Bibliothek

Notizen: Improvements in the 13C NMR Grant and Paul parameters as applied to polymers have been obtained by noting that the “corrective terms” can be temperature-sensitive. A corresponding temperature sensitivity was not observed for the primary parameters, α through ∊. Results are given for a hydrogenated polybutadiene and six different ethylene-1-olefin copolymers where the standard deviation between calculated and observed chemical shifts has been improved from 1.04 to 0.30 ppm. Since the “corrective terms” in the Grant and Paul empirical analyses of chemical shifts reflect the conformational character of polymers, it is shown that values for these terms can best be obtained directly from the system under study.

Notizen: Spin-lattice relaxation times (T1) for methyl, methylene, and methine carbons in an amorphous polypropylene have been measured as a function of temperature from 46 to 138°C. The carbons from isotactic sequences characteristically exhibited the longest T1's of those observed. The T1 differences increased with temperature with the largest difference occuring for methine carbons where a 32% difference was observed. Activation energies were determined for the motional processes affecting T1's for isotactic and syndiotactic sequences with essentially no dependence upon configuration noted.

Notizen: The 1,4-1,2 additions in three hydrogenated polybutadienes are described in terms of triads of monomer units, average sequence lengths, and branches per 1000 carbon atoms. A test for conformity with Markov statistical behavior shows that the measured triad distributions can be closely reproduced assuming a first-order model. Transition probabilities from the statistical analysis can be used to calculate the relative concentration of any desired sequence. Steric factors during polymerization may lead to the first-order bias in the monomer distribution.

Notizen: 13C NMR configurational assignments are made for an amorphous polystyrene sample examined at 25.2 MHz and 120°C. The assignments are based strictly on a one-parameter Bernoullian fit that was in satisfactory agreement with the nine observed methylene relative intensities. The methylene regions of the 13C NMR spectra of a polystyrene were examined before and after hydrogenation of the side-chain phenyl substituents. It was concluded that ring current effects have influenced the 13C methylene chemical shifts substantially and are limited largely to contributions from adjacent phenyl substituents. In addition, aromatic substituent parameters are reported that can be used in conjunction with the Grant and Paul parameters for calculating chemical shifts in aromatic hydrocarbons and polymers. Finally, it is concluded that free-radical and n-butyllithium-prepared polystyrenes have essentially atactic structures with meso additions favored over racemic additions by approximately 55/45.

Notizen: Sequence lengths of stereochemical additions in vinyl polymers are described in terms of the number average lengths of like configurational placements. Under these circumstances, a pure syndiotactic polymer has a number average sequence length of 1.0; a polymer with 50:50 meso, racemic additions has a number average sequence length of 2.0 and polymers with more meso than racemic additions have number average sequence lengths greater than 2. Amorphous and crystalline polypropylenes and an amorphous polystyrene are examined using 13C NMR as examples of the applicability of the average sequence length method. The results appear to be accurate for amorphous and semicrystalline polymers but limitations are present when this method is applied to highly stereoregular vinyl polymers containing predominantly isotactic sequences.

Notizen: Monomer distributions and number-average sequence lengths are determined from 13C nuclear magnetic resonance (NMR) data for 1,4- and 1,2-butadiene additions and styrene additions in a series of four hydrogenated butadiene-styrene copolymers. The monomer distribution is expressed in terms of the six unique days from which it is possible to calculate the number-average sequence length of each monomer type. Carbon-13 NMR spectral assignments are given and the techniques for making the assignments are discussed. The method presented could, in principle, be applied to any copolymer or terpolymer. Limitations that are encountered in the analysis of hydrogenated butadiene-styrene copolymers high in 1,2 additions are discussed.

Notizen: The terpolymerization composition equation has been modified to eliminate the consideration of interactions between monomers 2 and 3 when they are present in low concentration in the feed mixture relative to monomer 1. Terpolymers with a wide variety of comonomers and compositions have been synthesized and used to demonstrate that a simplified terpolymerization equation accurately predicts terpolymer composition.

