Jesus Christ, for His salvation and unlimited grace, and my family, for their unconditional love and support. Finally, I express my gratitude to my family and my wife Wei-Ling, for their unconditional love and support.
List of Illustrations and Tables
A-16 Figure A.8 Two-step synthesis of telechelic 1,4-PB with Mw = 204 kg/mol with difunctional chlorine-terminated dendritic end groups. A-17 Figure A.9 Synthesis of telechelic 1,4-PB with Mw = 22 kg/mol with tetrafunctional chlorine-terminated dendritic end groups.
Introduction
- Technological Need for Safer Kerosene-Based Fuel
- Development of Mist-control Additives for Fuels-A Historical Review .1 Early Research Efforts
- Renewed Motivation and Efforts After the 9-11 Terrorist Attack
- Overview of the Present Work
- Objective
- Organization
- Figures and Tables
Research efforts were focused on creating scientific understanding of the relationship between the molecular design of fog control polymers and dissolution behavior. As an intermediate step in fulfilling the molecular design in Figure 1.9, self-associative carboxyl group is used as the end-associative group in this chapter.
Air flow(432 m/s)
Azobisisobutyronitrile (AIBN) Tetrahydrofuran (THF), 55oC. Figure 1.4 Preparation of carboxyl- and tertiary amine-functionalized 1250KPBs with thiol-ene addition. Note that the ejected liquid breaks up so quickly that only a small number of individual droplets can be separated after 0.5 ms.
Opening the Way to Very Long Telechelic Polymers
Introduction
- Background
- Scope of the Present Work
CTA design is also an important factor in the development of high molecular weight telehelical 1,4-PBs as fuel mist control additives. The restoration of the carboxyl groups at the chain ends of the prepared telehelical 1,4-PB by removing the tert-butyl groups is also studied.
Experimental
- Materials and Instrumentation
- Synthesis of Chain Transfer Agents (CTAs) and Intermediates
- Polymer Synthesis (Scheme 2.2)
The solvent was removed under reduced pressure, and the residue was dissolved in 30 mL of DCM. The solvent was removed under reduced pressure, and the residue was dissolved in 5 mL of DCM.
Results
- Synthesis of Chain Transfer Agents (CTAs)
- Monomer Purification
- Polymer Synthesis
- Hydrolysis of tert-Butyl Ester Groups on Polymer Chain Ends
The family of well-defined CTAs 3, 5, 8 and 10 offered precise control of the number of tert-butyl ester groups on polymer chain ends (N), which later translated into the tunability of the strength of end association after the removal of tert-butyl groups (ref to Chapter 3). All tert-butyl ester-terminated telechelic 1,4-PBs synthesized in the present study were found to be transparent at room temperature. In the case of telechelic 1,4-PBs synthesized from GTAs 8 and 10, change in appearance was observed after TFA hydrolysis of tert-butyl ester groups on polymer chain ends: Their corresponding carboxyl-terminated polymers were observed to appear opaque at room temperature.
Discussion
- Aspects of the CTA Design
- Molecular-Weight Control
- Hydrolysis of tert-Butyl Ester Terminal Groups of Polymers
The polymer synthesis results in Table 2.2 and Appendix A illustrate the effects of the purity of COD on ROMP using Grubbs II. We found that the monomer purified according to the Macosko protocol contained sufficient amount of n-butanol and peroxides to completely quench the metathetic activity of the ruthenium. The occurrence of carboxyl-terminated 1,4-PBs with Mw ~ 230 kg/mol (Figure 2.7) illustrated the effect of the number of carboxyl groups on polymer chain ends (N).
Conclusions
Figures, Schemes, and Tables
Note that the G2 CTA is chosen as a representative example to demonstrate the ease of incorporating G0-G3 bis-dendritic CTAs into ROMP.
