The structure of random ethylene/propylene (EP) copolymers has been modeled using step polymerization chemistry. Six ethylene/propylene model copolymers have been prepared via acyclic diene metathesis (ADMET) polymerization and characterized for primary and higher level structure using in-depth NMR, IR, DSC, WAXD, and GPC analysis. These copolymers possess 1.5, 7.1, 13.6, 25.0, 43.3, and 55.6 methyl branches per 1000 carbons. Examination of these macromolecules by IR and WAXD analysis has demonstrated the first hexagonal phase in EP copolymers containing high ethylene content (90%) without the influence of sample manipulation (temperature, pressure, or radiation). Thermal behavior studies have shown that the melting point and heat of fusion decrease as the branch content increases. Further, comparisons have been made between these random ADMET EP copolymers, random EP copolymers made by typical chain addition techniques, and precisely branched ADMET EP copolymers.

A structural investigation of precise ethylene/1-butene (EB) copolymers has been completed using step polymerization chemistry. The synthetic methodology needed to generate four model copolymers is described; their primary and higher level structure is characterized. The copolymers possess an ethyl branch on every 9th, 15th, and 21st carbon along the backbone of linear polyethylene. Melting points and heats of fusion decrease with increased branch frequency. Differential scanning calorimetry and infrared spectroscopy show highly disordered crystal structures favoring ethyl branch inclusion. On the other hand, the EB copolymers contain high concentrations of kink and gauche defects independent of branch frequency. These model copolymers are compared with random copolymers produced using traditional chain chemistry and previously synthesized ADMET EP copolymers.

A deuterium labeling study was undertaken to determine the mechanism of olefin isomerization during the metathesis reactions catalyzed by a second-generation Grubbs catalyst (2). The reaction of allyl-1,1-d 2 methyl ether with 2 at 35 °C was followed by 1H and 2H NMR spectroscopy. The evidence of deuterium incorporation at the C-2 position of the isomerized product, trans-propenyl methyl ether, led to the conclusion that a metal hydride addition−elimination mechanism was operating under these conditions. Consequently, complex 8, an analogue of 2 bearing deuterated o-methyl groups on the aromatic rings of the NHC ligand, was synthesized to investigate the role of the NHC ligand in the formation of hydride species. Thermal decomposition of benzylidene 8 and methylidene 8‘ was monitored by 2H NMR spectroscopy; no deuteride complex was detected in either case. The decomposition mixtures were tested for isomerization activity with benchmark 1-octene but did not match the isomerization rates observed with 2 under similar metathesis conditions. Reaction of complex 8 with various olefinic substrates not only confirmed the formation of a deuteride complex but also revealed the existence of a competitive H/D exchange process between the CD3 groups on the NHC ligand and the C−H bonds of the substrate. We propose that the exchange is promoted by a ruthenium dihydride intermediate whose formation is closely related to the methylidene decomposition.

A kinetic study of three ruthenium carbene catalysts, (H 2IPr)(PCy3)(Cl)2RudCHPh, 3 (investigated extensively by Mol), (H2IMes)(Cl)2RudCH(o-iPrOC6H4), 4 (Hoveyda's catalyst), and (H2-IPr)(Cl)2RudCH(o-iPrOC6H4), 5 (a new catalyst structure), was conducted under ADMET polymerization conditions. The kinetic behavior of these catalysts was compared to the classical first-and secondgeneration Grubbs' complexes at 30, 45, and 60 °C. Complex 3 exhibits the highest initial ADMET rate (80 DP s -1 ) of any phosphine complex to date and efficiently promotes metathesis even at temperatures as low as 0 °C. Complex 4 alone does not polymerize 1,9-decadiene in the bulk; however, addition of a polar solvent induces polymerization. Combining elements of catalysts 3 and 4 yielded the new complex 5. This complex results in higher polycondensation rates than previous Hoveyda-type structures and exhibits an increased stability over its parent phosphine complex. The new catalyst polymerizes 1,9decadiene in the bulk to high polymer (M n ) 40 000 g/mol) using low catalyst loadings (0.1 mol %). The isomerization chemistry induced by complexes 3 and 5 was investigated using a model compound, 1-octene.

