Living cationic polymerization of isobutene

The polymerization of isobutene initiated by 1-chloro-1-phenylethane has been investigated, and molecular weight studies conducted using size exclusion chromatography. Polymerizations carried out in a 40/60 (v/v) mixture of dichloromethaneIcyclohexane, using titanium (IV) chloride as a catalyst in the presence of pyridine at -30 °C were found to be controlled and living. The number average molecular weights of the polymers increased linearly with monomer conversion, and the molecular weight distributions were between 1.15 and 1.20. Efficiencies of initiation were between 80 and 100%, and evidence was found to suggest that backbiting to the initiator had occurred, resulting in the formation of cyclic oligomers during the early stages of polymerization. The kinetics of polymerization can be explained in terms of active species in. equilibrium with dormant species. The effects of temperature. and dielectric constant on this equilibrium were studied and a model based upon the Fuoss equation was developed. Pyridine was found to behave as proton trap in the system, and when it was used in excess the rate of polymerization was retarded. By assuming that the catalyst and pyridine formed a one to one complex, it was possible to show that the reaction was second order with respect to the catalyst. The synthesis of low molecular weight polyisobutenes was studied. When the concentration of initiator was increased relative to that of the isobutene, such that the theoretical degree of polymerization was 20 or less, the rate of initiation was slow compared to propagation. The efficiency of initiation in these polymerizations was typically between 30 and 40 %. Optimal conditions of temperature. and.catalyst concentration were established, leading to a 60 % efficiency of initiation. A one-pot synthesis of phenol end-capped polyisobutene was attempted by adding phenol at the end of a living polymerization. Evidence to substantiate the existence of capped polymer chains in the resultant product was inconclusive. Block copolymerizations of oxetane and isobutene were conducted using 1-chloro-1phenylethane/TiCl4, but no copolymer or oxetane homopolymer could be isolated.

Living cationic polymerization of styrene monomers

This thesis describes an experimental investigation of synthesis of polystyrene under various polymerization conditions such as solvent polarity, temperature, initial concentrations of initiator, catalyst, monomer and added salts or co-catalyst, which was achieved using the living cationic polymerization technology in conjunction with gel permeation chromatography (GPC) and NMR spectroscopy. Polymerizations of styrene were conducted using 1-phenyl ethylchloride (1-PEC) as an initiator and tin tetrachloride (SnCI4) as a catalyst in the presence of tetra-n-Butylammonium chloride (nBu4NCI). Effects of solvent polarity varied by mixing dichloromethane (DCM) and less polar cyclohexane (C.hex), temperature, initial concentrations of SnC14, 1-PEC and nBu4NCI on the polymerizations were examined, and the conditions under which a living polymerization can be obtained were optimised as: [styrene]o ~ 0.75 - 2 M; [1-PEC]o ~ 0.005 - 0.05 M; [SnCI4Jo ~ 0.05 - 0.4 M; [nBu4NCIJo ~ 0.001 - 0.1 M; DCM/C.hex ~ 50/0 - 20/30 v/v; T ~ 0 to -45°C. Kinetic studies of styrene polymerization using the Omnifit sampling method showed that the number average molecular weight (Mn) of the polymers obtained increased in direct proportion to monomer conversion and agreed well with the theoretical Mn expected from the concentration ratios of monomer to initiator. The linearities of both the 1n([MJoI[M]) vs. time plot and the Mn vs. monomer conversion plot, and the narrow molecular weight distribution (MWD) measured using GPC demonstrated the livingness of the polymerizations, indicating the absence of irreversible termination and transfer within the lifetimes of the polymerizations. The proposed 'two species' propagation mechanism was found to apply for the styrene polymerization with 1-PEC/SnCI4 in the presence of nBu4NCl. The further kinetic experiments showed that living styrene polymerizations were achieved using the 1-PEC/SnCI4 initiating system in mixtures of DCM/C.hex 30/20 v/v at -15°C in the presence of various bromide salts, tetra-n-butylammonium bromide, tetra-n-pentylammonium bromide, tetra-n-heptylammonium bromide, and tetra-n-octylammonium bromide, respectively. The types of the bromide salts were found to have no significant effect on monomer conversion, Mn, polydispersity and initiation efficiency. Living polymerizations of styrene were also achieved using titanium tetrachloride (TiCI4) as a catalyst and 1-PEC as an initiator in the presence of a small amount of 2,6-di-tert-butylpyridine or pyridine instead of nBu4NCl. GPC analysis showed that the polymers obtained had narrow polydispersities (P.D. < 1.3), and the linearities of both the In([MJo/[MJ) vs. time plot and the Mn vs. monomer conversion plot demonstrated that the polymerizations are living, when the ratio of DCM and C.hex was less than 40 : 10 and the reaction temperature was not lower than -15°C. The reaction orders relative to TiCl4 and 1-PEC were estimated from the investigations into the rate of polymerization to be 2.56 and 1.0 respectively. lH and 13C NMR analysis of the resultant polystyrene would suggest the end-functionality of the product polymers is chlorine for all living polymerizations.

