3-di(2-chloro-2-propyl)benzene/TiCl4 in 60/ 40 hexane/methyl chloride: [IB]0 ) 1.0 M, [TiCl4] ) 0.12 or 0.24 M, [t-Bu-m-DCC] ) 0.0119 M, T ) -(40-80) °C. Most polymerizations contained a Lewis base or other additive, i.e., 2,4-dimethylpyridine, 2,6-di-tert-butylpyridine, tetra-n-butylammonium chloride, and/or pyridine hydrochloride. Polymerizations containing an additive yielded theoretical molecular weights, narrow polydispersity index, and apparent absence of irreversible chain termination (linear kinetic plots, ATR-FTIR spectroscopic data) and chain transfer, with two exceptions: coupled product was obtained at -40 °C, and protic initiation occurred with n-Bu4NCl alone. Polymerizations without an additive produced bimodal molecular weight distributions; however, essentially all chains were initiated from t-Bu-m-DCC. With an additive, Eact for propagation was -(5.3-5.5) kcal/mol. Removal of additives increased polymerization rate moderately at -80 °C but dramatically at -60 and -40 °C; this yielded higher Eact compared to that of systems containing additives. These results indicated that both paired and unpaired (free) ions are propagating species in absence of additives, with free ions less important at lower temperatures; free ion concentration and lifetime suggested the presence of adventitious common ions and chain transfer between free ions and tert-chloride-terminated PIB chains. The primary role of additives is suppression of free ions through in situ production, via the scavenging of protic impurities, of common ions.
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