Kinetic resolution of racemic {alpha}-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Copolymerization of racemic {alpha}-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2({eta}5-C5H-3,5-(CHMe2)2)({eta}5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-{alpha}-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin

Real-time depth sectioning: Isolating the effect of stress on structure development in pressure-driven flow

Transient structure development at a specific distance from the channel wall in a pressure-driven flow is obtained from a set of real-time measurements that integrate contributions throughout the thickness of a rectangular channel. This “depth sectioning method” retains the advantages of pressure-driven flow while revealing flow-induced structures as a function of stress. The method is illustrated by applying it to isothermal shear-induced crystallization of an isotactic polypropylene using both synchrotron x-ray scattering and optical retardance. Real-time, depth-resolved information about the development of oriented precursors reveals features that cannot be extracted from ex-situ observation of the final morphology and that are obscured in the depth-averaged in-situ measurements. For example, at 137 °C and at the highest shear stress examined (65 kPa), oriented thread-like nuclei formed rapidly, saturated within the first 7 s of flow, developed significant crystalline overgrowth during flow and did not relax after cessation of shear. At lower stresses, threads formed later and increased at a slower rate. The depth sectioning method can be applied to the flow-induced structure development in diverse complex fluids, including block copolymers, colloidal systems, and liquid-crystalline polymers

Improvement of the impact strength of ethylene-propylene random copolymers by nucleation

Five ethylene-propylene random copolymers were nucleated with two soluble nucleating agents. Ethylene content changed between 1.7 and 5.3 wt %, while nucleating agent content was adjusted according to the solubility of the additive. It changed from 0 to 5000 ppm for the sorbitol (1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol) and from 0 to 500 ppm for the trisamide compound (1,3,5-benzene-trisamide) used. Crystalline structure was analyzed in detail by various methods (DSC, XRD, and SEM). Mechanical properties were characterized by tensile and instrumented impact measurements. The results showed that most properties changed moderately upon nucleation, but impact resistance increased considerably. Spherulitic structure was not detected, but instead in the presence of the soluble nucleating agents used a microcrystalline structure formed. The large increase of impact resistance could not be related directly to changes in crystalline morphology. On the other hand, local rearrangement of morphology was detected by XRD and SEM analysis including an increase of lamella thickness, crystal orientation, and the formation of shish-kebab structures in the core of the injection molded specimens. A small increase in the γ-phase content of PP was also observed. These changes increased crack propagation energy considerably leading to the large improvement observed in impact resistance. Although the phenomenon could be related to ethylene content, differences in molecular weight also helped to explain the changes observed

THE ROLE OF CHAIN CONFIGURATION IN GOVERNING THE RATIONAL DESIGN OF POLYMERS FOR ADHESION

ABSTRACT THE ROLE OF CHAIN CONFIGURATION IN GOVERNING THE RATIONAL DESIGN OF POLYMERS FOR ADHESION SEPTEMBER 2017 ONYENKACHI C. WAMUO, B.Eng., FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI (FUTO), NIGERIA M.S., UNIVERSITY OF MASSACHUSETTS AMHERST Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Professor Shaw Ling Hsu The chain configurational control of polymers used in adhesion can be utilized as a means of tuning the cohesive properties of hot melt adhesives (HMAs). The cohesive properties control the solidification, strength, setting speed. Propylene-Ethylene copolymers (PP-co-PE) and thermoplastic polyurethanes (TPUs) were studied. In the first project, the effects of sequence distribution of the two types of the (PP-PE) copolymers, with propylene being the dominant component, on the associated crystallization behavior were analyzed. The average sequence lengths of the crystallizable propylene sequences in these copolymers are different, although the ethylene content was virtually identical. In one circumstance, the chain configuration was completely random with crystallizable propylene sequences following Bernouillian statistics. In the other case, we have used a bimodal distribution of crystallizable sequences. The crystallization kinetics, the crystallization temperature, and the degree of crystallinity were significantly higher for the latter sample as compared to the former. When crystallizing from the melt, the longest crystallizable propylene sequences crystallized first at any supercooling, thus controlling the segmental mobility of other segments in the distribution. This is especially evident in copolymers with the bimodal segmental distribution. The distribution of crystallizable polypropylene sequences also controls the size distribution and thermal stability of the crystallites formed. The elucidation of the crystallization behavior of these copolymers is crucial in defining the application driven setting speeds of hot melt adhesives, the principal application of interest in our laboratory. Due to its polarity and thermoplastic nature of its structure, TPUs can be used advantageously for binding a variety of substrates. The challenge with current polyurethanes based on conventional 1,4-butanediols is the long time dependency taken for their morphology and properties to set. These slow dynamics is unfavorable in HMAs where fast setting speeds are necessary and responsible for their widespread use in packaging applications. We hypothesize that the increased mobility and flexibility of the traditional 1,4-butanediol system enables slow morphology development in the traditional TPUs. We have therefore changed the mobility of the chain extenders by using a 1,2-propanediol chain extender which incorporates a methyl pendant group into the TPU structure. The presence of the pendant groups in this system incorporates rigidity to the chain extender and makes the HS made from it lack the mobility to move away from the SS matrix. We have shown this to be vital in creating stable domains whose properties do not change over time. Using DSC as well as LFNMR we established mobility differences between the symmetric and asymmetric chain extenders. Temporal DSC and FTIR were used to show the stable and time-independent morphologies associated with the 1,2-propanediol chain extender. In this study, we have achieved chain configurational control by changing the architecture of the chain extender. This concept of chain configurational control using chain extenders is highly useful in controlling the set speed for HMAs

