Miktoarm polymers constitute a relatively new entry to the macromolecular field. However, with the recent advances in the synthesis of these branched macromolecules and their intriguing supramolecular chemistry in a desired medium, the scope of their applications is fast expanding. Providing a detailed monograph on the topic, the book features chapters from experts actively working in this field, giving the reader a unique overview of the fundamental principles of this exciting macromolecular platform. Topics covered include the design, synthesis, characterization, self-assembly and applications of the polymers.
About the Author
King Abdullah University, Saudi Arabia
Tokyo Institute of Technology, Japan
Soochow University, China
Gebze Institute of Technology, Turkey
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Miktoarm Star Polymers
From Basics of Branched Architecture to Synthesis, Self-assembly and Applications
By Ashok Kakkar
The Royal Society of ChemistryCopyright © 2017 The Royal Society of Chemistry
All rights reserved.
Miktoarm Star (µ-Star) Polymers: A Successful Story
HERMIS IATROU, APOSTOLOS AVGEROPOULOS, GEORGIOS SAKELLARIOU, MARINOS PITSIKALIS AND NIKOS HADJICHRISTIDIS
1.1 The Genesis of Miktoarm (µ-Star) Star Polymers
Well-defined polymers with narrow molecular, structural, and compositional (in the case of copolymers) dispersity are essential for establishing structure or composition–property relationships and indispensable to accomplish one of the ultimate goals of polymer chemistry: designing macromolecules with predetermined properties/applications.
Among branched polymers, regular or symmetric stars consisting of several identical linear chains linked together at one chain-end initially attracted the attention of scientists since the star structure has the simplest form of branching. The earliest attempt to synthesize star polymers was that by Schaefgen and Flory in 1948, who synthesized the first 4- and 8-arm star homopolymers (polyamides) by polymerizing ε-caprolactam in the presence of either cyclohexanone-tetrapropionic or dicyclohexanone-octacarboxylic acid.
Fourteen years later, Morton and coworkers, taking advantage of the living character of anionic polymerization, succeeded to synthesize 4-arm star homopolystyrenes (PS) by 'terminating' living polystyryllithium with tetrachlorosilane (linking agent). Although the produced materials were mixtures of 3- and 4-arm PS, this work eventually led to the preparation of star polymers with up to 128 arms.
In 1963, Orofino and Wenger3 were the first to use tri(chloromethyl) benzene in combination with anionic polymerization as a linking agent to prepare 3-arm star PS. Mayer4 used 1,2,4,5-tetra-(chloromethyl)benzene to prepare 4-arm star di- and triblock copolymers of styrene and isoprene. It was difficult to extend the functionality (f) of stars beyond f = 6 with chloromethylbenzene derivatives due to the unavailability of chloromethylbenzene-based linking agents. Other compounds used as linking agents, such as the cyclic trimer of phosphonitrilic chloride, 2,4,6-tri(allyloxy) triazine, 1,1,4,4-tetraphenyl-1,4-bis (diallyloxytriazine)butane, tin tetrachloride, and phosphorus trichloride, suffer the same disadvantage. Decker and Rempp demonstrated for the first time the validity of divinylbenzene (DVB) as a linking agent by preparing and properly characterizing PS stars with 6 to 15 arms. The DVB method was apparently first alluded to by Milkovich but, unfortunately, in his patent there was no clear indication that star-branched polymers had been prepared. It should be noted that the DVB method does not allow the accurate control of the number of star arms since the polymerization of DVB (difunctional monomer) with the living chains is not well controlled. Considering the disadvantages of the aforementioned compounds as linking agents, multifunctional chlorosilane compounds became the reagents of choice for the preparation of well-defined stars. Table 1.1 summarizes the evolution of the synthesis of symmetric star polymers with chlorosilane linking agents.
In 1989, Roovers and collaborators, using a multifunctional linking agent designed/prepared by hydrosilylation of a low molecular weight linear or star 1,2-polybutadiene, succeeded to synthesize star polybutadienes (PB) with 200 and 270 arms. The exhaustive studies of the properties of these well-defined stars led to important conclusions concerning the influence of the star architecture on their properties in solution and bulk. In addition, these model polymers were used to test the existing related theories.
