Fundamentals of Controlled/Living Radical Polymerization: Rsc: Volume 4 (Polymer Chemistry Series) - Hardcover

Hardcover

ISBN 10: 1849734259 ISBN 13: 9781849734257

Publisher: Royal Society of Chemistry, 2013

Synopsis

Provides an in-depth history, description, and mechanistic understanding of each of the controlled/living radical polymerization techniques and practical details necessary to carry out the reactions.

About this title

About the Author

Nick Tsarevsky obtained M.S. degree in theoretical chemistry and chemical physics in 1999 from the University of Sofia, Bulgaria, and Ph.D. degree in chemistry in 2005 from Carnegie Mellon University under the supervision of Prof. Kris Matyjaszewski. He was Visiting Assistant Professor at the Department of Chemistry at Carnegie Mellon University (2005-6), and a member of the founding team of ATRP Solutions, Inc., of which he served as Chief Science Officer (2007-2010). He joined the Department of Chemistry at Southern Methodist University in the summer of 2010 as an assistant professor, and in 2016 he was promoted to associate professor with tenure. He has authored and coauthored over 90 peer-reviewed journal articles and book chapters, 1 textbook, and has served as the co-editor of 5 books. Research interests include polymerization techniques, functional materials, coordination chemistry and catalysis, and the chemistry hypervalent compounds, in addition to history of chemistry and chemical education.

From the Back Cover

Controlled/living radical polymerization (CRP) has revolutionized and revitalized the field of synthetic polymer chemistry over the last twenty years as it is now possible to prepare a wide variety of previously inaccessible macromolecules under relatively mild conditions. Fundamentals of Controlled/Living Radical Polymerization provides an in-depth coverage of the essential chemical principles that enable and govern each of the CRP methods. The book starts with a brief historical overview of the major findings in polymer science which eventually led to the development of living ionic and living radical systems. It then goes on to introduce the main CRP techniques including their mechanistic understanding. The book also provides the information needed to select the appropriate reagents and conditions to conduct various CRP methods in a practical setting. Therefore, in addition to a newcomer gaining an appreciation for what has already been accomplished, the reader will be armed with the tools needed to begin independent research. Fundamentals of Controlled/Living Radical Polymerization provides essential insight into a rapidly growing field that goes beyond a simple literature review of the area. Written by leading experts on the subject, the book is an indispensible resource for researchers, instructors, and students in polymer chemistry.

Excerpt. © Reprinted by permission. All rights reserved.

Fundamentals of Controlled/Living Radical Polymerization

By Nicolay V Tsarevsky, Brent S Sumerlin

The Royal Society of Chemistry

Copyright © 2013 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-425-7

Contents

Chapter 1 Kinetics and Thermodynamics of Radical Polymerization F. Ehlers, J. Barth and P. Vana, 1, 
Chapter 2 Fundamental Aspects of Living Polymerization Robson F. Storey, 60, 
Chapter 3 Controlled/Living Radical Polymerization in the Presence of Iniferters Mehmet Atilla Tasdelen and Yusuf Yagci, 78, 
Chapter 4 Controlled/Living Radical Polymerization Mediated by Stable Organic Radicals Peter Nesvadba, 112, 
Chapter 5 Mechanistic Aspects of Living Radical Polymerization Mediated by Organometallic Complexes Zichuan Ye and Bradford B. Wayland, 168, 
Chapter 6 Fundamentals of RAFT Polymerization Graeme Moad, Ezio Rizzardo and San H. Thang, 205, 
Chapter 7 Living Radical Polymerizations with Organic Catalysts A. Goto, Y. Tsujii and H. Kaji, 250, 
Chapter 8 Atom Transfer Radical Polymerization (ATRP) Nicolay V. Tsarevsky and Krzysztof Matyjaszewski, 287, 
Subject Index, 358, 

CHAPTER 1

Kinetics and Thermodynamics of Radical Polymerization

F. EHLERS, J. BARTH AND P. VANA

Georg-August-University of Göttingen, Institute of Physical Chemistry, Tammannstr. 6, D-37077 Göttingen, Germany

1.1 Introduction

Radical polymerization processes are of great scientific and economic importance. Knowledge about their kinetic principles is a prerequisite for the efficient synthesis of a wide range of polymeric products. Since the dawn of macromolecular chemistry in the 1920's, the study of these principles has been a central topic of academic research. Although a radical polymerization process is basically constituted by just four types of reactions, which are initiation, propagation, transfer and termination, the coupled nature of these reactions leads to a complexity that makes it difficult to determine the individual rate constants and to evaluate their effects on the properties of the final polymer, like its molecular weight distribution. Scheme 1.1 shows a set of fundamental reaction equations for a radical polymerization process.

