Hyperbranched Polymers: Macromolecules in between Deterministic Linear Chains and Dendrimer Structures (Polymer Chemistry Series, Volume 16) - Hardcover

Lederer, Albena; Burchard, Walther

Hardcover

ISBN 10: 1849733945 ISBN 13: 9781849733946

Publisher: Royal Society of Chemistry, 2015

Synopsis

There is great commercial interest in hyperbranched polymers from manufacturers of polymer formulations, additives and coatings, polymer electronics and pharmaceuticals. However, these polymers are difficult to characterize due to their very complex, multidimensional distribution and there is a great need to understand how to control their synthesis to obtain certain material properties. Hyperbranched Polymers is the first book to examine in detail the recent advances in hyperbranched polymers. Focusing on the structural characterization of hyperbranched polymers, the book summarizes the research in the field and makes a direct correlation between the chemical structure and global molecular properties. This correlation is essential for understanding the structure–properties relation and fills the gap between the synthetic advances and physico-chemical understanding of this polymer class. Written by acknowledged experts in the field, the book will appeal to both scientists working in fundamental research, as well as industrial manufacturers of dendritic polymers.

About this title

About the Author

Albena Lederer obtained her PhD in 1999 from the University of Mainz after her research in the field of physical chemistry of polymers at Max-Planck-Institute of Polymer Research Mainz. Since 2000 she is extensively investigating the physico-chemical properties of branched macromolecules. Since 2007 Albena Lederer is leader of the Polymer separation group at the Leibniz-Institute of Polymer Research Dresden. To her main research area belong the characterization of dendritic and multifunctional polymers in solution and the development of new separation methods for branched polymers.

From the Back Cover

There is great commercial interest in hyperbranched polymers from manufacturers of polymer formulations, additives and coatings, polymer electronics and pharmaceuticals. However, these polymers are difficult to characterize due to their very complex, multidimensional distribution and there is a great need to understand how to control their synthesis to obtain certain material properties. Hyperbranched Polymers is the first book to examine in detail the recent advances in hyperbranched polymers. Focusing on the structural characterization of hyperbranched polymers, the book summarizes the research in the field and makes a direct correlation between the chemical structure and global molecular properties. This correlation is essential for understanding the structure properties relation and fills the gap between the synthetic advances and physico-chemical understanding of this polymer class. Written by acknowledged experts in the field, the book will appeal to both scientists working in fundamental research, as well as industrial manufacturers of dendritic polymers.

Excerpt. © Reprinted by permission. All rights reserved.

Hyperbranched Polymers

Macromolecules in Between Deterministic Linear Chains and Dendrimer Structures

By Albena Lederer, Walther Burchard

The Royal Society of Chemistry

Copyright © 2015 Albena Lederer and Walther Burchard
All rights reserved.

ISBN: 978-1-84973-394-6

Contents

Acknowledgements, xiii, 
Abbreviations, xv, 
Chapter 1 Introduction, 1, 
Chapter 2 Degree of Branching, 20, 
Chapter 3 Control over the Conformation of Dendritic Polymers, 39, 
Chapter 4 Separation and Determination of Molar Mass, 69, 
Chapter 5 Solution Viscosity, 88, 
Chapter 6 Size Determination by Scattering Techniques, 136, 
Chapter 7 Model Calculation of Different Hyperbranched Structures, 193, 
Chapter 8 Scaling Conception and Interpretation of Structural Parameters and Scattering Experiments, 
227, 
Chapter 9 Correlation of Molecular and Bulk Properties, 269, 
Subject Index, 280, 

CHAPTER 1

Introduction

1.1 Overview

Macromolecular branching belongs to nature and is of significant importance for the life functions of organisms. Branching in polymer science has been known since almost the time when first synthesis of linear chains was carried out. No doubt when Staudinger and Schulz carried out the synthesis of styrene they also tried to add divinyl benzene as a second monomer for a co-monomer. The effect was dramatic and rather unexpected. In those days besides the monomer conversion the reaction was also followed by a change in viscosity because the increase of viscosity was a qualitative mean for the increase in molar mass. In the corresponding co-polymerization the increase in the viscosity was significantly lower but after certain time a sudden increase occurred and an non-measurable high value was attained. A highly elastic, gel-like material was obtained whose behaviour greatly resembled that of natural rubber.

The sudden change from a low solution viscosity to an elastic gel remained unclear for some time until, in 1941, Paul J. Flory put forward the mathematically simple expression for the condition of the gelation point:

αc = 1/ ƒ - 1 (1:1)

He described the extent of reaction by the probability α and found a critical conversion αc when gelation takes place. f is the number of functional groups per monomer of the same reactivity, and solubility remains in the pre-gel state of α(f - 1)<1. Branching takes place only if f > 2 and gelation will occur at a connversion beyond the critical point defined by eq (1.1).

