Chemistry & Biochemistry Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2752

Browse

Subject: search.filters.subject.Chemistry, Polymer

Search Results

Now showing 1 - 7 of 7
  • Thumbnail Image
    Item
    Micelle and Aggregate Formation in Amphiphilic Block Copolymer Solutions
    (2010) Clover, Bryna Christine; Greer, Sandra C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The amphiphilic nature of many block copolymers causes self-aggregation and micelle formation in solvents that are miscible with only one of the block polymers (selective solvents). Micelle and aggregate formation of amphiphilic block copolymers in selective solvents is a function of temperature and concentration. Such self-aggregation has been examined here in a variety of block copolymer systems. In dilute solutions of Pluronic P85 (PEO26PPO40PEO26) (where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)) in D2O, transitions between clustered unimers, spherical micelles, cylindrical micelles, and finally lamellar micelles were observed with increasing temperature. The effect of pressure on this system was examined through small angle neutron scattering (SANS) techniques. At temperatures above 95 oC, a new phase of “demixed lamellae” was observed. Pressure effects on the transition temperatures between the phases of this system were investigated. The self-aggregation of Reverse Pluronic 17R4 (PPO14PEO24PPO14) in D2O has also been examined. The phase diagram of this system was determined through visual cloud-point techniques. Three distinct regions have been observed in solutions of this system, as a function of temperature and concentration: a cloudy, one-phase region; a clear, one-phase system; and a region of phase separation. Copolymer structures were examined in the clear and cloudy one-phase regions through SANS and dynamic light scattering (DLS) techniques. A network, or clustering, of unimers was observed in the cloudy phase. Aggregates in the clear, one-phase region could not be identified definitively as micelles. Finally, micellization of PEO132-PB89 (where PB is polybutadiene) has been studied in solutions of deuterated methanol and deuterated cyclohexane. Spherical micelles were observed in solutions of deuterated methanol. These micelles change little in size or shape over a 50 oC temperature span. The difference in aggregates in protonated and deuterated solvents was also examined. In deuterated cyclohexane, the copolymer formed flexible, cylindrical micelles below 40 oC. These micelles became spherical in shape at higher temperatures.
  • Thumbnail Image
    Item
    Living Coordinative Chain Transfer Polymerization of 1-Alkenes
    (2008-12-05) Zhang, Wei; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A novel polymerization method, living coordinative chain transfer polymerization (CCTP), was recently developed with monocyclopentadienyl monoamidinate (CpAm) Group 4 metal complexes, which were previously applied for the traditional living coordination polymerization (TLCP) and stereomodulated degenerative transfer living (SDTL) coordination polymerization. In addition to a CpAm precatalyst and a cocatalyst, a chain transfer agent (CTA) was also added to the CCTP system. The CTA undergoes a rapid and reversible chain transfer with the Group 4 metal catalyst, which results in chain growth on an inexpensive main group metal alkyl. This new CCTP technique provides a practical solution towards the intrinsic problem, one chain per catalytic center, for a TLCP polymerization process. The first example of living CCTP was provided with ZnEt2 via Cp*HfMe2[N(Et)C(Me)N(Et)] (35) activated by [PhNHMe2][B(C6F5)4]. It was very efficient for the polymerization of ethene, propene, higher α-olefins and α,ω-nonconjugated dienes, and copolymerization of these monomers. The (co)polymers obtained possess very narrow polydispersity (PDI 1.03-1.10) and tunable molecular weights by several factors including a wide range of equivalents of ZnEt2. The living property of this CCTP system was further confirmed by kinetic studies and end group functionalization. The quantitative chain extension on zinc was clearly shown by in situ NMR spectroscopy. The coordinative chain shuttling polymerization (CCSP) was also studied while binary precatalysts, cocatalysts, or chain transfer agents were applied. The TLCP, SDTL and CCTP of propene via some new CpAm complexes other than 35 were also studied, including the zirconium analogue of 35, Cp*ZrMe2[N(Et)C(Me)N(Et)], and a series of binuclear complexes which have the common structure of [Cp*ZrMe2]2[N(tBu)C(Me)N(CH2)xNC(Me)N(tBu)] (26, x = 8; 27, x = 6; 28, x = 4). The formamidinate precatalyst Cp*ZrMe2[N(tBu)C(H)N(Et)] (12) was also covered in this study. Under both SDTL and CCTP conditions, the binuclear catalysts showed a tether-length dependent chain transfer process as observed by the polymerization results especially by the tacticity of resulting polypropene. Using CCSP process, multi-stereoblock polypropene was successfully prepared via 12 and 27. The structures and properties of these new complexes and (co)polymers were fully characterized by X-ray crystallography, elemental analysis, GPC, DSC, GC and high field NMR spectroscopy.
