Ionic polymerization

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IUPAC definition

Chain polymerization in which active centers are ions or ion pairs.

Contents

Note 1: Usually the chain-ends are ions, although ions can also be located ionic

on the monomer molecules, as in an activated-monomer polymerization.

Note 2: The ions may also be present in the form of higher aggregates

that usually are less reactive than non-aggregated species.

Modified from the earlier definition. [1]

Penczek S.; Moad, G. Pure Appl. Chem., 2008, 80(10), 2163-2193

In polymer chemistry, ionic polymerization is a chain-growth polymerization in which active centers are ions or ion pairs. [2] It can be considered as an alternative to radical polymerization, and may refer to anionic polymerization or cationic polymerization. [3]

As with radical polymerization, reactions are initiated by a reactive compound. For cationic polymerization, titanium-, boron-, aluminum-, and tin-halide complexes with water, alcohols, or oxonium salts are useful as initiators, as well as strong acids and salts such as KHSO4. [4] Meanwhile, group 1 metals such as lithium, sodium, and potassium, and their organic compounds (e.g. sodium naphthalene) serve as effective anionic initiators. In both anionic and cationic polymerization, each charged chain end (negative and positive, respectively) is matched by a counterion of opposite charge that originates from the initiator. Because of the charge stability necessary in ionic polymerization, monomers which may be polymerized by this method are few compared to those available for free radical polymerization. Stable polymerizing cations are only possible using monomers with electron-releasing groups, and stable anions with monomers with electron-withdrawing groups as substituents.

While radical polymerization rate is governed nearly exclusively by monomer chemistry and radical stability, successful ionic polymerization is as strongly related to reaction conditions. Poor monomer purity quickly leads to early termination, and solvent polarity has a great effect on reaction rate. Loosely-coordinated and solvated ion pairs promote more reactive, fast-polymerizing chains, unencumbered by their counterions. Unfortunately, molecules that are polar enough to support these solvated ion pairs often interrupt the polymerization in other ways, such as by destroying propagating species or coordinating with initiator ions, and so they are seldom utilized. Typical solvents for ionic polymerization include non-polar molecules such as pentane, or moderately polar molecules such as chloroform.

History

The potential utility of ionic polymerization was first recorded by Michael Szwarc after a conversation with Samuel Weissman. [5] He and a team, composed of Moshe Levy and Ralph Milkovich, attempted to recreate an experiment performed by Weissman to study the electron affinity of styrene. By adding styrene monomer to a solution of sodium naphthalenide and Tetrahydrofuran, the "olive-green" solution became a "cherry-red" and appeared to continue to react with new additions of styrene even minutes after the last. This observation, coupled with the determination that the product was polystyrene, indicated that a living, anionic polymerization had been initiated by the addition of electrons.

Applications

Because of the polarity of the active group on each polymerizing radical, termination by chain combination is not seen in ionic polymerization. Furthermore, because charge propagation can only occur by covalent bond formation with the compatible monomer species, termination by chain transfer or disproportionation is impossible. This means that all polymerizing ions, unlike in radical polymerization, grow and maintain their chain lengths throughout the reaction duration (so-called "living" polymer chains), until termination by the addition of a terminating molecule such as water. This leads to virtually monodisperse polymer products, which have many applications in material analysis and product design. Furthermore, because the ions do not self-terminate, block copolymers may be formed by the addition of a new monomer species.

A few important uses of anionic polymerization include the following:

Related Research Articles

<span class="mw-page-title-main">Polymerization</span> Chemical reaction to form polymer chains

In polymer chemistry, polymerization, or polymerisation, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them.

In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.

In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer. The reactive center can be radical, anionic or cationic. Some cyclic monomers such as norbornene or cyclooctadiene can be polymerized to high molecular weight polymers by using metal catalysts. ROP is a versatile method for the synthesis of biopolymers.

In polymer chemistry, emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomers, and surfactants. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

In polymer chemistry, an addition polymer is a polymer that forms by simple linking of monomers without the co-generation of other products. Addition polymerization differs from condensation polymerization, which does co-generate a product, usually water. Addition polymers can be formed by chain polymerization, when the polymer is formed by the sequential addition of monomer units to an active site in a chain reaction, or by polyaddition, when the polymer is formed by addition reactions between species of all degrees of polymerization. Addition polymers are formed by the addition of some simple monomer units repeatedly. Generally polymers are unsaturated compounds like alkenes, alkalines etc. The addition polymerization mainly takes place in free radical mechanism. The free radical mechanism of addition polymerization completed by three steps i.e. Initiation of free radical, Chain propagation, Termination of chain.