TFA Hydrolysis of tert-butyl Ester Polymer End Groups
End-Associative Polymers in Apolar Aprotic Solvents
Introduction
Using the methods presented in Chapter 2 to synthesize telechelic 1,4-PBs with Mw up to 430 kg/mol capped at each end with well-defined tert-butyl ester-terminated dendrons (Scheme 2.2) provides easy access to matched pairs of non -associative and associative telechelic 1,4-PBs (Scheme 2.3). In this chapter, we use these model polymers to study the relationship between molecular properties (eg, the molecular weight of the polymer and the number of carboxyl groups on chain ends) and association behavior, particularly its effects on the rheological properties in solution. The present study of the self-association behavior of carboxyl-terminated telechelic 1,4-PBs provides a basis for comparative studies of complementary association illustrated in Figure 1.9 and considered further in Chapter 4.
Experimental .1 Materials
- Procedure for Sample Preparation
- Viscosity Measurements
Solutions of carboxyl-terminated polymers were prepared as follows: To 150 to 200 mg of carboxyl-terminated polymer in a 50-ml Schlenk flask was added the required amount of solvent for 1 wt% stock. Solutions of tert-butyl ester-terminated polymers were examined logarithmically in the shear rate range 1 - 200 s-1 (5 shear rates per decade). The range was extended to 3000 s-1 for carboxyl-terminated polymers to better capture the shear-thinning behavior.
Results
- Dissolution Behavior
- Steady-Flow Shear Viscosity of 1wt% Polymer Solutions
- Concentration Dependence of Specific Viscosity
- Shear-Thinning Behavior of Solutions of Carboxyl-Terminated Polymers The onset and magnitude of shear-thinning depend on the molecular weight and
Thus, it appears that there is a minimum number of carboxyl groups on polymer chain ends to achieve the intermolecular association required for viscosity modification (N>2) and a maximum number imposed by the solubility limit (N<8). With decreasing concentration, the extent of shear thinning decreased and the shear rate required to elicit it increased (eg, relative to the 1 wt% solution, at 0.7 wt%, the extent of shear thinning observed in both CDD and TL less significant and the onset shifted to >100 s-1) (Figures 3.6 (b) and 3.7 (b)). Similar trends were observed for solutions of 430K di-TA 1,4-PBs, with a greater degree of shear thinning and onset of shear thinning at lower shear rates compared to their 76K and 230K counterparts (in both CDD and TL, Figures 3.6 (c) and 3.7 (c), respectively).
Discussions
- Importance of Precise Control of Chain-End Structure
- Relationship between Rheology Modification and Supramolecular Structures One of the complications in using self-association (rather than complementary
In contrast, the dendritic CTA approach (see Scheme 2.1) offers precise control over the number of carboxyl groups at the ends of polymer chains. We expect that this molecular toolkit will prove equally useful as we continue to investigate the use of complementary end-association in the development of telehelical polymers as haze control additives (see Chapter 4). The difference discussed above indicates the presence of bridging chains in the supramolecular structures of the two di-TA 1,4-PB with high Mw in CDD and TL at concentrations ≥ 0.4 wt%.
Conclusions
As indicated in numerous reports, bridge chains, which constitute a small part of the temporary network and are only visible when the polymer concentration is high enough, are the active parts of the temporary network of interconnected flower-like micelles; Telechelic chains trapped in loop structures contribute little to the rheological properties of the solution.10,11,24 In other words, a significant portion of the telechelic chains are lost in the flower-like micelles. Tripathi and co-workers reported that bridging chains also carry most of the tensile stress in the transient network of self-associating telechelic polymers in extensional flow.21 Therefore, we can expect di-TA 1,4-PBs with Mw >400 kg/ mol to be used as additives for anti-fogging in kerosene, a relatively high polymer concentration, for example on the order of 0.5% by weight, will be required to enable the formation of bridging chains. The non-directional self-association of TA end groups allows aggregation numbers larger than pairwise possible and therefore many of the chains adopt unproductive configurations (loops).
Figures and Tables
Ester-terminated in CDD Carboxyl-terminated in CDD Ester-terminated in TL Carboxyl-terminated in TL. Data are not available for octa-carboxyl end groups (N = 8) due to insolubility of the material in both CDD and TL. The overlap concentration of the tert-butyl ester form of each polymer is indicated by the marks on the concentration axis, red for tetralin (TL) and blue for 1-chlorododecane (CDD); for 76K di-TE in CDD c*=1.4 wt% (scaled down).