  • Thumbnail Image
    Item
    Toward Optimization of Photomodulation of Azobenzene-modified PPV Derivatives
    (2007-10-01) Grimes, Amy Frances; English, Douglas S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Photophysical characterization of a family of photoswitchable conjugated polymers is presented in this work. Additionally the instrumentation constructed for these studies, a time-correlated single photon counting spectrometer, is discussed along with the methods used to characterize the instrument. Sample data and fitting procedures are presented. Overall instrument capabilities are also presented, specifically the use of the spectrometer to measure time-resolved fluorescence anisotropy. An example study probing the interactions between charged fluorophores and surfactant vesicles is included as a demonstration of a time-resolved fluorescence anisotropy application. Understanding the effect of side chain modifications on the emission of light from conjugated polymers is useful in the design of new polymers for applications in sensing and photovoltaics. This thesis focuses on determining the photophysical interactions between a photochromic side chain, azobenzene, covalently bound to a poly(p-phenylenevinylene) (PPV) derivative. Time-resolved and steady-state fluorescence measurements were employed in these studies. The photochromic azobenzene quenches emission from the PPV backbone differentially in its' two isomeric states. Both static quenching and non-radiative energy transfer were found to play important roles in the differential quenching of PPV emission by azobenzene. These studies led to the definition of a parameter to judge degree of difference in quenching between the two isomeric forms. This parameter, modulation efficiency (Emod), serves throughout the studies as an important figure of merit for the depth of modulation observed for structurally modified derivatives of the original azobenzene-modified PPV derivative. Maximizing the modulation efficiency was the aim of the PPV studies. The results presented here elucidated the complex photophysical processes that influence the emission properties of this family of azobenzene-modified PPV derivatives. Important guidelines to maximize modulation efficiency were determined based on these results which will aid researchers in the design of photomodulated conjugated polymers.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Item
    DEGENERATIVE TRANSFER LIVING ZIEGLER-NATTA POLYMERIZATION OF α-OLEFINS
    (2005-12-04) Zhang, Yonghui; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A degenerative transfer (DT) living Ziegler-Natta polymerization system based on the cyclopentadienyl amidinate zirconium complex, (h5-C5Me5)ZrMe2[N(t-Bu)C(Me)N(Et)] (19), has been studied in detail. Compound 19 is an active pre-catalyst for the living and stereospecific polymerization of a-olefins, upon activation by a stoichiometric amount of [PhNHMe2][B(C6F5)4] (18). With a substoichiometric amount of 18, degenerative transfer living Ziegler-Natta polymerization occurs. The extremely fast interconversion between an active cationic species and a dormant neutral species via a methyl (MeDeT) or chloride (ChloDeT) group is essential for degenerative transfer to occur (Rex>>Rp). Under MeDeT conditions, atactic polyolefins are produced due to the configurational instability of the amidinate ligand in its dormant state, with the rate of epimerization being much faster than the rate of propagation (Repi >>Rp). The configurational stability of all the zirconium species involved in ChloDeT ensures that this degenerative transfer living polymerization is stereospecific. Methylating reagents (h5-C5Me5)ZrMe2[N(t-Bu)C(t-Bu)N(Et)] (34) and (h5-C5Me5)ZrMe2[NCH2(t-Bu)C(Me)N(Et)] (39) were synthesized to provide a means by which a fully activated polymerization system could be brought under degenerative transfer conditions. Adding 18 and 34 or 39 alternatively to a living polymerization system produces well-defined atactic-isotactic stereoblock polyolefins. This allows, for the first time, the production of stereoblock polyolefins with controlled block length, block tacticity and block placement within a polymer backbone. Living stereoselective propylene polymerizations were carried out by {(h5-C5Me5)ZrMe[N(t-Bu)C(Me)N(Et)]}[B(C6F5)4] (32). A mmmm of 73% and a s value of 94% were determined, which is the highest among living propylene polymerizations reported to date. Under degenerative transfer conditions, atactic PP was produced. Using the developed methodology, a family of different atactic-isotactic stereoblock polypropylene (sb-PP) was synthesized with controlled block length, block tacticity for the first time. Preliminary data shows excellent elastomeric properties for the sb-PPs that are synthesized. The amidinate zirconium initiator was chemically immobilized to polystyrene-based solid support via deprotonation and nucleophilic addition reactions at the distal position of (h5-C5Me5)ZrCl2[N(t-Bu)C(Me)N(Et)] (40). Heterogeneous initiator (58) polymerizes a-olefins in a living and isospecific fashion. Also, a long shelf time was observed for 58 at room temperature. This is the first heterogeneous living Ziegler-Natta stereospecific catalyst reported to date.