Many other interesting linking systems using cationic, group transfer, or living ring-opening metathesis polymerization were later developed, leading to symmetric star vinyl ethers, isobutylenes, methacrylates, and norbornenes.
Later on, the synthesis of stars with different arms either in molecular weight (molecular weight asymmetry; asymmetric stars) or chemistry (chemical asymmetry; miktoarm stars) was achieved (Scheme 1.1a and 1.1b). The term miktoarm stars (coming from the Greek word [TEXT NOT REPRODUCIBLE IN ASCII.] meaning mixed) was adopted by our group for stars with chemical asymmetry. The term heteroarm stars (hetero coming from the Greek word [TEXT NOT REPRODUCIBLE IN ASCII.], meaning the other) is not appropriate for this class of polymers, since it does not convey the concept of a star composed of dissimilar arms. Later, the term was expanded to stars with molecular weight asymmetry, which can be considered miktoarm homopolymers. Stars having arms of similar chemical nature but different end-functional groups also belong to this category. Finally, topologically asymmetric stars are also µ-Stars. They consist of diblock copolymer arms attached by different chain-ends to the star center. π-Shaped, H-shaped, super-H or pom-pom copolymers can be considered as double miktoarm stars (Scheme 1.1c).
Pennisi and Fetters were the first to report the synthesis of 3-arm asymmetric star homopolymers of PB and PS. Mays and our group, based on the work of Fetters, were the first to synthesize a 3µ-Star copolymer consisting of two polyisoprene (PI) and one PS arm and a 3µ-Star terpolymer of PS, PI, and polybutadiene (PBd or PB), respectively. Later, different miktoarm stars were synthesized by anionic polymerization and selective chlorosilane-based linking chemistry, as reported mainly by our group, representative examples of which are given in Scheme 1.1. The evolution of the synthesis of different miktoarm structures are summarized in Table 1.2.
Synthetic efforts based on anionic polymerization leading to miktoarm stars (µ-Stars) are reviewed in this chapter. In addition, a few examples of the striking influence of the star structure on the morphology of block co- and terpolymers are given. The structures synthesized by anionic polymerization guided scientists working with other types of polymerization techniques, such as controlled/living, ring opening, catalytic, ring opening metathesis polymerization reactions, etc., towards the synthesis of star architectures. The tremendous influence of miktoarm stars on polymer science is evidenced by: (i) the need for a book on miktoarm stars, (ii) the high number of references miktoarm stars have produced in the last years (h-index = 70, references = 21 275, from our first paper to August 2016, source: ISI web of science), and (iii) the significant amount of novel nanostructures reported by this class of materials.
1.2 Synthesis of Miktoarm Star (µ-Star) Polymers
Two general strategies have been developed for the synthesis of miktoarm stars via living anionic polymerization. The first one is based on divinyl compounds, either homopolymerizables (e.g., divinylbenzene) or non-homopolymerizables (e.g., double diphenylethylene, DDPE), and the second one on multifunctional linking agents. Several linking agents have been used for the synthesis of star polymers. The most commonly used ones are chlorosilanes, adopted mostly for non-polar chains, and chloro-/bromomethyl benzenes, adopted for polar ones. A few more complex linking agents have also been used in the synthesis of star-like copolymers.
1.2.1 Divinylbenzene (Homopolymerizable Linking Agent)
The use of DVB for the synthesis of miktoarm stars was first recognized by Eschwey and Burchardand developed mainly by Rempp and colleagues. The general route is given in Scheme 1.2.
The living macroinitiator/precursor ALi polymerizes a small amount of DVB, leading to the formation of a star molecule bearing a number of active sites within its core (microgel nodule of DVB), which is theoretically equal to the number of incorporated A arms of the star polymer. Subsequent addition of another monomer, B, or the same monomer A, yields a µ-star copolymer or asymmetric homopolymer star, respectively. The growing B arms have anionic sites at their outer ends, thus providing the possibility of reacting with electrophilic compounds or other monomers towards the preparation of end-functionalized stars or star-block copolymers. Because of its simplicity, this method can be carried out under inert atmosphere, thereby avoiding the use of highly demanding and time-consuming vacuum techniques. The DVB method has been applied in the synthesis of µ-Star copolymers of the AnBn type, with the A arm being polystyrene and the B arm poly(tert-butyl methacrylate), poly(tert-butyl acrylate), poly(ethylene oxide), or poly(2-vinyl pyridine). Usually, n varies between 6 and 20. PS µ-star homopolymers of the type AnA'n have also been prepared by this method.