Flory intensely tried to derive the molar mass distributions of such randomly branched polymers. It was Walter H. Stockmayer who, in 1943–1944, found the solution and with this distribution he could derive the weight average molar mass expressed in term of the probability of reaction:

Mw/M0 = 1 + αƒ/1 - α(ƒ - 1) = 1 + α/1 - α(ƒ - 1), β = α/2 (1.2)

with M0 the molar mass of the monomer. Flory, as well as other researchers, was excited because this equation now gave a mathematical proof for his derivation of the prediction for the critical point of gelation. A lifelong friendship between the two pioneers, Flory and Stockmayer, began here.

The derivation of the so-simple looking equation was a hard challenge for Stockmayer. He was frustrated that he could not find a solution. It was Maria Goeppert Mayer (the later Nobel Prize winner in 1963 and wife and co-author with Joseph E. Mayer) who gave Stockmayer the decisive hint. She observed Stockmayer's attempt and after a while she said, 'Stocky, why don't you try to use our cluster description when we treated the interparticle interactions between colloidal spheres?'

Flory confirmed the Stockmayer' distribution by applying combinatoral mathematics which will be given in Chapter 7 of this book. In the course of the time Flory also derived a molar mass distribution for the polymerization of AB2 monomers and the corresponding weight average degree of polymerization. No critical conversion for gelation was obtained.

This result surprised even Flory, because he realized that a divergence can be obtained only at full conversion, i.e. α = 1 or in other words, the branching of such AB2samples can never lead to a gel or network. This discovery was the birth date for hyperbranching.

Flory's interest in this type of structure arose from a book by K. H. Meyer in 1950 and from Staudinger. The structure of natural polymers was one of the reasons why Staudinger, with co-workers, turned to chemical synthesis of model polymers in the hope of gaining a deeper understanding of these structures. Amylopectin, the branched component of starch, was one of them.

It took more than 20 years before the subject of AB2 polymers was taken up but the decisive change was made by Kim and Webster by coining the name 'hyperbranching'. These two authors were trying to find a less laborious synthesis for dendrimers, which were known to possess advantageous properties as nano-objects. Although Kim and Webster could not achieve the high accuracy of a perfect structure, so they stated, at least the samples are hyperbranched. This now generally used notation is rather unfortunate because also co-polymers of the A3

1.2 Branched Polymers

In 1935 Staudinger and Schulz observed that the changes in the viscosity of polystyrene solutions depended on the reaction conditions during synthesis. Later, Staudinger and Husemann found a higher number of end groups than expected during their studies on the natural polymers amylopectin and glycogen, which was explained by the existence of branching. Both low viscosity and high number of end groups are typical characteristics of branched polymers. In addition, impeded crystallinity and poor mechanical strength are very common in branched polymers. These properties become more pronounced with branching density and depend on branching topology.

The type of branching can strongly influence polymer properties. Examples include natural polysaccharides, which occur after enzymatic polymerization of glucose, forming different types of branching. Glycogen is formed of glucose monomers and possesses high number of branching points (Figure 1.1). There are up to 50 000 monomer units of glucose on glycogen coupled together. The glucose units are arranged in a tree-like branched structure with a branching point every 8–12 glucose units. Glycogen is an energy supplier for animal cells and is the product of an enzymatic decomposition of starch. Starch is composed of amylose and amylopectin, which are also formed of glucose units, but while amylose possesses linear or very slightly branched chains, in amylopectin branching occurs every 25 units. Starch is the product of photosynthesis in plants. Despite the similar chemical character of the monomer units, the properties of starch and glycogen are different; starch is not soluble in cold water and is semi-crystalline, while glycogen is soluble in water and is not crystalline.

Today the variety of branching in synthetic polymers is manifold and 'branching' becomes a generic term for a huge class of polymer structures. The systematic development in branched polymers started with the discovery of long-chain branched polymers in the 1940s, the synthesis of the first star and graft polymers in the 1960s and the invention of dendrimers in the 1980s and of hyperbranched polymers in the 1990s. Nowadays, the combination of different branched topologies in one macromolecule becomes a common approach to control material properties. Some of these hybrid structures are overviewed in part 1.2.3 of this chapter. Roughly, the structure of branched polymers could be classified depending on different characteristics and could be illustrated as a multidimensional property.

The regularity of branching is significant for calculating global and local properties as well as for further calculation of structural parameters. Branching can be regular or random with regard to either the length of the branches or the frequency of branching points. Figure 1.2 elucidates the differences in stars, comb polymers and dendritic polymers.

The quantification of branching is of immense importance for estimation of structure–property relationships, although, its definition is substantially influenced by the type of branching. Stars can be classified by the functionality of the core possessing only one branching point; comb-like and classical long-chain branched polymers are classified by the number of branches per molecule or per 1000 C-atoms; dendritic branching is characterized by the relative number of branched units, e.g. degree of branching, due to the significantly higher density compared to the above branching types. The calculation and determination of branching degree is described in detail in Chapter 2.

Other characteristics of branched polymers are the length of branches and the functionality of the branching point. Both are variables which have to be taken into account calculating scaling characteristics of the polymers. The length of the branches also defines their properties in solution and in bulk. Long-chain branched polymers can possess properties typical for entangled macromolecules influencing flow and crystallinity properties. Short-chain branched polymers and dendritic structures are typically non-entangled and non-crystalline objects.