  • Thumbnail Image
    Item
    Ethylene Polymerization Using a Zirconium Amidinate Supported Catalyst
    (2004-05-07) Young, Andrea Elise; Sita, Lawrence; Chemistry
    A series of W. R. Grace Davison IOLA(TM), methylaluminoxane-silica (MAO/Silica) and MAO/IOLA support materials were used to activate and immobilize a zirconium amidinate single site catalyst of the formula Cp*ZrMe2[tBuNC(Me)NCEt]. Ethylene homo-polymerizations and co-polymerizations with 1-hexene were conducted in heptane and compared. The catalysts activity was investigated under varying condition such as pre-catalyst loading, pressure, temperature, co-monomer incorporation and additives. The catalyst supported on the MAO/IOLA B support material proved to be more active than the IOLA and m-IOLA support activators, and the MAO/Silica and MAO/IOLA A support materials. A difference in activity of as much as 1015 gPE/gcat.h-1 was noted. The catalyst sensitivity to varying ethylene polymerization conditions such as temperature and pressure were investigated for the MAO/IOLA B supported catalyst. Catalyst activities of more than 2100 gPE/gcat.h-1 were achieved. Homo-polymer and co-polymer samples were characterized and compared with respect to their melting temperature, molecular weights and polydispersities.
  • Thumbnail Image
    Item
    Amidinate Based Catalysts for the Stereospecific and Living Ziegler-Natta Polymerization of Alpha-Olefins
    (2003-12-04) Keaton, Richard J; Sita, Lawrence R; Chemistry
    The living Ziegler-Natta polymerization of alpha-olefins has been accomplished with a series of precatalysts based on cyclopentadienyl and pentamethylcyclopentadienyl zirconium amidinates (Cp and Cp*ZA's, respectively) upon activation by a borate cocatalyst at 10 °C in chlorobenzene. For the latter, the symmetry of the precatalyst determines the polymer microstructure: C(1)-symmetry gives isotactic polymer, while C(s)-symmetry gives nearly atactic material. The living behavior has been proven through kinetic analyses, narrow molecular weight distribution polymers, formation of telechelic polymers, and synthesis of well defined block copolymers. Aside from simple straight chain alpha-olefins, non-conjugated dienes and vinylcyclohexane have also been polymerized in a living fashion with this series of precatalysts. Characterization of several catalytically active derivatives through solution NMR studies and single crystal X-ray analyses were successful. In the solid state, the initiator appears as a Zr-Me cation that is involved in a doubly methyl bridged dimeric structure. The presence of ether is sufficient to break up the dimer affording a monomeric species. Also, substantially increasing the steric hindrance of the amidinate ligand yields a monomeric structure. The Zr-Me cations undergo rapid methyl group exchange as evidenced through a crossover experiment between C(1)- and C(s)-symmetric initiators. Similarly, the methyl cations can engage in methyl-polymer group exchange, thus providing a new method toward stereoblock copolymer production. Insertion of cyclopentene is also successful into the Zr-Me bond, though further propagation steps do not occur. The product of initiation is the previously unobserved cis-1,2-product, which upon warming quantitatively isomerizes to the cis-1,3-product. The former features a beta-hydrogen agostic interaction with a low J value of 87.7 Hz. Upon isomerization, two beta-agostic hydrogens are present, with J values of 97.5 and 107.2 Hz. Oligomeric polymers were prepared from low ratios of monomer to initiator. Extensive NMR studies showed a 9 : 1 selectivity for the enantiofacial selectivity of the initiation step and perfect stereospecificity thereafter. Quenching the polymerization after extremely long times, or performing the polymerization at higher temperatures, afforded evidence that beta-hydride elimination was a rare, yet active, path, and that chain-walking occurs along the alkyl chain of the last inserted monomer unit.