Crystallization studies of propylene copolymer fractions

The aim of this work was to study the effect of comonomer content and tacticity on the crystallization behavior of propylene copolymer fractions. Initially two random copolymers of propylene and ethylene with similar molecular weight, but different ethylene (comonomer) content, were fractionated successively in a soxhlet apparatus with solvents of increasing solvent power. These fractions varied in comonomer content and tacticity. The thermal properties from DSC, the morphologial studies from light microscopy, polymorphism from WAXD and the linear growth rates from hot stage microscope were determined. The comonomer content had a significant effect on the thermal, structural and morphological properties of the copolymer. The total impurity content of the copolymers was determined by calculating the defects caused by the effects of tacticity and comonomer. The equilibrium thermodynamic properties of these fractions were determined using different extrapolation techniques. The total defect content obeyed the Flory\u27s total exclusion model. The effect of comonomer content on the regimes of crystallization for these fractions was also evaluated

Crystallization and melting studies of branched isotactic polypropylenes

Crystallization, melting and structural studies were conducted on iostactic polypropylenes treated with varying dosages of electron beam radiation and an untreated iPP. Through FTIR methods, all specimens were found to be greater than 99% isotactic. Crystallization and melting studies were performed using light depolarizing microscopy (LDM) and other melting experiments were conducted using differential scanning calorimetry (DSC). Structural studies were conducted by use of a wide-angle x-ray diffractometer (WAXD). Through isothermal crystallization studies it was found that at the highest supercoolings all specimens had approximately the same half-time of crystallization values, t½, attributed to increased nucleation by increased supercooling. At higher temperatures of crystallization, Tc, it was observed that t½ varied for the specimens. This was attributed to the effects of branching on primary nucleation and to the size of the spherulites. All specimens were observed to nucleate in the heterogeneous mode, meaning that nuclei density stayed constant throughout the isothermal crystallization process. Average spherulite growth geometry (Avrami) exponent, n, values were in the range of 2.2 and 2.5. These low values were a consequence of the amount of branching and stereoregularity of the polymer chains and secondary crystallization. The spherulite growth rates, k, for all the samples decreased with decreasing supercooling, resulting from the decrease in the number of nuclei forming into spherulites. Through x-ray studies the predominant crystal form was found to be of the α modification, with some β and γ modifications observed. No structural changes at the crystal lattice level were detected. The degree of crystallization was seen to decrease as a result of increased branching in the treated specimens and attributed to thermal degradation in the untreated one. From the DSC endotherms small melting peaks in the range of 140 °C to approximately 145 °C was noticed in some of the specimens and attributed to the β modification as a consequence of nucleating agent(s) and stresses induced during sample film preparations. The equilibrium melting points taken from the highest peak and the return to baseline of the endothermic curves showed that the treated samples had lower points than the untreated one. This was due to branching and degradation from the irradiation process. The melting ranges of the treated specimens were shifted to lower values as compared to the untreated specimen, as a consequence of branching and degradation The temperature ranges for the irradiated specimens were broader than the melt range of the untreated sample The α peak also showed broadening as a result of branching

Influence of E/P copolymer composition on impact copolymer properties

Houževnatý E/P kopolymer se vyrábí polymerací v plynné fázi, která je katalyzovaná Ziegler–Nattovými katalyzátory. Tento materiál s výbornými mechanickými vlastnostmi je používán v mnoha průmyslových odvětvích, např. v automobilovém průmyslu na výrobu nárazníků aut a palubních desek. Jeho vlastnosti závisí na chemické struktuře, která byla zkoumána pomocí metod DSC, SSA, 13C NMR, ATREF a FTIR. Výsledky analýz byly porovnány s mechanickými vlastnostmi vzorků - modulem pružnosti v ohybu a nízkoteplotní vrubovou houževnatostí dle Charpyho. Pro studium vlivu složení na mechanické vlastnosti bylo použito 5 různých vybraných vzorků houževnatého kopolymeru E/P s různým poměrem ethylenu a propylenu v kopolymerní fázi, které byly frakcionovány pomocí o-xylenové extrakce za účelem rozdělení materiálu na krystalickou část, složenou z krystalického E/P kopolymeru a PP, a amorfní část, tvořenou E/P kaučukem. Získané frakce i původní vzorky byly následně analyzovány výše uvedenými metodami.Impact E/P copolymer is produced using the gas-phase polymerization, which is catalysed by Ziegler-Natta catalyst. This material, which has excellent mechanical properties, is used in many industries, for example in automotive industry to produce bumpers or instrument panels. Its properties depend on chemical structure, which was analysed using DSC, SSA, 13C NMR, ATREF and FTIR. The results were compared with the mechanical properties of the samples – flexural modulus and low-temperature Charpy impact strength. 5 different samples were used for studies, the chosen samples contain different ratio between ethylene and propylene in their copolymer phase. The samples were fractionated using o-xylene extraction to divide the material into crystalline part, composed of crystalline E/P copolymer and PP, and into amorphous phase, formed by E/P rubber. The obtained fractions and the original samples were subsequently analysed using above-mentioned methods.