Block co-polymerization by transformation reactions

The aim of this study was to use the transformation of anionic to metathesis polymerization to produce block co-polymers of styrene-b-pentenylene using WC16 /PStLi and WC16/PStLi/ AlEtC12 catalyst systems. Analysis of the products using SEC and 1H and 13C NMR spectroscopy enabled mechanisms for metathesis initiation reactions to be proposed. The initial work involved preparation of the constituent homo-polymers. Solutions of polystyryllithium in cyclohexane were prepared and diluted so that the [PStLi]o<2x10-3M. The dilution produced initial rapid decay of the active species, followed by slower spontaneous decay within a period of days. This was investigated using UV / visible spectrophotometry and the wavelength of maximum absorbance of the PStLi was found to change with the decay from an initial value of 328mn. to λmax of approximately 340nm. after 4-7 days. SEC analysis of solutions of polystyrene, using RI and UV / visible (set at 254nm.) detectors; showed the UV:RI peak area was constant for a range of polystyrene samples of different moleculor weight. Samples of polypentenylene were prepared and analysed using SEC. Unexpectedly the solutions showed an absorbance at 254nm. which had to be considered when this technique was used subsequently to analyse polymer samples to determine their styrene/ pentenylene co-polymer composition. Cyclohexane was found to be a poor solvent for these ring-opening metathesis polymerizations of cyclopentene. Attempts to produce styrene-b-pentenylene block co-polymers, using a range of co-catalyst systems, were generally unsuccessful as the products were shown to be mainly homopolymers. The character of the polymers did suggest that several catalytic species are present in these systems and mechanisms have been suggested for the formation of initiating carbenes. Evidence of some low molecular weight product with co-polymer character has been obtained. Further investigation indicated that this is most likely to be ABA block copolymer, which led to a mechanism being proposed for the termination of the polymerization.

The cationic ring-opening polymerization of cyclic ethers

The kinetics and mechanisms of ring opening polymerization and copolymerizntion of different cyclic ethers were studied using mainly a cationic system of iinitiation. BF30Et2/ethanediol. The cyclic ethers reacted differently showing that ring strain and basicity are the main driving forces in cationic ring opening polymerizaion. In most cases it was found that the degree of polymerization is controlled kinetically via terminations with the counterion and the monomers, and that the contribution of each type of reaction to the overall termination differs markedly. The Gel permeation chromatography studies showed that the molecular weight distribution of the samples of polyoxetanes were bimodal. This was in accordance with previous work establishing that the cyclic tetramer is found in much higher proportions than any of the other cyclic oligomers. However the molecular weight distribution of the copolymers made from oxetane and THF or from oxetane and oxepane were shown to be unimodal. These observations could be explained by a change in the structure of the growing end involved in the cationic polymerization. In addition crown ethers like dibenzo-crown-6 and compounds such as veratrole are believed to stabilise the propagating end and promote the formation of living polymers from oxetane.

The ring opening polymerization of ring strained cyclic ethers

The kinetics and mechanisms of the ring-opening polymerization of oxetane were studied using cationic and coordinated anionic catalysts. The cationic initiators used were BF30Et2!/ethanol, BF30Et2!/ethanediol and BF30Et2/propantriol. Kinetic determinations with the BF30Et2/diol system indicated that a 1: 1 BF3:0H ratio gave the maximum rate of polymerization and this ratio was employed to detenmne the overall rates of polymerization. An overall second-order dependence was obtained when the system involved ethanediol or propantriol as co-catalyst and a 3/2-order dependence with ethanol, in each case the monomer gave a first-order relationship. This suggested that two mechanisms accounted for the cationic polymerization. These mechanisms were investigated and further evidence for these was obtained from the study of the complex formation of BF30Et2 and the co-catalysts by 1H NMR. Molecular weight studies (using size-exclusion chromatography) indicated that the hydroxyl ion acted as a chain transfer reagent when the [OH] > [BF3]. A linear relationship was observed when the number average molecular weight was plotted against [oxetane] at constant [BF3:0H], and similarly a linear dependency was observed on the BF3:0H 1:1 adduct at constant oxetane concentration. Copolymerization of oxetane and THF was carried out using BF30Et2/ethanol system. The reactivity ratios were calculated as rOXT = 1.2 ± 0.30 and rTHF = 0.14 ± 0.03. These copolymers were random copolymers with no evidence of oligomer formation. The coordinated anionic catalyst, porphinato-aluminium chloride [(TPP)AICl], was used to produce a living polymerization of oxetane. An overall third-order kinetics was obtained, with a second-order with respect to the [(TPP)AICl] and a first-order with respect to the [oxetane] and a mechanism was postulated using these results. The stereochemistry of [(TPP)AlCl] catalyst was investigated using cyclohexene and cyclopentene oxide monomers, using extensive 1H NMR, 2-D COSY and decoupling NMR techniques it was concluded that [(TPP)AlCl] gave rise to stereoregular polymers.