<span class="mw-page-title-main">Copolymer</span> Polymer derived from more than one species of monomer

In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively. Copolymers can be characterized by a variety of techniques such as NMR spectroscopy and size-exclusion chromatography to determine the molecular size, weight, properties, and composition of the material.

Chain-growth polymerization (AE) or chain-growth polymerisation (BE) is a polymerization technique where unsaturated monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.

<span class="mw-page-title-main">End group</span> Functional group at the extremity of an oligomer or other macromolecule

End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, they are functional groups that are at the very ends of a macromolecule or oligomer (IUPAC). In polymer synthesis, like condensation polymerization and free-radical types of polymerization, end-groups are commonly used and can be analyzed by nuclear magnetic resonance (NMR) to determine the average length of the polymer. Other methods for characterization of polymers where end-groups are used are mass spectrometry and vibrational spectrometry, like infrared and raman spectroscopy. These groups are important for the analysis of polymers and for grafting to and from a polymer chain to create a new copolymer. One example of an end group is in the polymer poly(ethylene glycol) diacrylate where the end-groups are circled.

In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.

In polymer chemistry, anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but traditionally vinyl monomers are used. Often anionic polymerization involves living polymerizations, which allows control of structure and composition.

<span class="mw-page-title-main">Reversible addition−fragmentation chain-transfer polymerization</span>

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 to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (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 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.

Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step propagation cycle involving as chain carriers a chlorine atom and a methyl radical which are regenerated alternately:

In polymer chemistry, chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.

<span class="mw-page-title-main">Photopolymer</span>

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.

Living cationic polymerization is a living polymerization technique involving cationic propagating species. It enables the synthesis of very well defined polymers and of polymers with unusual architecture such as star polymers and block copolymers and living cationic polymerization is therefore as such of commercial and academic interest.

In polymer chemistry, cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer, which then becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer. The types of monomers necessary for cationic polymerization are limited to alkenes with electron-donating substituents and heterocycles. Similar to anionic polymerization reactions, cationic polymerization reactions are very sensitive to the type of solvent used. Specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. Cationic polymerization is used in the production of polyisobutylene and poly(N-vinylcarbazole) (PVK).

Reversible deactivation radical polymerizations (RDRPs) are members of the class of reversible deactivation polymerizations which exhibit much of the character of living polymerizations, but cannot be categorized as such as they are not without chain transfer or chain termination reactions. Several different names have been used in literature, which are:

In polymer chemistry, degenerative chain transfer is a process that can occur in a radical polymerization whereby reactivity of active centres are changed, hence significantly influencing the molecular weight distribution of the resulting product.

<span class="mw-page-title-main">Graft polymer</span> Polymer with a backbone of one composite and random branches of another composite

In polymer chemistry, graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. The picture labeled "graft polymer" shows how grafted chains of species B are covalently bonded to polymer species A. Although the side chains are structurally distinct from the main chain, the individual grafted chains may be homopolymers or copolymers. Graft polymers have been synthesized for many decades and are especially used as impact resistant materials, thermoplastic elastomers, compatibilizers, or emulsifiers for the preparation of stable blends or alloys. One of the better-known examples of a graft polymer is a component used in high impact polystyrene, consisting of a polystyrene backbone with polybutadiene grafted chains.

References

  1. ↑ Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". Pure and Applied Chemistry . 68 (12): 2287–2311. doi: 10.1351/pac199668122287 .
  2. ↑ Penczek, S.; Moad, G. (2008). "Glossary of terms related to kinetics, thermodynamics, and mechanisms of polymerization (IUPAC Recommendations 2008)". Pure and Applied Chemistry . 80 (10): 2163–2193. doi: 10.1351/pac200880102163 .
  3. ↑ Chang, Feng-Chi. "Ionic Polymerization: Anionic and Cationic Polymerization" (PDF). Polymer Research Center, National Chiao Tung University. Archived from the original (PDF) on 12 May 2013. Retrieved 27 May 2013.
  4. ↑ Chanda, Manas (2013). Introduction to Polymer Science and Chemistry: A Problem-Solving Approach, 2nd Edition. Boca Raton: CRC Press. pp. 429–482. ISBN   978-1-4665-5384-2.
  5. ↑ Szwarc, M. (1998-01-15). "Living polymers. Their discovery, characterization, and properties". Journal of Polymer Science Part A: Polymer Chemistry. 36 (1): IX–XV. Bibcode:1998JPoSA..36D...9S. doi: 10.1002/(sici)1099-0518(19980115)36:1<ix::aid-pola2>3.0.co;2-9 . ISSN   1099-0518.
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