The DVB method is characterized by several disadvantages, the foremost being architectural limitations. Only stars with equal number of arms different in chemical nature or molecular weight, AnBn, can be prepared. In fact, since the polymerization of DVB by living chains is not controllable, n is an average value influenced by several parameters. Specifically, n increases with the decreasing molecular weight of the precursor A and the molar ratio of DVB to living chains. Another disadvantage is that the B arms cannot be isolated and characterized independently. Finally, reaction of the living ends with the remaining double bond of the DVB nodule can lead to the formation of loops (intramolecular reaction) or networks (intermolecular reaction).
1.2.2 Double Diphenylethylenes (Non-Homopolymerizable Linking Agents)
Hocker and Latterman recognized in 1976 the usefulness of the addition of living chains to non-homopolymerizable divinyl compounds. They were the first to propose that 4µ-stars could be obtained by adding living chains to bis(1-phenylvinyl)benzenes, followed by the subsequent addition/polymerization of another monomer. In 1978, Szwarc and coworkers studied the kinetics of addition of PSLi to several divinyl compounds in benzene by UV spectroscopy. They found that, in the case of the para-double diphenyl ethylene (PDDPE) 1,4-bis(1-phenylethenyl)benzene, the ratio of the rate constants of the first and second additions was equal to 13. In 1983, Leitz and Hocker reported that the reaction of two moles of sec-BuLi with the meta -double diphenylethylene (MDDPE) 1,3-bis(1-phenylethenyl)benzene proceeds rapidly and efficiently to produce a dilithium initiator. The ratio of the rate constants of the first and second additions is almost identical in toluene.
Quirk and coworkers have further developed this 'living linking method' for the synthesis of 3µ- and 4µ-stars. The general reactions are given in Schemes 1.3 and 1.4, respectively. PDDPE is usually employed for the synthesis of 3µ-AµB and 3µ-ABC, whereas MDDPE is used for A2B2 4µ-stars. More recently, Quirk's group extended this method to 6µ-stars by using a triple diphenylethylene, 1,3,5-tris(1-phenylethyl)benzene.
The key to the living linking procedure is the control of the stoichiometry of the reaction between the living A chains and the DDPE; otherwise, a mixture of star and linear polymers is produced. A major problem is that the rate constants of initiation of the two new active sites differ, resulting in a bimodal distribution. To overcome this problem, polar compounds have to be added. It is well known that they dramatically affect the microstructure of the polydienes formed in later stages. However, addition of lithium sec-butoxide to the living DDPE derivative prior to the addition of the diene monomer was found to produce monomodal well-defined µ-stars with high 1,4 content. Again, the B arms cannot be isolated from the reaction mixture and characterized separately. Nevertheless, Quirk's method is valuable for the synthesis of ω-functionalized µ-stars.
Regular homopolymer and block copolymer stars can be synthesized by reaction of an excess of living chains, prepared by anionic polymerization, with the appropriate chlorosilane. An excess of the living polymer is needed to force the linking reaction to completion. For the synthesis of µ-stars, each chlorine atom should be replaced stepwise by a different chain. To achieve this goal, the different reactivity of the living chain ends towards the SiCl group must be taken into consideration. The reactivity of the living chain end decreases with the charge delocalization and by increasing the steric hindrance as follows: butadienyl lithium (BdLi) > isoprenyl lithium (IsLi) > styryl lithium (SLi) > diphenyl ethylenyl lithium (DPELi). The reactivity of the living ends is also affected by the chain molecular weight (the lower the molecular weight, the lower the steric hindrance and, consequently, the higher the reactivity), the polarity of the environment (the higher the polarity, the lower the association of the living chains and, consequently, the higher the reactivity), and the temperature (same as the environment).