There is a kinetic rate law expression for each of these reactions. Determination of the corresponding rate coefficients is the main task of all kinetic experiments in this field. The employed experimental techniques can roughly be separated into two classes: one approach focuses on accurate measurements of the overall polymerization rate or time-resolved species concentration, while the other one is based on the analysis of the resulting molecular weight distributions. Provided that all relevant rate coefficients for a certain polymerizing system are known, it is possible to make precise predictions about the kinetics of the entire process, and therefore also about the molecular weight distribution. Today, computer simulations are an important tool in polymer research, allowing for precise numerical simulations of even very complex polymerizing systems and thus contributing to a deeper understanding of radical polymerization kinetics.

The intention of the following chapter is to give a general overview of our knowledge about the kinetics of conventional radical polymerization and its implications for the process and the formed product. This basic knowledge is also mandatory for the understanding and optimization of controlled polymerization processes.

1.2 Initiation

For a radical polymerization to occur, the first thing needed are free radicals. These are initially provided by some agent, the initiator, during the reaction step called initiation. The initiation step is commonly characterized by two coefficients, the initiation rate coefficient ki and the initiator efficiency [Florin]. Knowledge of these parameters is of crucial importance for both industrial applications and theoretical studies of radical polymerizations.

The vast majority of initiators belong to one of two groups, thermal initiators or photoinitiators. Thermal initiators form radical species upon heating, while photoinitiators decompose when exposed to visible or UV light. While in commercial processes mainly thermal initiators are used, kinetic studies are preferentially performed using photoinitiators. This is because the irradiation can precisely be timed, defining a sharp starting point for the polymerization reaction. A general scheme for the decomposition reaction, regardless of the type of initiator, is given in Scheme 1.2.

The initiator (I) decay, be it caused by light absorption or heating, follows a first order rate law:

- dcI/dt = kdcI (1.1)

For the polymerization kinetics, the initiator concentration is not the important quantity. More important is the concentration of primary radicals formed by the initiation process. The rate Rd of formation of radicals that can start chain growth can be expressed by the following first order rate law:

Rd = dcI•/dt = 2[Florin] dcI/dt = 2[Florin]kdcI (1.2)

where kd is the rate coefficient for the initiator decomposition reaction, and [Florin] is the initiator efficiency. Integration of eqn (1.2) leads to the following expression, showing the exponential decrease of initiator concentration with time:

[MATHEMATICAL EXPRESSION OMITTED] (1.3)

Initiator decay alone is not sufficient to start a new polymer chain. The formed radical has to react with a monomer unit. Right after decay, the (usually two) freshly formed radicals I1• and I2• are still in close proximity of each other and surrounded by solvent molecules. The primary radicals' ability to leave the solvent cage unreacted and then react with a monomer is expressed by the initiator efficiency [Florin], with values ranging from zero (no initiation) to unity. In a real system, not every primary radical will actually start a polymer chain. Radicals can recombine before leaving the solvent cage or undergo a side reaction before they encounter a monomer molecule. Typically, [Florin] has a value between 0.5 and 0.8 and depends on the viscosity of the system, indicating that the diffusion-controlled escape from the solvent cage is the crucial factor.

If the initiator molecule is asymmetric, i.e. I1• ≠ I2•, the formed radical species generally do not show identical reactivity towards the monomer. Thus, the initiation process will take on the form shown in Scheme 1.3, where M indicates a monomer molecule, R1• refers to a macroradical of chain length 1 and ki(1) and ki(2) refer to the rate coefficients of initiation for the respective initiator fragments. The overall rate of initiation, Ri, can be calculated according to eqn (1.4):

[MATHEMATICAL EXPRESSION OMITTED] (1.4)

The rate coefficient of initiation ki can be expressed as the arithmetic mean of the coefficients for the individual fragments, since [MATHEMATICAL EXPRESSION OMITTED]

Ri = kicMcI• with ki = ki(1) + ki(2)/2 (1.5)

1.2.1 Thermal Initiation

There are mainly two types of thermally activated initiators: azo-type, and peroxy-type. Their general structures are shown in Scheme 1.4.

Thermal initiators decompose in a first order reaction upon heating, displaying a characteristic half life at a certain temperature. It is correlated to the decomposition rate coefficient by eqn (1.6). The half-life, t1/2, is the amount of time it takes for half of a sample of initiator to decompose.

t1/2 = ln 2/kd (1.6)

A large variety of initiators has been described in the literature, and many of them are available commercially, so that the initiator can be chosen according to the desired decomposition rate. For practical use, initiators are often characterized by the temperature where they show a half life of 10 h. Typical values for these temperatures lie in the range from 20 to 120 °C. A large collection of data on initiator properties has been published in the Polymer Handbook. However, the decomposition rate is not the only property to consider when selecting an initiator. Possible side reactions with monomer or solvent might also play a role, or the ability of the initiator to act as a transfer agent (compare Section 4.2).

