Chapter II Opening the Way to Very Long Telechelic Polymers
2.1 Introduction
2.1.1 Background
association of the end groups, telechelic associative polymers can act, even at low concentrations, as effective modifiers for low-shear rheological properties without considerably altering the high-shear properties.18,19 They therefore have been used for a number of industrial applications where careful control of rheological properties is required, e.g., paints, coating, cosmetics, foods, and pharmaceuticals.19,20 A representative example of a telechelic associative polymer used in aqueous systems is hydrophobically-modified ethoxylated urethanes (HEURs), where the main chain is water-soluble poly(ethylene oxide) (PEO) and the chain ends are capped with hydrophobic groups such as long alkyl or fluoroalkyl groups through urethane groups.15 As for apolar systems, linear hydrophobic chains (such as polystyrene, polyisoprene, and poly(ethylene butylene)) end-capped with ionic groups (also known as ionomers) serve as good examples of telechelic associative polymers.21,22 A variety of techniques have been reported in the literature to prepare telechelic polymers, including anionic and cationic polymerizations, conventional radical polymerization, controlled radical polymerizations, polycondensation, and ring-opening metathesis polymerization.12,23
The demand for an effective mist-control fuel additive calls for telechelic associative polymers with high-molecular-weight (100 kg/mol << Mw < 1000 kg/mol), fuel-soluble backbones, and terminal groups that can effectively associate in fuels, as well as an economically feasible route to prepare these polymers (see Chapter 1). These requirements distinguish ring-open metathesis polymerization (ROMP), one of many valuable applications of the highly recognized olefin metathesis chemistry (2005 Nobel Prize in Chemistry), from the other methods discussed above as the ideal approach to prepare such high-molecular-weight telechelic associative polymers. In ROMP, cyclic
olefin monomers are polymerized into unsaturated linear polyolefins that are chemically similar to fuels.24,25 When acyclic functional olefins are used in small quantities as chain- transfer agents (CTAs) along with cyclic monomers in ROMP, they not only permit the regulation of polymer molecular weight, but also effectively transfer functional groups to the ends of the polymer chains, yielding a high degree of end-functionalization (> 95%) (see Figure 2.1).6,9,10 The built-in ability of ROMP to directly end-functionalize a growing polymer chain provides a huge advantage over the other competing techniques, since most, if not all, of them require post-polymerization functionalization, which can be inefficient and expensive for polymer chains with molecular weights in the range of interest for mist-control applications.23 Recent advances in ruthenium-based catalysts for olefin metathesis, also known as Grubbs catalysts, have achieved high tolerance towards a variety of functional groups and allowed ROMP to be performed under extremely mild and user-friendly conditions.26 Consequently, ROMP has become a popular approach in recent years to prepare polymers with well-defined and complex structures.27-30
Key aspects of the design of telechelic associative polymers for mist-control of fuels include the size and chemical nature of the backbone and structures of the terminal associative groups. The former factors are determined by the selection of cyclic olefin monomers, the metathesis catalyst, and the monomer:CTA:catalyst ratio; while the latter solely rely on the design of CTAs. Mechanistically, ROMP is thermodynamically driven by the release of ring strain in cyclic olefins, and thus a highly strained monomer is preferred to achieve polymer backbones with high molecular weights (cyclic olefins with ring strains less than 5 kcal/mol have been reported to not readily polymerize).31-33 Representative examples of commercially available cyclic olefins that possess sufficient