Chlorosilanes cannot be combined with macroanions of polar monomers, such as (meth)acrylates and 2-vinyl pyridine, since the linking reaction either leads to unstable products [poly(meth)acrylates] or it does not occur at all [poly(2-vinyl pyridine), P2VP]. Instead, linking agents including chloro-/ bromomethyl benzene derivatives are used although, unfortunately, they display significant drawbacks such as lithium–halogen exchange, leading to linking agents with higher functionalities and, consequently, to a mixture of stars with different functionalities. To overcome this problem, potassium (instead of lithium) counter ions and polar solvents at low temperature (-78 °C) should be used.
126.96.36.199 Trichloromethylsilane and Tetrachlorosilane
Trichloromethylsilane (CH3SiCl3) and tetrachlorosilane (SiCl4) are appropriate linking agents for the synthesis of 3µ- and 4µ-stars. The replacement of only one Cl by one chain can be achieved by very fast addition of the living polymer into a large excess of the chlorosilane. Before the addition of the second living chain, the unreacted chlorosilane is removed to avoid contamination of the µ-stars with the homostars with three (CH3SiCl3) or four (SiCl4) arms. The above chlorosilanes can be easily removed on a high vacuum line owing to their low boiling point (CH3SiCl3, b.p.: 66 °C and SiCl4, 57.6 °C). This method was developed by Pennisi and Fetters, and was used for the synthesis of 3µ-Star homopolymers of styrene and butadiene with arm molecular weight asymmetry. The general reactions for the synthesis of 3µ-Star homopolymers are given in Scheme 1.5.
The excess A'Li needed for completion of the linking reaction is removed by fractionation, as in the case of regular stars. In the case of asymmetric PS stars, to ensure complete reaction of PSLi with AS i(CH3)Cl2, the living PS is end-capped with a few units of butadiene to increase its reactivity towards the chlorosilane.
Mays prepared one sample of 3µ-Star PS(PI)2 by this method. The method was further developed by our group to all possible combinations of A2B µ-stars, where A and B were PS, PI, or PB. Furthermore, by using SiCl4, we prepared PS(PI)3 4µ-stars. A more sophisticated high vacuum technique was used to ensure the synthesis of well-defined µ-stars. A high degree of molecular and compositional homogeneity was identified by size exclusion chromatography (SEC), with refractive index and UV-detectors, low-angle laser light-scattering (LALLS), membrane (MO) as well as vapor-pressure osmometry (VPO ), and NMR spectroscopy. The chlorosilane approach was also adopted for the synthesis of µ-stars of the PS(PI-b-PS)2 or 3 type.
When only one chlorine needs to be replaced by a highly reactive living chain, for example low molecular-weight PBLi or PI, the living end has to be transformed by end-capping into a less reactive carbanion. By decreasing the reactivity, the selectivity is increased and the replacement of only one chlorine is achieved. Using DPE, we prepared model mono- and difunctional 3µ-Star PBs and (d-PB)2(PI), (d-PB)(PI)2, where d-PB is deuterated PB, having arm molecular weights below 10 000 g mol-1. The same goal could be achieved more easily (avoiding the necessity for end-capping) by linking the first arm or arms at a temperature low enough to create selectivity between the successive steps of replacement of chlorines.
Excerpted from Miktoarm Star Polymers by Ashok Kakkar. Copyright © 2017 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Miktoarm Star Polymers: A Successful Story; Precise Synthesis of Multi-Component Miktoarm Star Polymers by a New Conceptual Iterative Methodology Using Living Anionic Polymerization; Facile Synthesis of Multicomponent Star Copolymers via Controlled Polymerization and Click Chemistry; Use of Click Chemistry as a Coupling Strategy for the Synthesis of Miktoarm Star Polymers; Micellar and Emulsion-Assisted Drug Delivery: Comparison of Miktoarm Star Polymers and Block Copolymers; Synthetic Articulation of Miktoarm Polymers for Applications in Biology; Supramolecular (Miktoarm) Star Polymers: Self-Assembly and Applications