Propylene polymerization, catalyst analysis

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

A kinetic model has been proposed based on microstructural analysis, including both chain-epimerization and site-epimerization reactions in both C2- and C.-symmctric metallocenes, and rationalizing the observed pseudo-second-order kinetics of propylene polymerization promoted by C2-symmetric metallocene catalysts. This point has been extended to co-polymers.298 A thorough study of propylene polymerization with the Me2C(Cp)(9-Flu)ZrCl2 system in the presence of a large series of different counterions that rationalized the correlation between the nature of ion pair and the microstructure of the resulting PPs has been performed.104... 

As mentioned above, the activation of the metallocene dichlorides 1 and 2 with MAO provides very efficient catalysts for polymerization of propylene to high molecular weight, highly crystalline s-PP polymers. Detailed propylene polymerization conditions, results, and polymer analysis with 1 and 2/MAO catalyst systems are presented in Tables 1 and 2. Inspection of the data presented in Table 1 reveals, aside from the fact that all the produced polyqjropylene polymer samples are syndiotactic in nature (apparent from the large mx pentads), other important informatirHi. 

As an example of the use of MIXCO.TRIAD, an analysis of comonomer triad distribution of several ethylene-propylene copolymer samples will be delineated. The theoretical triad Intensities corresponding to the 2-state B/B and 3-state B/B/B mixture models are given In Table VI. Abls, et al (19) had earlier published the HMR triad data on ethylene-propylene samples made through continuous polymerization with heterogeneous titanium catalysts. The data can be readily fitted to the two-state B/B model. The results for samples 2 and 5 are shown In Table VII. The mean deviation (R) between the observed and the calculated Intensities Is less than 1% absolute, and certainly less than the experimental error In the HMR Intensity determination. 

It has been suggested however that isotacticity derives from polymerization occurring on colloidal particles formed by thermal decomposition of the catalysts. As stated previously, in the presence of the monomer even the allyl compounds are stable at 65°C and none of the thermal decomposition products (black to yellow solids) could be detected. As a check on these results a polymerization of propylene was carried out with Zr (benzyl) 4 in toluene at 0°C in a sealed tube. The reaction was very slow and analytical quantities of polymer could be obtained only after 312 hr. NMR analysis showed peaks assignable to isotactic sequences, and these were much stronger than the peaks assignable to syndiotactic diads. It was concluded... 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Another example is illustrated in the relationship between the specific rotation and the microstructure of polypropylene oxide reported by Price. Optically active propylene oxide and racemic propylene oxide-a-d were polymerized under otherwise identical conditions by the freeze-dried ZnEt2-H20 (1 0.7) catalyst system containing varying amounts of ZnEt2. A linear relationship was observed between specific rotation of the former polymer and the tail-to-tail dyad content of the latter (Fig. 14). This result proves quantitatively that the decrease in the specific rotation of polymer prepared by several catalysts is due to the presence of head-to-head and tail-to-tail linkages, and also provides supporting evidence for our microstructure analysis. 

Added in proof. Recently the analysis of 13C-nmr spectra of polypropylene oxide provided a new information about the triad of methine carbon atom and dyad of methylene carbon in the main chain (9/). Applications of this analysis should give a valuable contribution to the more detailed understanding of the polymerization reaction of propylene oxide by organometallic catalysts. 

From the analysis of 13C NMR spectra of polypropylenes, Doi96) found that the sequence distribution of inverted propylene units follows first-order Markov statistics. Table 4 lists the two reactivity ratios rQ and rt, for the polymerization of propylene with the soluble catalysts composed of VC14 and alkylaluminums at — 78 °C ... 

Graft copolymer, polyclhylene-gra/f-poly(propylcnc oxide) (PE-g-PPG), has been synthesized by ring opening anionic polymerization of propylene oxide with a phosphazene catalyst and hydroxylated polyethylene (Mn = 12400, [OH] = 5 units/chain). Polymerization of propylene oxide was carried out in tetraline at 120 °C for 20 hours. The 13C NMR analysis of PE-g-PPG suggested that all the hydroxyl groups were consumed for propylene oxide polymerization (Fig. 6). 

When polymerization is carried out in a solvent, and a solid catalyst is employed, the gaseous poison is distributed among gaseous, liquid and solid phases and the equilibration may take some time. The adsorbed amount of the poison usually forms a minor part of the injected quantity. Thus, its consumption by side processes makes the determination of the adsorbed amount of the poison less certain. A more favourable case is the gas phase polymerization, where a more suitable (much lower) ratio of the poison amount in the gaseous and solid phases can be achieved. Doi et al.102) document a fast and quantitative insertion of CO into transition metal-carbon bonds on the basis of the GC analysis in the gas phase polymerization of propylene. These data may require an additional study of other potential reactions of CO in the system. 

In an attempt to prepare polycyclopentadiene which would be stable in toluene solution, the polymer was hydrogenated over a platinum oxide catalyst in a Parr bomb immediately after the completion of the polymerization reaction. Infrared analysis indicated the presence of residual unsaturation and the polymer became insolubilized on standing. An attempted copolymerization of cyclo-pentadiene with propylene gave a product whose infrared spectrum indicated the presence of C-methyl groups but which was still insoluble in toluene. No attempt was made to determine whether copolymerization had occurred. 

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