Tetrahydrofuran polymerization, determination

Gel Permeation Chromatography. The instrument used for GPC analysis was a Waters Associates Model ALC - 201 gel permeation chromatograph equipped with a R401 differential refractometer. For population density determination, polystyrene powder was dissolved in tetrahydrofuran (THF), 75 mg of polystyrene to SO ml THF. Three y -styragel columns of 10, 10, 10 A were used. Effluent flow rate was set at 2.2 ml/min. Total cumulative molar concentration and population density distribution of polymeric species were obtained from the observed chromatogram using the computer program developed by Timm and Rachow (16). 

Both polymers 10 and 11 are soluble in common organic solvents, melt without decomposition, and can be drawn into the fibers. Molecular weights of the polymers 10 and 11, determined by gel permeation chromatography with tetrahydrofuran as the eluant after purification by reprecipitation from benzene-ethanol, showed a broad monomodal molecular weight distribution. The degree of polymerization depends on particle size of sodium metal. Polymers with molecular weights of 23,000-34,000 are always obtained, if fine sodium particles are used. 

Hiller and Funke obtained easily dissolvable linear macromolecules of PVS by anionic polymerization of 1,4-DVB up to conversions of 80-90% [230,231]. In these experiments very low concentrations of n-butyl lithium (n-BuLi) were used and tetrahydrofuran (THF) as solvent. The reactions were carried out at -78 °C and for 7 min. The contents of pendant vinyl groups in the polymer were determined by infrared spectroscopy, mercury-II-acetate addition and catalytic... 

Fig. 7-1 Determination of the equilibrium monomer concentration [M]c for the (CiHs O1 (Blvi) initiated polymerization of tetrahydrofuran in dichloroethane at 0°C. After Vofsi and Tobolsky [1965] (by...

Small amounts of polar solvents such as tetrahydrofuran, ether, dioxane and triethylamine have been shown to break down the association of organo-lithium compounds in non-polar solvents, and to greatly increase the rate of chain initiation. In polar solvents, therefore, one expects rapid initiation and a polymerization rate essentially determined by the rate of chain propagation of solvated ion-pairs. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

In the polymerization of 1,3-dioxolane and tetrahydrofuran it has been shown additionally that concentration of active centers is constant throughout the polymerization (both by direct determination and from analysis of polymerization kinetics). In some other polymerizations, believed to proceed as living processes, only the moderate molecular weights regions (M < 105) were studied thus, for example, no very high molecular weight polymers were obtained in the polymerization of oxazolines. 

This phenomenon is in remarkable contrast to the cases of alanine NCA and 7-glutamate NCA, where a selection of monomer antipodes by a-helical polymer chain is observed. The lack of selectivity of the growing poiy(valine) chains cannot be attributed to the heterogeneity of the reaction mixture due to the insolubility of the polymer, since the polymerization of alanine NCA in tetrahydrofuran does demonstrate the selectivity, as mentioned previously, despite the heterogeneous reaction observed. The determining factor seems to be the different conformation acquired by the growing poly(valine) as compared to the a-helices of poly(7-benzyl glutamate) and poly(alanine), both in solution and solid state. 

Only one kinetic study exists on initiation of methacrylate polymerization by a sodium compound. The initiator was the disodium oligomer ( tetramer ) of a-methylstyrene and polymerization was investigated at 25°C in toluene in presence of 0.05—0.2 mole fraction of tetrahydrofuran [181]. An internal first order disappearance of monomer was observed, the first order coefficient being directly proportional to active chain and tetrahydrofuran concentrations. The rate coefficients evaluated, e.g. fep = 3.1—13 X 10 1 mole sec at various tetrahydrofuran concentrations, are much lower than those for lithium initiators. They were, however, evaluated using a methyl iodide titration technique to estimate the active chain concentration. In view of the reactivity of tritiated acetic acid with many short chains which are clearly not active in chain propagation, there must be suspicion of similar behaviour with methyl iodide. If this happens, the active chain concentration would be over-estimated and the derived fep value would be too low. Unfortunately no molecular weights of the precipitable polymer were determined, so that it is impossible to check on active chain concentration using this alternative method. 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

In a more recent study Sigwalt et al. [41] investigated the use of carbazyl sodium to initiate PS polymerization in tetrahydrofuran (THF). This initiator produced only one living end per chain. Excellent agreement with the results obtained on amphianionic polymer was obtained. At levels of living ends below 10 molel the rate coefficient found for ion pairs was 3 x 10" 1 mole sec and for free ions 4 1 mole sec . The dissociation constant (determined kinetically) was 6.4 x 10 mole 1 . ... 

The amyl alcohol method showed that at —78°C only diethyl ether/ SnCl4, diethyl ether/HC104 and THF/SnCl4 gave fast and complete initiation. After a polymerization time of 2—5 min and a conversion of less than 10%, 75% of the active cations could be determined. Under these conditions initiation is much faster than propagation. The amount of available cations in tetrahydrofuran did not change in hours and even with diethyl ether only a very small decrease was detectable. In contrast, with BF3. etherate in diethyl ether at —78°C only 10—20% active polymeric cations could be detected, and with toluene/SnCl4 only 2—4%, after 5 min. 

Studies of such systems are reported in the literature. Worsfold and Bywater (28) determined kp for the anionic homopolymerization of a-methylstyrene in tetrahydrofuran solution and Allen, Gee, and Stretch (I, 2) studied the polymerization of styrene in dioxane. Both groups utilized the dilatometric technique to follow the reaction and show the absence of termination. 

The activation energy of anionic propagation in the homopolymerization of styrene was determined to be about 1 kcal. per mole. This value refers to the reaction proceeding in tetrahydrofuran solution. The activation energy for the same reaction in dioxane was reported (1, 2) to be 9 3 kcal. per mole. This is one of many examples which stresses the importance of a solvent in ionic polymerization. 

As stated above. It has been established that carboxylate Ions are the true Initiators in B lactone polymerization. In an early study. Hall ( ) stated that In Intermediate pH ranges anions or water Itself attack the CH2 group with alkyl-oxygen cleavage by SN2 reaction that proceeds most rapidly In anhydrous acetonitrile or tetrahydrofuran. From heats of combustion he determined the heat of polymerization of plvalolactone AH j. to be -20.1 kcal mol l. 

Problem 8.11 Polymerization of styrene with sodium naphthalene initiator was performed at 25°C in tetrahydrofuran (THF) using a static technique [8] that is suitable for monitoring fast reactions. The conversion was determined by monitoring the residual styrene monomer spectrophotometrically during polymerization and the concentration of living ends [M ] was determined spectrophotometrically at the end of the experiment. In independent experimental series, the overall rate constant kp was obtained [cf. Eq. (P8.10.2)] both at different concentrations of initiator (and hence [M ]) without addition of electrolyte and at different concentrations of sodium ions from externally added sodium tetraphenyl borate (NaBPh4) salt and constant concentration of initiator. The data are given below ... 

Problem 10.4 (a) The kinetics of polymerization of tetrahydrofuran was studied [6] with the use of triethyloxonium tetrafluoroborate, (C2Hs)30+BF4, as im tiator and dichloromethane as solvent. Conversion versus time was measured at 0°C with initial catalyst concentration [I]o = 0.61 x 10 mol/L and monomer concentration [M] varying from 3 to 9 mol/L. The initial rates, Rp, determined from these data are given in Table A. 

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