In the next section of this introductory chapter a short historical and structural background of dendritic polymers and their hybrids is given. The correlation to other types of branching is part of this book as far as a systematic comparison of properties is needed for better understanding the particularities of hyperbranched polymers. Additionally, the theoretical considerations developed for long-chain branched or star polymers are critically validated for the case of dendritic polymers.

1.2.1 Dendrimers

The term 'dendritic' comes from the Greek word for tree, [TEXT NOT REPRODUCIBLE IN ASCII] and is related to the branching character of polymers. The ideal examples for dendritic polymers are dendrimers (Figure 1.3), which are built step-wise and have a branching point at every monomer unit. Due to their perfectly branched nature, aesthetic 'look', complete dimensional control and variety of functional possibilities, dendrimers have fascinated the scientific community for several decades, especially in their role as nano-objects for high-end applications. Organic chemists were attracted by the challenging synthesis, and highly interesting potential applications in the fields of biomaterials and nanotechnology have been found. A drawback of these perfect polymers is their limited industrial relevance because of high manufacturing costs.

The history of dendrimers started in 1978 with the cascade polymers of Buhleier et al., while iterative synthetic concepts were reported in the early 1970s by Lehn and Cram et al. In 1982, Aharoni et al. studied lysine trees for peptide synthesis. In 1985 the first concepts of dendrimers, or arborols, were reported, giving birth to a new polymer topology class which expanded quickly to a huge variety of chemical structures, modifications and applications.

The special properties of dendritic polymers could easily be explained on the basis of the very well defined structure of dendrimers reflecting complete branching and regularity, which could be achieved in a three-dimensional sense. The formation of dendrimers starts with a core with functionality Nc. The functionality of the branching site Nb depends on the monomer functionality. Based on these values the development of the dendrimer structure can be simply calculated for every generation G. The number of end groups z increases according to

z = NcNGb (1.3)

which means that for the simplest case of a three-functional core and two-functional branching site (Nc = 3, Nb = 2) the number of end groups increases from 2 in the core to 48 in the fourth generation.

The number of repeating units Nr increases greatly with the generation number even for the simplest case:

Nr = Nc NG + 1b - 1/ Nb - 1 (1.4)

and the molar mass M accordingly increases exponentially with every generation:

M = Mc + Nc(Nr NG + 1b- 1/ Nb -1 + MtNG + 1b (1.5)

with Mc the mass of the core and Mt, the mass of the terminal units.

The synthesis of dendrimers is well controlled and could proceed according to a divergent or convergent approach, described in Chapter 3. Dendrimers show very interesting materials properties because of their globular, very compact structure and very high functionality, especially due to their fully defined structure. However, their synthesis is elaborate and very time consuming, which makes their implementation in largescale applications very complicated and expensive. This fact stimulated scientists and polymer manufacturers to search for alternative routes to produce polymers with similar properties as dendrimers but with easier synthesis.

1.2.2 Hyperbranched Polymers

For applications in which easy and efficient synthesis is more important than a perfectly branched architecture, hyperbranched polymers prepared by only one synthesis step are considered. This was the reason why Kim and Webster, researchers at Du Pont, developed new dendritic polymers that can be synthesized rapidly, in large quantities, for application as rheology modifiers and multifunctional initiators. In 1990 they reported on the first hyperbranched polymers, the polyphenylenes, which possess irregular structures with linear segments within the dendritic structure and a high polydispersity.

In fact the existence of synthetically produced, highly branched architectures started much earlier, in the 19th century. The first works are not directly available, but they were the basis of later developments on the polymerization of multifunctional monomers (A2 + B3) based on phthalic anhydride or phthalic acid and glycerol. In 1909, Baekeland used the A2 + B3 reaction between formaldehyde and phenol to produce the first commercialized polymer, a cross-linked material, which exists as an irregularly branched structure just before the network formation.

The theory of polycondensation between multifunctional monomers was extensively treated in the works of Flory in the 1940s and 1950s. Flory was interested in the statistics of molar mass distribution before gel formation in polymers with trifunctional and tetrafunctional branching points. Before reaching the so-called gel point, e.g. the critical conditions for cross-linking, highly branched structures with a certain branching degree are formed. Flory found that, theoretically, the combination of both reaction functionalities in one monomer of type ABx (x ≥ 2) should prevent gelation if A reacts only with B (condition 1). The one-pot synthetic route yields dendritic structures that possess highly dense branching with the potential of a branching point at each repeat unit. Simplification of the calculation, which corresponds also to conditions leading to ideal statistics, requires the assumptions that no cyclization or side reactions occur (condition 2) and that the reactivity of B is independent of the degree of polymerization (condition 3).

(Continues...)

Excerpted from Hyperbranched Polymers by Albena Lederer, Walther Burchard. Copyright © 2015 Albena Lederer and Walther Burchard. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2015
• ISBN 10: 1849733945
• ISBN 13: 9781849733946
• Binding: Hardcover
• Language: English
• Edition number: 1
• Number of pages: 297