ring strain as ROMP monomers are given in Table 2.1.31 Among these monomers, cis,cis- 1,5-cyclooctadiene (referred to as COD hereinafter) is considered the most appropriate one for building mist-control polymers as additives for fuels, because it possesses a high ring strain (13.3 kcal/mol) and its corresponding polymer backbone, 1,4-polybutadiene (1,4-PB), has been shown to have excellent solubility in fuels over a wide range of temperature.34 Although norborene has an even higher ring strain (27.2 kcal/mol) and can be easily polymerized into very high molecular weights (>1,000 kg/mol), the poor solubility of its corresponding polymer, polynorbornene, in common organic solvents renders it an inappropriate building block for mist-control polymers.35 Despite of the advantages provided by COD, the monomer has historically been shown to be not viable to polymerize into polymer with Mw >100 kg/mol due to the interference with the metathesis catalyst by the isomer of COD, 4-vinylcyclohexene (VCH) (which is usually found at 0.05-0.1 mol% in commercial “redistilled” grade of COD), and the difference in boiling point between COD (150oC) and VCH (129oC) is too small to allow effective removal of VCH by means of fractional distillation on a bench-top scale. 36,37 In 2004, the Macosko group at the University of Minnesota reported a ground-breaking approach to synthesize telechelic 1,4-PBs with Mw up to 260 kg/mol via ROMP of chemically purified COD. Specifically, they found that after treating redistilled-grade COD with ≥12 mol% of BH3·THF complex, the level of VCH was reduced below the detection limit of
1H NMR (<100 ppm), and the subsequent polymerization of purified COD could easily afford the aforementioned high polymer when a 2000:1 monomer:CTA ratio was used;
nevertheless a higher monomer:CTA ratio did not yield higher molecular weight.38 The effects of other contaminants in COD (such as peroxides39 and n-butanol introduced by
BH3·THF complex40) on the highest achievable molecular weight of telechelic 1,4-PBs, however, were not addressed in the work by the Macosko group. To the best of our knowledge, the Mw of telechelic 1,4-PB reported by the Macosko group (260 kg/mol) is the highest ever in the literature. The value, however, merely reaches the lower bound for mist-control applications. A method to prepare telechelic 1,4-PB with even higher molecular weights, for instance, 500 kg/mol, is needed.
The design of CTA is also an important factor in the development of high- molecular-weight telechelic 1,4-PBs as mist-control additives for fuels. As addressed previously, CTAs regulate the polymer molecular weight and transfer functional groups (in this case, associative groups or their precursors), to polymer chain ends in ROMP.
Carboxyl groups are ideal associative groups for self-associative telechelic 1,4-PBs because they are able to perform pair-wise self-association in apolar media, and they can be burnt in an engine cleanly.34,41 In practice, they need to be “protected” by functional groups that can be easily removed afterwards, for instance, tert-butyl group, because of the poor solubility of carboxylic acids in solvents suitable for ROMP of COD (e.g., dichloromethane (DCM) and toluene) and the interference of carboxyl groups with metathesis catalysts.6
It should be noted that the binding strength provided by a pair of associative carboxylic acid groups may not be strong enough to hold polymer chains with Mw >> 100 kg/mol together, therefore ideal CTAs for preparing self-associative telechelic polymers as mist-control additives will possess multiple tert-butyl ester groups, and the resultant telechelic 1,4-PBs end-capped with multiple tert-butyl ester groups will need to be
deprotected so as to recover the carboxyl groups on chain ends. There are two possible configurations to cluster multiple tert-butyl ester groups: (1) a short, narrow-disperse functional block of tert-butyl ester (e.g., poly(tert-butyl acrylate), PtBA); (2) a dendron terminated with tert-butyl ester groups (Figure 2.2). A dendron configuration is preferable due to (1) the unparalleled structural uniformity of dendrons which provides precise control of the number of self-associative groups on polymer chain ends, and (2) the dendritic configuration allows very low number of self-associative groups (e.g., 1, 2, and 4) on polymer chain ends that cannot be achieved in the configuration of short functional block, a feature that could potentially prevent the poor solubility of the resultant telechelic polymers in fuels due to having too many associative groups on chain ends. An example of such dendritic CTAs and corresponding telechelic polymers by ROMP was reported by Sill and Emrick in 2005.42 In their work, they prepared bis- dendritic CTAs with 1st to 3rd generations (G1-G3) of benzyl bromide-terminated poly(benzyl ether) dendrons (or “Fréchet” dendrons, Figure 2.2 (c)) and used these CTAs in ROMP of cyclooctene to prepare dendron-poly(cyclooctene)-dendron ABA triblock copolymers. Such a molecular architecture, to the best of our knowledge, has never been employed to prepare fuel-soluble telechelic polymers with well-defined, dendritic end- associative units.