Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item End-Group Functionalized Poly(α-olefinates) as Modular Building Blocks(2016) Thomas, Tessy S.; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of living coordination chain transfer polymerization (LCCTP) has provided a viable path to practical and scalable bulk quantities of structurally well-defined polyolefins that are further characterized by having tunable molecular weight, narrow polydispersity and end-group moieties through the functionalization of the Zn(polymeryl)2 intermediate. These low molecular weight, atactic poly(α-olefinates) are attractive non-polar building blocks for new types of self-assembling polyolefin materials with tunable occupied volumes and lengths. In the present work, the investigation of self-assembled morphology manipulation through tunable occupied volume required a model α-olefin. Living coordinative group-transfer polymerization techniques were employed to determine the viability of low molecular weight, atactic poly(α-olefinates) (X-PAOs) as building blocks with bulky pendent groups. Application of these X-PAOs for the synthesis and self-assembly of high χ, low N amphiphilic diblock copolymers demonstrated the ability to manipulate the morphology of the thin film nanostructures through variation in occupied volume of the X-PAO domain. The resulting materials proved the combination of X-PAOs and polyester blocks provided a high enough χ in order to demonstrate self-assembly with an N as low as 50 monomer units while maintaining sub-20 nm domain spacings. It was of significant interest to develop a higher χ, lower N system to achieve sub-10 nm domain spacings. As a result a novel system using sugar-hybrid-PAO conjugates was developed. This system also demonstrated that variation in occupied volume of the X-PAO domain could influence thin-film morphology. The sugar-hybrid-PAO conjugates also demonstrated the ability to self-assemble in solution and encapsulate hydrophobic molecules. The sugar-hybrid-PAO conjugates proved to be a highly versatile system that has simplified the polysaccharide/synthetic block copolymer designs that have been previously used in the literature to obtain sub-10 nm domain spacings. The ability to generate hydrophobic building blocks using LCCTP has opened up the possibility of further investigations of new, advanced self-assembling materials and applications thanks to these readily-available X-PAOs modular building blocks.Item Examination of Dynamic Processes in Living Ziegler-Natta Polymerization and New Polypropylene Architectures through Bimolecular Control(2006-11-24) Harney, Matthew Brian; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The cationic cyclopentadienyl zirconium acetamidinate (CPZA) complex {Cp*ZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6 Fsub5)sub4], (Cp* = ηsup5-Csub5Mesub5) generated through activation of Cp*ZrMesub2[N(Et)C(Me)N(tBu)] via protonolysis of a methyl group with one equivalent of [PhNMesub2H][B(Csub6Fsub5)sub4], has been shown to be a highly active initiator for living alpha-olefin polymerization. Discrete cationic CPZA alkyl complexes of the general structure {Cp*ZrR[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] were studied as model complexes for living polymers derived from this system. Detailed analysis of models in which R = nPr, iPr, iBu, and 2-Et-Bu, as well as living isotactic poly(1-butene) and living isotactic poly(1-sup13C-decene) revealed significant differences with respect to isomerization and decomposition. Studies carried out with various isotopically labeled iso-butyl derivatives revealed isomerization concurrent with decomposition, while the only case which did not provide evidence for isomerization was an isotopically labeled n-propyl derivative. Products of decomposition in cases of R = iBu and 2-Et-Bu included not only the expected alkenes from isomerization/beta-hydride elimination, but significant quantities of alkane. This is proposed to arise from competitive intramolecular abstraction of a hydrogen from the Cp* ligand. During decomposition, all species mentioned above disappeared in a first order manner, and all products of decomposition/isomerization appeared in a zero order manner. This is consistent with slow beta-hydride elimination followed by rapid isomerization, decomposition or reinsertion. Possible mechanisms of isomerization are discussed. The {Cp*ZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] initiator was found to be an active initiator for the living isotactic polymerization of propylene ([mmmm] = 0.71). When substoichiometrically activated with only 0.5 equivalents of [PhNMesub2H][B(Csub6Fsub5)sub4], atactic polypropylene was produced due to the rapid degenerative transfer of methyl groups between active and dormant centers, which results in the racemization of active sites by virtue of the configurational instability of the dormant state. Successive additions of [PhNMesub2H][B(Csub6Fsub5)sub4] and a methylating agent were used to modulate the conditions of the polymerization system between degenerative transfer (at 50% activation) and fully activated conditions to produce the first discrete, homogeneous isotactic-atactic stereoblock polypropylene (sb-PP) materials. Preliminary tensile testing of three unique sb-PP materials displayed elastomeric properties that were heavily dependant on the stereoblock architecture. The synthesis of a wide range of polypropylene stereochemical microstructures between isotactic and atactic through bimolecular control by substoichiometric activation was also demonstrated, leading to the development of a fundamentally new architecture: stereogradient polypropylene.