Notizen: More than a hundred propylene polymerizations were carried out with the CW catalyst (our particular MgCl2/ethylbenzoate/p-cresol/AlEt3/TiCl4 supported high mileage catalyst). Highest I.I. index (% yield of boiling heptane insoluble product) of 96.2 ± 0.9 was obtained at [Ti] = 2.4 × 10-4 M, A/T (amount of AlEt3 with 0.33 equivalent of methyl-p-toluate to amount of Ti in the catalyst) = 167 at 50°C. The I.I. values became lower when any one of these variables was changed. The I.I. values did not change with time of polymerization, indicating that both stereospecific and nonstereospecific sites were produced at the same time and polymerized monomers during the course of a polymerization. Estimates of maximum active site concentrations, [Ti*]0,Because of the complexity of the catalyst system, the active sites are designated as follows: [Ti*], active sites of all kinds at a given time; [Ti*]0 active sites of all kinds at time zero; [Ti1*], active sites of the first kind formed initially upon activation; [Ti2], active sites of the second kind, which were transformed from the former, and are responsible for olefin polymerization after the initial phase of rapid decay of activity; [Ti*]i, stereospecific active sites; [Ti*]a, nonstereospecific active sites, and [Ti*]t = [Ti*]i + [Ti*]a Similarly, the subscripts 1 and 2 for the various rate constants refer to active sites Ti1* and Ti2*, respectively. Finally [Ti] is the concentration of total titanium in the amount of catalyst used. [Ti-P] is the titanium polymer bond concentration as determined by 14CO tagging; [Ti-P] (1 h) and [Ti-P] (48 h) are the values obtained with indicated time of contact of 14CO with the polymerization mixture in the obsence of monomer. were obtained from a variation of vn-1 versus t-1. The values of [Ti*]0.i and [Ti*]0,a for the stereospecific and nonstereospecific sites, respectively, are in excellent agreement with those values of [MPB]0 (metal polymer bond concentrations at t = 0) determined earlier by radiotagging with tritiated methanol. The rate of formation of [Ti*]1 (the initial active site) is first order with respect to [Ti] and [A] with an activation energy of 12 kcal mol-1 where [A] is the AlEt3 concentration. The rate constants of propagation at 50°C are kp,i ∼ 160M-1s-1 and kp,a ∼ 11M-1s-1. The activation energy for the stereospecific propagation is about 4.1 kcal mol-1. At 50°C the rate of polymerization decreases according to second order kinetics suggesting bimolecular processes which transform one-half to one-fourth of the Ti1* site to Ti2* types depending upon experimental conditions, while the remainder decay to inactive species. The values of kt1 lie between 19 and 61M-1s-1. These processes are more complicated at 70°C involving two consecutive reactions; at low [A], the data fits better with first order decay kinetics. Comparison of the [Ti2*] values and the values of [Ti-P] obtained by 14CO tagging suggests that CO reacts primarily with the Ti2* sites and very little with the initially formed Ti1* sites. The Ti2* sites are slightly less active than the Ti1* sites having kp2,i ∼ 86M-1s-1 and kp2,a ∼ 7M-1s-1 at 50°C.

Notizen: The following criteria are proposed to judge whether a coordination polymerization may be diffusion controlled or not: (1) If the number-average molecular weight and polydispersity of the polymer calculated from kinetic rate constants as a function of time agree with the experimental values, the polymerization is not diffusion controlled. (2) The polymerization may be diffusion controlled if the Thiele modulus, the ratio of the characteristic diffusion time to the characteristic reaction time, is much greater than unity; if it is much smaller than unity, the polymerization is reaction controlled. (3) If an initial linear dependence of rate of polymerization on catalyst concentration changes over to a square-root dependence, the polymerization may be diffusion limited. (4) The polymerization is likely to be diffusion limited if the instantaneous rate of polymerization is proportional to the rate of particle growth when the proportionality coefficient is the surface area of the particle. Criterion (1) is a necessary and sufficient condition as stated, as its converse is not true. All the other criteria are merely necessary but not sufficient conditions. The established Ziegler-Natta catalysts have activities too low to cause diffusion limitation; the Phillips catalyst system is likely to be diffusion limited. The polydispersity of polyolefins produced with Ziegler-Natta catalysts are not the consequence of diffusion control but are the characteristics of the catalysts in their kinetics of initiation, propagation, chain transfer, and termination.

Notizen: Two methods were used in an attempt to determine by radioquenching the active site concentration, [Ti*], in a MgCl2 supported high activity catalyst. For the reactions of tritium labelled methanol, the kinetic isotope effects were first determined: kH/kT = 1.63 for the total polymer and 1.67 for the isotactic polypropylene fraction. Polymerizations were quenched with an excess of isotopic CH3OH after various lengths of time, at different A/T (amount of AlEt3 with 0.33 equivalent of methyl-p-toluate to amount of Ti in the catalyst) ratios, and temperatures. From the known specific activity of tritium in CH3OH and radioassay of the polymer, value of the total metal polymer bond, [MPB], can be obtained. [MPB] increases linearly with polymerization time. Extrapolation to t = 0 gives [MPB]0, which should be close to [Ti*] because chain transfer with aluminum alkyls to produce Al-P bonds is negligible during very early stage of the polymerization. The values of [MPB]0 range from 7-30% of the total Ti; the number of MPB is nearly equally distributed in the amorphous and isotactic fractions of polypropylene in most runs. The rate of incorporation of radioactive CO into polymers produced by the MgCl2 supported high mileage catalyst is far slower than that claimed by some investigators for TiCl3 type catalysts. There is an initial rapid phase of incorporation of CO which lasts for about 1 hr of contact time. The subsequent rate of CO incorporation steadily declines, yet there is no constant maximum value of radioactivity even after 48 h of reaction in the absence of monomer. Radioquenching of polymerizations with CO was also performed at several temperatures and A/T ratios. In all cases, the maximum [Ti-P] was reached after 30-40 min of polymerization, whereas the maximum rates of polymerization, Rp,m, occurred within 3-10 min. In fact, the rate of polymerization decays to a small fraction of Rp,m after 30-40 min. Furthermore, this maximum value of [Ti-P] remains constant until the end of polymerization (t = 90 min). Therefore, isotopic CO is not reacting with the initially formed active sites Ti1*, but only with those sites, Ti2*, which predominate during the later stage of polymerization.