The pH-induced swelling and collapse of a polybase brush synthesized by atom transfer radical polymerization

We have used neutron reflectometry to characterize the swelling behaviour of brushes of poly[2-(diethyl amino)ethyl methacrylate], a polybase, as a function of pH. The brushes, synthesized by the "grafting from" method of atom transfer radical polymerization, were observed to approximately double their thickness in low pH solutions, although the pK is shifted to a lower pH than in dilute solution. The composition-depth profile obtained from the reflectometry experiments for the swollen brushes reveals a region depleted in polymer between the substrate and the extended part of the brush.

Controlled RAFT polymerization and zinc binding performance of catechol-inspired homopolymers

Incorporation of catechols into polymers has long been of interest due to their ability to chelate heavy metals and their use in the design of adhesives, metal-polymer nanocomposites, antifouling coatings, and so on. This paper reports, for the first time, the reversible addition-fragmentation chain transfer (RAFT) polymerization of a protected catechol-inspired monomer, 3,4-dimethoxystyrene (DMS), using commercially available trithiocarbonate, 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT), as a chain transfer agent. Our identified RAFT system produces well-defined polymers across a range of molecular weights (5-50 kg/mol) with low molar mass dispersities (Mw/Mn < 1.3). Subsequent facile demethylation of poly(3,4-dimethoxystyrene) (PDMS) yields poly(3,4-dihydroxystyrene) (PDHS), a catechol-bearing polymer, in quantitative yields. Semiquantitative zinc binding capacity analysis of both polymers using SEM/EDXA has demonstrated that both PDMS and PDHS have considerable surface binding (65% and 87%, respectively), although the films deposited from PDMS are of a better quality and processability due to solubility and lower processing temperatures. © 2014 American Chemical Society.

Competitive processes in controlled cationic ring-opening polymerization of oxetane:a Lotka-Volterra predator-prey model of two growing species competing for the same resources

The activation-deactivation pseudo-equilibrium coefficient Qt and constant K0 (=Qt x PaT1,t = ([A1]x[Ox])/([T1]x[T])) as well as the factor of activation (PaT1,t) and rate constants of elementary steps reactions that govern the increase of Mn with conversion in controlled cationic ring-opening polymerization of oxetane (Ox) in 1,4-dioxane (1,4-D) and in tetrahydropyran (THP) (i.e. cyclic ethers which have no homopolymerizability (T)) were determined using terminal-model kinetics. We show analytically that the dynamic behavior of the two growing species (A1 and T1) competing for the same resources (Ox and T) follows a Lotka-Volterra model of predator-prey interactions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Enzymatic Polymerization of High Molecular Weight DNA

The use of DNA as a polymeric building material transcends its function in biology and is exciting in bionanotechnology for applications ranging from biosensing, to diagnostics, and to targeted drug delivery. These applications are enabled by DNA’s unique structural and chemical properties, embodied as a directional polyanion that exhibits molecular recognition capabilities. Hence, the efficient and precise synthesis of high molecular weight DNA materials has become key to advance DNA bionanotechnology. Current synthesis methods largely rely on either solid phase chemical synthesis or template-dependent polymerase amplification. The inherent step-by-step fashion of solid phase synthesis limits the length of the resulting DNA to typically less than 150 nucleotides. In contrast, polymerase based enzymatic synthesis methods (e.g., polymerase chain reaction) are not limited by product length, but require a DNA template to guide the synthesis. Furthermore, advanced DNA bionanotechnology requires tailorable structural and self-assembly properties. Current synthesis methods, however, often involve multiple conjugating reactions and extensive purification steps.