Structure-Property Relationships for Hetero- Complementary Hydrogen-Bonding Partners
Introduction
- Directional Noncovalent Bonding
- Methods to Characterize Noncovalent Bonding
- Scope of the Present Work
Among the directional interactions, hydrogen bonding is particularly important in the development of responsive supramolecular materials due to its high sensitivity to various external stimuli (eg, solvent polarity, temperature, shear, and pH).19,21, 25 Many creative applications of hetero-complementary covalent pairs in supramolecular polymer chemistry have been reported, for example, reversible and responsive polymer networks,26 single-chain self-assembly,27 self-healing materials,18 multi-supramolecular block copolymers . Thus, a hydrogen bond can be characterized as a proton shared by two lone pairs of electrons.2 The strength of a lone hydrogen bond can be tuned by the design of the chemical structure of the donor and acceptor. One of the important design principles is that the strength of a single hydrogen bond is related to the difference in pKa value between the proton donor and proton acceptor conjugate acid, pKa.
Results
- Synthesis of Tertiary Amine-Terminated Telechelic 1,4-PBs
- Synthesis of Complementary Hydrogen-Bonding Polymer Pairs Triple-hydrogen-bonding THY/DAAP pair
- Shear Viscometric Study of Complementary End-Association
In the case of the HR/CA pair, Grubbs III was also used to synthesize telechelic associative polymers 21 and 30 (Schemes 4.4 and 4.5). In the absence of its complementary unit, the signal of the imide proton of THY end groups was observed at 8.05 ppm (Figure 4.7 (a)). A strong improvement in shear viscosity was observed due to complementary TA/TB association: at 1 wt%, the 1:1 mixture of 430K di-TA and 430K di-TB became 1,4-PBs in Jet-A found to be significantly more viscous than the individual's Jet-A solutions.
Discussions
- Insights into Hydrogen-Bonding-Based Complementary Association
- Reconciliation between 1 H NMR Results and Viscosity Data
- Connection between Shear Viscosity and Supramolecular Assembly
Similarly, the six hydrogen bonds in the HR/CA pair contribute -47.4 kJ/mol to Gasso, while the eight repulsive SEIs contribute +23.2 kJ/mol, which is also 48.9% of the contribution from primary interactions. The trend observed in the shear viscosity data is not surprising, since the binding strength of the TA/TB pair is, as mentioned earlier, much higher than that of THY/DAP and HR/CA. Another feature of the equilibrium nature of THY/DAAP and HR/CA complementary associations related to the discrepancy between 1H NMR conclusions and shear viscometry data is that the size of the polymer backbone and the concentration of the end group can affect the strength of the complementary associations.
Conclusions
Further characterizations, for example, dynamic and static light scattering (DLS and SLS), and small-angle neutron scattering (SANS), will be useful to provide deeper insights into the supramolecular self-assembly of complementary associative pairs di-DA/di-DB and di-TA/ di-TB 1,4-PBs in nonpolar media.
Experimental
- Triple Hydrogen-Bonding THY/DAAP Pair
- Sextuple Hydrogen-Bonding HR/CA Pair
- CAHB-Based DA/DB and TA/TB Pairs
- Shear Viscometric Study of Hetero-Complementary End-Association
The reaction mixture was stirred at 40 °C for 16 h and precipitated in 300 mL of methanol at room temperature. After cooling to room temperature, the reaction mixture was precipitated in 100 mL of acetone at room temperature. After cooling to room temperature, the reaction mixture was precipitated in 300 mL of acetone at room temperature.
Figures, Schemes, and Tables
Note that the overlay shows the complete consumption of the di-THY macro CTA in the synthesis of 288K di-THY 1,4-PB. Note that the overlay shows the complete consumption of the di-DAAP macro CTA in the synthesis of 219K di-DAAP 1,4-PB. Note that the overlay shows the complete consumption of the di-CA macro CTA in the synthesis of 200K di-CA 1,4-PB.