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Three linear polyethylenes with branches at every 21st backbone atom have been analyzed by differential scanning calorimetry (DSC) and quasi-isothermal, temperature-modulated DSC. The branches were methyl (PE1M), dimethyl (PE2M), and ethyl groups (PE1E). Linear polyethylene (HDPE) and atactic poly-(octadecyl acrylate) (PODA) were also analyzed. All were compared to a random poly(ethylene-co-octene-1) of similar branch concentration (LLDPE) and poly(4,4′-phthaloimidobenzoyldoeicosyleneoxycarbonyl) (PEIM-22). The HDPE has the highest melting temperature and crystallinity with relatively large contributions of reversing melting when grown as folded-chain crystals. The precisely branched polyethylenes and copolymers have lower melting temperatures and heats of fusion. Of the branched samples, PE1M crystallizes more readily, followed by PE1E and PE2M, with PE2M showing cold crystallization. In contrast to paraffins of equal length which melt fully reversibly, the precisely designed, branched polymers melt largely irreversibly with small amounts of reversing melting, which is least for the best-grown crystals. The PE1M forms monoclinic, PE1E, pseudohexagonal, or triclinic crystals, and PE2M has a multitude of crystal structures.

Since the initial reports by Ziegler 1 on the "aufbau" reaction, or stepwise insertion of ethene into the aluminum-carbon bond of alkylaluminum compounds, it has been widely believed that because of the accompanying displacement reaction the products of this reaction were limited to oligoethenes, with few reports of the preparation of high molecular weight polyethene at an aluminum center. Martin has described the preparation of polyethene by exposing ethene to a heptane solution of either bis-(dichloroaluminum)ethane or trialkylaluminum over several days. 2 Recently, Jordan and Gibson have reported the polymerization of ethene using chelated alkylaluminum complexes activated by a Lewis acid. 3 On the other hand, the polymerization of higher alkenes, such as propene, by an aluminum-based system has neWer been reported. Herein, we report that high molecular weight, linear homo-and copolymers of ethene and propene can be prepared, in the absence of any transition metal species, via a catalyst system consisting of simple alkylaluminum compounds activated by Lewis acids.The homo-and copolymerization of ethene and propene were carried out in 125 mL glass-lined reactors and our results are summarized in Tables 1-3. As can be seen, several systems based on the combination of an alkylaluminum compound and a Lewis acid are effective. Of note is the observation that methylaluminoxane (MAO) can act as either one of the two components (presumably because of the presence of small amounts of trimethylaluminum in the commercial sample). Several features of these systems are of interest. First, the molecular weights are high and 1 H and 13 C NMR spectroscopy indicates that the polymers are highly linear. The linearity of the polyethene formed was further supported by its high melting point (T m > 135°C). The polypropene is atactic. For the ethene-propene copolymer, the melting point was found to decrease with increasing propene content in the copolymer.Narrow polydispersities (approximately 2) were observed for the polyethene and polypropene formed, suggesting a single-site catalyst. Finally, in the polymerization of ethene, the polymer molecular weight was found to increase with increasing reaction time (first entry in Table 1), indicating some degree of "livingness" to the system. This may be due to the lack of d-orbitals on aluminum that are necessary for chain transfer through facile -hydrogen abstraction ( -hydrogen abstraction from neutral aluminum alkyls occurs only at elevated temperatures). The transfer of a growing polymer chain from a transition metal center to aluminum to form a stable aluminum-terminated polymer has been reported. 4A critically important issue that must be addressed for all transition metal-free polymerization systems is whether trace amounts of transition metal impurities are actually responsible for the polymerization. For reasons given below, this appears to be unlikely for the present systems. First, the two components used in our systems were analyzed for Ti, Zr, and V by AA spectroscopy and we...

A structural investigation into model linear low-density polyethylene containing precise hexyl branches has been completed using metathesis chemistry. These models based on ethylene/1-octene (EO) copolymers are versions of industrially produced metallocene copolymers; however, they contain exact primary structures and constant methylene sequence lengths. Acyclic diene metathesis (ADMET) polymerization has been used to produce copolymers containing only hexyl branches on every 9th, 15th, or 21st carbon along the backbone of polyethylene. Thermal examination of these macromolecules has demonstrated the first narrow melting profile for EO copolymers with high 1-octene incorporation (111, 67, and 48 hexyl branches per 1000 carbons). Further, a new synthetic methodology has been developed to produce branched, pure diene functional monomers with the ability to produce any model LLDPE in good yields.