Precisely branched polyolefins

Recently, a new synthetic route (Figure 10.5) to symmetric alkyl branched dienes has been presented and utilized in the synthesis of a whole family of precision alkyl-branched polyolefins (Rojas et al, 2007). This general route, which can be used to create dienes with virtually any branch, involves only two steps with quantitative yields. The first step is the dialkylation of a primary nitrile using freshly prepared lithium diisopropylamide (LDA). Alkylation in this fashion affords a symmetric diene premonomer with the desired alkyl branch and a nitrile group on the central carbon. In the next step, this nitrile is removed by reductive elimination in the presence of potassium metal, HMPA, and r-butanol. 

Figure 10.7 TEM images clearly show the effects of precise branch placement random polyolefins show a broad distribution of lamellar thicknesses precision polyolefins feature lamella of uniform thickness. (Reprinted from S. Hosoda, Y. Nozue, Y. Kawashima et al, Perfectly controlled lamella thickness and thickness distribution A morphological study on ADMET polyolefins, Macromolecular Symposia, 282, 50-64 2009, with the permission of John Wiley Sons, Inc.)...

Figure 10.9 Synthesis and thermal behavior of polyolefins with precise ether-branch placement. (Reprinted from E.B. Berda, T.W. Baughman and K.B. Wagener, Precision branching in ethylene copolymers Synthesis and thermal behavior, Journal of Polymer Science Part A Polymer Chemistry, 44, 17, 4981-4989, 2006, with the permission of John Wiley Sons, Inc.)...

These materials can be considered linear copolymers of ethylene and propylene or precisely methyl-branched polyethylene. In addition, copolymerizations of the methyl-containing monomers with 1,9-decadierie yield polymers with lower propylene content [50]. These materials are of great interest to the polyolefin community, especially in the physical understanding of the effects of branching on physical properties. Polyethylenes with a variety of main chain functionality have also been synthesized and analyzed [51-54]. 

In contrast to Group IV-based polymerization catalysts, late transition metal complexes can carry out a number of useful transformations above and beyond the polyinsertion reaction. These include isomerization reactions and the incorporation of polar monomers, which have allowed the synthesis of branched polymer chains from ethylene alone, and of functional polyolefins via direct copolymerization. The rational design of metallocene catalysts allowed, for the first time, a precise correlation between the structure of the single site catalyst and the mi-crostructure of the olefin homo- or copolymer chain. A similar relationship does not yet exist for late transition metal complexes. This goal, however, and the enormous opportunities that may result from new monomer combinations, provide the direction and the vision for future developments. 

Y.Y. Wei, R. Graf, J.C. Sworen, C.Y. Cheng, C.R. Bowers, K.B. Wagener, H.W. Spiess, Local and collective motions in precise polyolefins with alkyl branches a combination of H-2 and C-13 solid-state NMR spectroscopy, Angew. Chem. Int. Ed. Engl. 48 (2009) 4617-4620. 

Thermal behavior of precision polyolefins with increasing alkyl branch size... 

Increasing the Spacing Between Alkyl Branches in Precision Polyolefins... 

Figure 10.15 Thermal behavior of precision amphiphilic copolymers with precisely placed amphiphilic branches simple structural modifications at the PEG branch terminus result in significant changes in behavior and morphology. (Reprinted with permission from E.B. Berda and K.B. Wagener, Inducing pendant group interactions in precision polyolefins Synthesis and thermal behavior, Macromolecules, 41, 5116-5122, 2008. 2008 American Chemical Society.)...

Rojas, G., Berda, E.B., and Wagener, K.B. (2008) Precision polyolefin structure modeling polyethylene containing alkyl branches. Polymer, 49,2985-2995. 

The use of NMR to elucidate the structure of polyolefins was pioneered by Randall, who has reviewed the capabilities of this technique [89]. Structural features that can be probed include the identity of the repeat unit and its chirality, copolymer sequence structures and their distributions, identity of end groups, degree of polymerization, and branching. Carbon-thirteen is a naturally-occurring isotope that represents about one percent of the carbon atoms present in a polymer. In a typical experiment, a sample is excited by a series of radio frequency pulses, which alter local nuclear moments by a certain angle, after which they decay to their undisturbed (equilibrium) state. The decay is monitored by a detector coil, and the output curve shows the frequencies at which resonance occurred. In order to achieve sufficient precision for the applications mentioned above, experiments of quite long duration are required, often many hours, or even several days. 

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