1.2.2 Photoinitiation

Photoinitiators are seldomly used in commercial applications, because large reaction volumes cannot be irradiated easily in a uniform way. Still, there are some applications in the area of UV hardening lacquers and printing inks. In research, on the other hand, photoinitiators are used frequently, for they allow to precisely define the beginning and end of the initiation process by flash photolysis of the initiator. Also, most photoinitiators show almost no temperature dependence of the decomposition rate, but a strong dependence on the (UV) light intensity.

A good photoinitiator for a given polymerization should have the following properties:

1. An irradiation wavelength should be available were the initiator shows strong absorption, but the monomer and the solvent show almost no absorption.

2. The initiator should show a high efficiency.

3. At best it should only generate a single radical species.

There are two groups of photoinitiators that differ in the mechanism of radical formation. Type I photoinitiators generate radicals via unimolecular bond cleavage after irradiation, similar to thermal initiators. Type II initiators show no bond cleavage directly after irradiation. Instead, they enter a bound excited state which reacts with a so-called co-initiator molecule to generate free radicals, mostly via H-abstraction. Most visible light active photoinitiators belong to type II.

Type I photoinitiators show considerable structural variety, one of the most common motives being the acetophenone group. The general structure of this kind of photoinitiators is shown in Scheme 1.5.

Upon irradiation, the initiator molecules will absorb a certain amount of energy changing from the electronic ground state to an excited state. The accessible electronic states of a molecule are usually shown in a Jablonski diagram. An example of such a diagram is shown in Figure 1.1.

In most cases, absorption will cause the initiator molecule to enter the first singlet excited state, commonly denoted as S1. In general, more than one deactivation channel will be active for the excited species, and not all of them do necessarily lead to free radical generation. The fraction of the excited molecules that are actually converted into primary radicals is expressed by the overall quatum yield Φ, which is the product of the quantum yields of three successive elementary processes: intersystem crossing from the lowest excited singlet to the lowest triplet state (1.8), bond scission in the triplet state (1.9), and reaction of the formed radical with a monomer molecule (1.10).

[MATHEMATICAL EXPRESSION OMITTED] (1.7)

[MATHEMATICAL EXPRESSION OMITTED] (1.8)

[MATHEMATICAL EXPRESSION OMITTED] (1.9)

[MATHEMATICAL EXPRESSION OMITTED] (1.10)

where Φ = ΦISC ΦR ΦRM ≤ 1.

For ketones, the intersystem crossing step usually has a rather high quantum yield, so setting the value of ΦISC to one is a good approximation in most cases. The overall quantum yield is then determined by the values of ΦR and ΦRM. ΦR may be reduced due to alternative deactivation pathways from the T1 triplet state. The dominating deactivation reactions in free radical polymerizations are the reaction with molecular oxygen and the deactivation by monomer molecules (compare ref. 3). The influence of the former may be reduced by thorough degassing of the polymerization mixture prior to initiation.

A long life time of the triplet state tends to lead to a reduced overall quantum yield, because chances are higher that alternative deactivation routes may successfully compete with radical formation. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) is an example for an initiator with a rather short-lived first triplet state (τ < 0.1 ns), causing a high quantum yield ΦR. The quantum yield ΦRM for the formation of macroradicals from the initiator fragment radicals is also known as the initiator efficiency, [Florin], which is defined analogously to the efficiency of thermal initiators.

Most acetophenone-type initiators decompose into more than one kind of radical upon irradiation. Typically, two different radical species R1• and R2• are formed, showing very different reactivities towards the monomer. While the carbonyl radical is efficiently forming macroradicals, the additional radical does not add much to the initiator efficiency. On the contrary, for DMPA the methoxy radical is thought to be involved exclusively in termination steps. In such a case, it is common to say that the carbonyl radical is the "effective" and the methoxy radical the "ineffective" primary radical. Their initial concentrations are equal and denoted as ρ, while the overall initial radical concentration is 2ρ. Polymerization kinetics can strongly be influenced by the formation of two sorts of primary radicals with markedly different reactivities.

Azoinitiators may also be used as photoinitiators. They decompose upon irradiation by a mechanism that is markedly different from the acetophenone type. By example of 2,2-azobisisobutyronitrile (AIBN), this mechanism is shown in Scheme 1.6.

UV irradiation leads to a cis-trans isomerization reaction. Since the rate of this isomerization reaction is finite, it is observed in experiments that the time of incidence of the laser pulse is not identical with the time of primary radical formation, but there is a certain delay, usually in the order of microseconds.