The research described in this dissertation aims to develop a facile method to synthesize high molecular weight, single stranded DNA (or polynucleotide) with versatile functionalities. We exploit the ability of a template-independent DNA polymerase−terminal deoxynucleotidyl transferase (TdT) to catalyze the polymerization of 2’-deoxyribonucleoside 5’-triphosphates (dNTP, monomer) from the 3’-hydroxyl group of an oligodeoxyribonucleotide (initiator). We termed this enzymatic synthesis method: TdT catalyzed enzymatic polymerization, or TcEP.

Specifically, this dissertation is structured to address three specific research aims. With the objective to generate high molecular weight polynucleotides, Specific Aim 1 studies the reaction kinetics of TcEP by investigating the polymerization of 2’-deoxythymidine 5’-triphosphates (monomer) from the 3’-hydroxyl group of oligodeoxyribothymidine (initiator) using in situ 1H NMR and fluorescent gel electrophoresis. We found that TcEP kinetics follows the “living” chain-growth polycondensation mechanism, and like in “living” polymerizations, the molecular weight of the final product is determined by the starting molar ratio of monomer to initiator. The distribution of the molecular weight is crucially influenced by the molar ratio of initiator to TdT. We developed a reaction kinetics model that allows us to quantitatively describe the reaction and predict the molecular weight of the reaction products.

Specific Aim 2 further explores TcEP’s ability to transcend homo-polynucleotide synthesis by varying the choices of initiators and monomers. We investigated the effects of initiator length and sequence on TcEP, and found that the minimum length of an effective initiator should be 10 nucleotides and that the formation of secondary structures close to the 3’-hydroxyl group can impede the polymerization reaction. We also demonstrated TcEP’s capacity to incorporate a wide range of unnatural dNTPs into the growing chain, such as, hydrophobic fluorescent dNTP and fluoro modified dNTP. By harnessing the encoded nucleotide sequence of an initiator and the chemical diversity of monomers, TcEP enables us to introduce molecular recognition capabilities and chemical functionalities on the 5’-terminus and 3’-terminus, respectively.

Building on TcEP’s synthesis capacities, in Specific Aim 3 we invented a two-step strategy to synthesize diblock amphiphilic polynucleotides, in which the first, hydrophilic block serves as a macro-initiator for the growth of the second block, comprised of natural and/or unnatural nucleotides. By tuning the hydrophilic length, we synthesized the amphiphilic diblock polynucleotides that can self-assemble into micellar structures ranging from star-like to crew-cut morphologies. The observed self-assembly behaviors agree with predictions from dissipative particle dynamics simulations as well as scaling law for polyelectrolyte block copolymers.

In summary, we developed an enzymatic synthesis method (i.e., TcEP) that enables the facile synthesis of high molecular weight polynucleotides with low polydispersity. Although we can control the nucleotide sequence only to a limited extent, TcEP offers a method to integrate an oligodeoxyribonucleotide with specific sequence at the 5’-terminus and to incorporate functional groups along the growing chains simultaneously. Additionally, we used TcEP to synthesize amphiphilic polynucleotides that display self-assemble ability. We anticipate that our facile synthesis method will not only advance molecular biology, but also invigorate materials science and bionanotechnology.

Preparation of Polymer Nanoparticles by Ring Opening Metathesis Polymerization in Aqueous Dispersed Systems with Water-Soluble Ruthenium-based Catalyst

Ring opening metathesis polymerization (ROMP) is a variant of olefin metathesis used to polymerize strained cyclic olefins. Ruthenium-based Grubbs’ catalysts are widely used in ROMP to produce industrially important products. While highly efficient in organic solvents such as dichloromethane and toluene, these hydrophobic catalysts are not typically applied in aqueous systems. With the advancements in emulsion and miniemulsion polymerization, it is promising to conduct ROMP in an aqueous dispersed phase to generate well-defined latex nanoparticles while improving heat transfer and reducing the use of volatile organic solvents (VOCs). Herein I report the efforts made using a PEGylated ruthenium alkylidene as the catalyst to initiate ROMP in an oil-in-water miniemulsion. 1H NMR revealed that the synthesized PEGylated catalyst was stable and reactive in water. Using 1,5-cyclooctadiene (COD) as monomer, we showed the highly efficient catalyst yielded colloidally stable polymer latexes with ~ 100% conversion at room temperature. Kinetic studies demonstrated first-order kinetics with good livingness as confirmed by the shift of gel permeation chromatography (GPC) traces. Depending on the surfactants used, the particle sizes ranged from 100 to 300 nm with monomodal distributions. The more strained cyclic olefin norbornene (NB) could also be efficiently polymerized with a PEGylated ruthenium alkylidene in miniemulsion to full conversion and with minimal coagulum formation.