chemistry

Reversible addition−fragmentation chain-transfer polymerization

Reversible addition−fragmentation chain-transfer polymerization is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Reversible addition−fragmentation chain-transfer polymerization rather than just read about it. In short: Reversible addition−fragmentation chain transfer or RAFT polymerization is one of several kinds of reversible deactivation radical polymerization. It makes use of a chain transfer agent (CTA) in the form of a thiocarbonylthio compound (or similar, from here on referred to as a RAFT agent, see Figure 1) to afford control over the generated molecular weight and dispersity during a free-radical polymerization.

Key takeaways

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Reference excerpt

Reversible addition−fragmentation chain transfer or RAFT polymerization is one of several kinds of reversible deactivation radical polymerization. It makes use of a chain transfer agent (CTA) in the form of a thiocarbonylthio compound (or similar, from here on referred to as a RAFT agent, see Figure 1) to afford control over the generated molecular weight and dispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity Đ (narrow molecular weight distribution) and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.

Overview

History The addition−fragmentation chain transfer process was first reported in the early 1970s. However, the technique was irreversible, so the transfer reagents could not be used to control radical polymerization at this time. For the first few years addition−fragmentation chain transfer was used to help synthesize end-functionalized polymers. Scientists began to realize the potential of RAFT in controlled radical polymerization in the 1980s. Macromonomers were known as reversible chain transfer agents during this time, but had limited applications on controlled radical polymerization. In 1995, a key step in the "degenerate" reversible chain transfer step for chain equilibration was brought to attention. The essential feature is that the product of chain transfer is also a chain transfer agent with similar activity to the precursor transfer agent. RAFT polymerization today is mainly carried out by thiocarbonylthio chain transfer agents. It was first reported by Rizzardo et al. in 1998. RAFT is one of the most versatile methods of controlled radical polymerization because it is tolerant of a very wide range of functionality in the monomer and solvent, including aqueous solutions. RAFT polymerization has also been effectively carried out over a wide temperature range.

Important components of RAFT

Typically, a RAFT polymerization system consists of:

a radical source (e.g. thermochemical initiator or the interaction of gamma radiation with some reagent) monomer RAFT agent solvent (not strictly required if the monomer is a liquid) A temperature is chosen such that (a) chain growth occurs at an appropriate rate, (b) the chemical initiator (radical source) delivers radicals at an appropriate rate and (c) the central RAFT equilibrium (see later) favors the active rather than dormant state to an acceptable extent. RAFT polymerization can be performed by adding a chosen quantity of an appropriate RAFT agent to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Radical initiators such as azobisisobutyronitrile (AIBN) and 4,4'-azobis(4-cyanovaleric acid) (ACVA), also called 4,4'-azobis(4-cyanopentanoic acid), are widely used as the initiator in RAFT. Figure 3 provides a visual description of RAFT polymerizations of poly(methyl methacrylate) and polyacrylic acid using AIBN as the initiator and two RAFT agents.

RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)acrylamides, acrylonitrile, styrene and derivatives, butadiene, vinyl acetate and N-vinylpyrrolidone. The process is also suitable for use under a wide range of reaction parameters such as temperature or the level of impurities, as compared to NMP or ATRP. The Z and R group of a RAFT agent must be chosen according to a number of considerations. The Z group primarily affects the stability of the S=C bond and the stability of the adduct radical (Polymer-S-C•(Z)-S-Polymer, see section on Mechanism). These in turn affect the position of and rates of the elementary reactions in the pre- and main-equilibrium. The R group must be able to stabilize a radical such that the right hand side of the pre-equilibrium is favored, but unstable enough that it can reinitiate growth of a new polymer chain. As such, a RAFT agent must be designed with consideration of the monomer and temperature, since both these parameters also strongly influence the kinetics and thermodynamics of the RAFT equilibria.

Products The desired product of a RAFT polymerization is typically linear polymer with an R-group at one end and a dithiocarbonate moiety at the other end. Figure 4 depicts the major and minor products of a RAFT polymerization. All other products arise from (a) biradical termination events or (b) reactions of chemical species that originate from initiator fragments, denoted by I in the figure. (Note that categories (a) and (b) intersect). The selectivity towards the desired product can be increased by increasing the concentration of RAFT agent relative to the quantity of free radicals delivered during the polymerization. This can be done either directly (i.e. by increasing the RAFT agent concentration) or by decreasing the rate of decomposition of or concentration of initiator.

RAFT mechanism

Kinetics overview

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Illustrations

Reversible addition−fragmentation chain-transfer polymerization: Figure 4. Major product of a RAFT polymerization (left) and other byproducts, arranged in order of decreasing prevalence. — Figure 4. Major product of a RAFT polymerization (left) and other byproducts, arranged in order of decreasing prevalence.

Worked examples

Example 1 — a first encounter with Reversible addition−fragmentation chain-transfer polymerization

Start with the simplest possible case. Write down what Reversible addition−fragmentation chain-transfer polymerization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Reversible addition−fragmentation chain-transfer polymerization before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Reversible addition−fragmentation chain-transfer polymerization ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Reversible addition−fragmentation chain-transfer polymerization

In research: Reversible addition−fragmentation chain-transfer polymerization appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Reversible addition−fragmentation chain-transfer polymerization in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Reversible addition−fragmentation chain-transfer polymerization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Polymerization reactions, so understanding it makes those chapters shorter.
In everyday life: Look for Reversible addition−fragmentation chain-transfer polymerization outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

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