The concentration of effective primary free radicals ρ that is generated by irradiation may be calculated by:

ρ = 2Φnabs/V (1.11)

with Φ the primary quantum yield, nabs the number of absorbed photons and V the volume considered.

The number of absorbed photons is given by the Beer–Lambert Law:

nabs = Ep/Eλ (1 - 10-εcd) (1.12)

Ep: energy absorbed by the sample

Eλ: molar energy of photons at the irradiation wavelength λ

ε: molar absorption coefficient of the initiator molecule at wavelength λ

c: photoinitiator concentration

d: optical path length

Detailed information about different photoinitiators and their decomposition behaviour has been gathered by Gruber.

1.2.3 Self-initiation

It is not strictly necessary for a radical polymerization to be started by an initiator. It might also be initiated by impurities, by peroxy compounds that are formed in the presence of molecular oxygen, or even by the monomer itself. A prominent example for the latter is the self-initiation of styrene, which proceeds via Diels–Alder reaction of two monomers, as depicted in Scheme 1.7. Such self-initiated polymerization processes are typically limited to elevated temperatures, and can often be prevented under very pure conditions. Few monomers are capable of self-initiation even under very pure conditions, one of which is styrene, reacting via a self Diels–Alder cycloaddition mechanism. The self-initiated bulk polymerization of styrene has a substantial activation energy: a 50% monomer conversion needs 400 days at 29 °C, but only 4 h at 127 °C. However, the produced polystyrene is very pure due to the absence of initiators and other additives.

1.2.4 Redox-initiation

Redox-initiation is most frequently used in polymerizations in aqueous systems but may be used in organic solvents as well. A redox-initiator consists of an oxidizing and a reducing agent. In most redox initiators, the redox reaction leads to the formation of only one radical, avoiding cage termination processes and thus enhancing the initiator efficiency. As an example, Scheme 1.8 shows the radical forming reaction in an iron-(II)-peroxide system.

For more information on redox-initiation, the reader is referred to more specialized literature, for example the review article of Sarac and the sources cited therein.

1.3 Propagation

The propagation step, that is the addition of another monomer unit to a macroradical, can be described by a rate law expression as shown in eqn (1.13).

- dcM/dt = kp cR • cM (1.13)

where kp is the rate coefficient of propagation of a macroradical. It is well known that up to high monomer conversions, the propagation step is controlled chemically. Therefore, the rate coefficient is independent of monomer conversion as long as it stays lower than about 80%. The chemical control of the propagation reaction becomes obvious when comparing the frequency of propagation reactions, typically on the order of 103 s-1, to the average collision frequency in a liquid at room temperature, which is about 1012 s-1, meaning that only one in 109 collisions is reactive.

1.3.1 Chain Length Dependence

While the rate coefficient of propagation does not depend on conversion (except in the high viscosity regime), it is doubtlessly dependent on chain length. The first few addition steps are particularly fast and may reach rate coefficients many times the long chain limit, as in the case of methyl methacrylates, where the first propagation step at 60 °C exceeds the long chain limit by a factor of about 16. It has also been found that the product of monomer concentration and propagation rate coefficient is strongly dependent on chain length up to several hundred monomer units. Since currently no method is known to determine the propagation rate coefficient independently of the concentration of monomer, the apparent chain length dependence of kp might actually reflect a structuring of monomer concentration in the surrounding of the propagating chain end, rather than a chemical effect.

1.3.2 Monomer Effects

The absolute value of the propagation rate coefficient is determined by the reactivity of the propagating radical as well as by the properties of the monomer unit. The relationship between monomer reactivity and reactivity of the propagating radical is roughly reciprocal, i.e. a very reactive monomer corresponds to a rather unreactive radical and vice versa.

An important condition for chain growth is that the macroradical is sufficiently stable, which means it has to survive a large number of ineffective collisions with monomer or solvent in order to finally react with a monomer molecule. Any possible decomposition or side reaction has to be sufficiently slow so that it will not compete with propagation.

When going from ethene as the simplest possible monomer for radical polymerization to monomers with higher reactivity, one or more hydrogen atoms are formally substituted by groups activating the double bond and stabilizing the macroradical. Of course, steric hindrance is added at the same time, so a complete separation of entropic and enthalpic effects on the propagation rate seems impossible.

(Continues...)

Excerpted from Fundamentals of Controlled/Living Radical Polymerization by Nicolay V Tsarevsky, Brent S Sumerlin. Copyright © 2013 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2013
• ISBN 10: 1849734259
• ISBN 13: 9781849734257
• Binding: Hardcover
• Language: English
• Number of pages: 364
• Editor: Nicolay V Tsarevsky