Miktoarm Star Polymers

From Basics of Branched Architecture to Synthesis, Self-assembly and Applications

Ashok Kakkar

• Publisher: Royal Society of Chemistry
• Year: 2017
• Edition: 1
• eBook ISBN:
  9781788011259
• Topic: Technology & Engineering
• Subtopic: Physical & Theoretical Chemistry
• Index: Technology & Engineering

The term 'miktoarm polymers' refers to asymmetric branched macromolecules, a relatively new entry to the macromolecular field. Recent advances in their synthesis and intriguing supramolecular chemistry in a desired medium has seen a fast expansion of their applications. The composition of miktoarm polymers can be tailored and even pre-defined to allow a desired combination of functions, meaning polymer chemists can have complete control of the overall architecture of these macromolecules. By carefully selecting the composition, they can create supramolecular structures with intriguing properties, particularly for applications in biology.
Miktoarm Star Polymers features chapters from experts actively working in this field, and provides the reader with a unique introduction to the fundamental principles of this exciting macromolecular system. Topics covered include the design, synthesis, characterization, self-assembly and applications of miktoarm polymers.
The book is an excellent overview and up to date guide to those working in research in polymer chemistry, materials science, and polymers for medical applications.

CHAPTER 1

Miktoarm Star (µ-Star) Polymers: A Successful Story

HERMIS IATROU^a, APOSTOLOS AVGEROPOULOS^1,b,c, GEORGIOS SAKELLARIOU^a, MARINOS PITSIKALIS^a AND NIKOS HADJICHRISTIDIS^2*a,b

^a University of Athens, Department of Chemistry, Industrial Chemistry Laboratory, Panepistimiopolis Zografou, 15771, Athens, Greece

^b King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, Thuwal, 23955, Saudi Arabia

^c Department of Materials Science Engineering, University of Ioannina, University Campus-Dourouti, Ioannina 45110, Greece

^* E-mail: [email protected]

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 1948,¹ 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,² 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.⁵ Other compounds used as linking agents, such as the cyclic trimer of phosphonitrilic chloride,⁶ 2,4,6-tri(allyloxy)triazine,⁷ 1,1,4,4-tetraphenyl-1,4-bis (diallyloxytriazine)butane,⁸ tin tetrachloride,⁹ and phosphorus trichloride,¹⁰ suffer the same disadvantage. Decker and Rempp¹¹ 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 Milkovich¹² 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.¹³ 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.

Table 1.1 Evolution of the synthesis of symmetric star polymers with various functionalities (f) from anionic polymerization and chlorosilane linking agents

Year	Author	Monomer	Functionality (f)
1962	Morton, Helminiak, Gadkary, Bueche^2	Styrene	Mixt. of 3/4
1965	Zelinski, Wofford^14	Butadiene	3, 4
1972	Roovers, Bywater^15	Styrene	4
1974	Roovers, Bywater^16	Styrene	6
1974	Hadjichristidis, Roovers^17	Isoprene	4, 6
1978	Hadjichristidis, Guyot, Fetters^18	Isoprene	8, 12
1980	Hadjichristidis, Fetters^19	Isoprene	18
1983	Roovers, Hadjichristidis, Fetters^20	Styrene	12, 18
1986	Nguyen, Hadjichristidis, Fetters^21	Styrene-b-isoprene	18
1992	Zhou, Hadjichristidis, Toporowski, Roovers^22	Butadiene	32
1993	Roovers, Zhou, Toporowski, van der Zwan, Iatrou, Hadjichristidis^23	Butadiene	64, 128

In 1989, Roovers and collaborators,²⁴ 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.²⁵ In addition, these model polymers were used to test the existing related theories.²⁶

Many other interesting linking systems using cationic, group transfer, or living ring-opening metathesis polymerization were later developed, leading to symmetric star vinyl ethers,^27–31 isobutylenes,^32–36 methacrylates,³⁷ and norbornenes.^38,39

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 µικτός meaning mixed) was adopted by our group for stars with chemical asymmetry. The term heteroarm stars (hetero coming from the Greek word έτερος, meaning the other) is not appropriate for ...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Miktoarm Star (µ-Star) Polymers: A Successful Story
• Chapter 2 Precise Synthesis of Multi-Component Miktoarm Star Polymers by a New Conceptual Iterative Methodology Using Living Anionic Polymerization
• Chapter 3 Facile Synthesis of Multicomponent Star Copolymers via Controlled Polymerization and Click Chemistry
• Chapter 4 Use of Click Chemistry as a Coupling Strategy for the Synthesis of Miktoarm Star Polymers
• Chapter 5 Micellar and Emulsion-Assisted Drug Delivery: Comparison of Miktoarm Star Polymers and Block Copolymers
• Chapter 6 Synthetic Articulation of Miktoarm Polymers for Applications in Biology
• Chapter 7 Supramolecular (Miktoarm) Star Polymers: Self-Assembly and Applications
• Subject Index