A Smart HTE Approach to Sustainable Polyolefin Materials

There is little doubt that High Throughput Experimentation (HTE) will ultimately become the gold standard of chemical R&D. On the other hand, until now the technical complexity and high Capex and Opex of HTE tools and methods have hampered a broad dissemination in several important areas of the chemical sciences. In particular, HTE approaches to organometallic catalysis began to spread in academia only recently. The general aim of the present PhD project was to implement and apply 'smart' HTE protocols for tackling complex problems in olefin polymerization catalysis, with special focus on polyolefin sustainability. The main case history was Coordinative Chain Transfer Polymerization (CCTP) and its Chain Shuttling Polymerization (CSP) variant: unraveling the complex kinetics governing this elusive chemistry and expanding its scope to novel monomers and materials are important open challenges. We have also addressed questions of relevance for the recycling of polyolefin wastes in the context of a circular economy. The HTE toolkit is introduced in Chapter 2. Despite the extensive robotic automation, a HTE platform is not a push-button setup. A complete HTE workflow can include several reaction platforms and an array of integrated analytical tools amenable to high-throughput operation and yet ensuring the precision and accuracy of conventional high-end tools. Chapter 3 illustrates the implementation of HTE protocols for parallel olefin CSP experiments. We successfully downscaled the high-temperature and high-pressure synthesis of statistical Olefin Block Copolymers (OBC) according to the Dow InfuseTM technology. A systematic exploration of the multi-dimensional variables hyperspace of ethene/1-alkene copolymerization under tandem catalysis conditions led us to elucidate unambiguously for the first time the microstructure and architecture of these advanced materials, that found commercial applications as unique thermoplastic elastomers and also as effective phase compatibilizers in immiscible polyolefin blends. Chapter 4 illustrates a systematic and thorough search for catalyst systems amenable to CCTP/CSP other than those originally introduced by Dow Chemical. Notwithstanding the several claims in the literature, our study led us to conclude that reversible trans-alkylation in catalytic olefin polymerizations is exceedingly rare, and therefore expanding the scope of CSP via catalyst diversification is problematic. Moving from this negative conclusion, in Chapter 5 we explored the alternative option of OBC diversification by using unconventional comonomers. Two new classes of OBCs were prepared by CSP of ethene with 4-methyl-1-pentene or 1-hexadecene, respectively. Both comonomers are expected to provide block copolymers with unusual and interesting physical properties. In Chapter 6 we report how the HTE workflow was utilized to explore the possibility to introduce a fluorescent tag into polyethylene and polypropylene chains via copolymerization, for diagnostic purposes. The idea was to make different polyolefin grades identifiable post-mortem with a simple, cheap and fast optical measurement. Series of ethene and propene copolymerizations with 1-pyrenylheptene, a fluorescent comonomer prepared ad hoc, demonstrated that the concept works very well down to incorporations of the tag at which the thermal and physico-mechanical properties of the copolymers are practically identical to those of the corresponding homopolymers. Chapter 7 investigates catalytic depolymerization as a possible route of polymer waste recycling. It has long been known that polyolefin can be cleaved under comparatively mild conditions in the presence of certain heterogeneous transition metal catalysts. Recently, this has also been shown for polydienes with a homogeneous catalyst. In the framework of the present thesis we explored the depolymerization of 1,4-cis-polybutadiene mediated by a large library of Group 4 metallocene and post-metallocene complexes. A strong dependence of molecular kinetics on catalyst structure was highlighted, and efficient catalysts were identified. This part of the project was a collaboration with Prof. Adam S. Veige at the University of Florida (Gainesville, FL). From the conclusions of the project, which are summarized in Chapter 8, it is well evident that 'smart' HTE methodologies are ideally suited to rapidly identify novel and convenient routes of olefin polymerization and polyolefin/polydiene depolymerization that can improve the sustainability of these ubiquitous and important industrial processes and materials, making them ultimately better suited to a circular economy