Chain-growth polymerization

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Chain-growth polymerization (AE) or chain-growth polymerisation (BE) is a polymerization technique where monomer molecules add onto the active site on a growing polymer chain one at a time. [1] There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.

Contents

Chain-growth polymerization involves 3 types of reactions :

  1. Initiation: An active species I* is formed by some decomposition of an initiator molecule I
  2. Propagation: The initiator fragment reacts with a monomer M to begin the conversion to the polymer; the center of activity is retained in the adduct. Monomers continue to add in the same way until polymers Pi* are formed with the degree of polymerization i
  3. Termination: By some reaction generally involving two polymers containing active centers, the growth center is deactivated, resulting in dead polymer

Introduction

IUPAC definition

chain polymerization: A chain reaction in which the growth of a polymer chain proceeds exclusively by reaction(s) between monomer and reactive site(s) on the polymer chain with regeneration of the reactive site(s) at the end of each growth step. (See Gold Book entry for note.) [2]

An example of chain-growth polymerization by ring opening to polycaprolactone PolycaprolactoneSynthesis.svg
An example of chain-growth polymerization by ring opening to polycaprolactone

In 1953, Paul Flory first classified polymerization as "step-growth polymerization" and "chain-growth polymerization". [3] IUPAC recommends to further simplify "chain-growth polymerization" to "chain polymerization". It is a kind of polymerization where an active center (free radical or ion) is formed, and a plurality of monomers can be polymerized together in a short period of time to form a macromolecule having a large molecular weight. In addition to the regenerated active sites of each monomer unit, polymer growth will only occur at one (or possibly more) endpoint. [4]

Many common polymers can be obtained by chain polymerization such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polyvinyl acetate (PVA). [5]

Typically, chain-growth polymerization can be understood with the chemical equation:

In this equation, P is the polymer while x represents degree of polymerization, * means active center of chain-growth polymerization, M is the monomer which will react with active center, and L may be a low-molar-mass by-product obtained during chain propagation. For most chain-growth polymerizations, there is no by-product L formed. However there are some exceptions, such as the polymerization of amino acid N-carboxyanhydrides to oxazolidine-2,5-diones.

This type of polymerization is described as "chain" or "chain-growth" because the reaction mechanism is a chemical chain reaction with an initiation step in which an active center is formed, followed by a rapid sequence of chain propagation steps in which the polymer molecule grows by addition of one monomer molecule to the active center in each step. The word "chain" here does not refer to the fact that polymer molecules form long chains. [6] Some polymers are formed instead by a second type of mechanism known as step-growth polymerization without rapid chain propagation steps.

Reaction steps

All chain-growth polymerization reactions must include chain initiation and chain propagation. Chain transfer and chain termination steps also occur in many but not all chain-growth polymerizations.

Chain initiation

Chain initiation is the initial generation of a chain carrier, which is an intermediate such as a radical or an ion which can continue the reaction by chain propagation. Initiation steps are classified according to the way that energy is provided: thermal initiation, high energy initiation, and chemical initiation, etc. Thermal initiation uses molecular thermal motion to dissociate a molecule and form active centers. High energy initiation refers to the generation of chain carriers by radiation. Chemical initiation is due to a chemical initiator.

For the case of radical polymerization as an example, chain initiation involves the dissociation of a radical initiator molecule (I) which is easily dissociated by heat or light into two free radicals (2 R°). Each radical R° then adds a first monomer molecule (M) to start a chain which terminates with a monomer activated by the presence of an unpaired electron (RM1°). [7]

Chain propagation

IUPAC defines chain propagation as a reaction of an active center on the growing polymer molecule, which adds one monomer molecule to form a new polymer molecule (RM1°) one repeat unit longer.

For radical polymerization, the active center remains an atom with an unpaired electron. The addition of the second monomer and a typical later addition step are [8]

For some polymers, chains of over 1000 monomer units can be formed in milliseconds. [8]

Chain termination

In a chain termination step, the active center disappears, resulting in the termination of chain propagation. This is different from chain transfer in which the active center only shifts to another molecule but does not disappear.

For radical polymerization, termination involves a reaction of two growing polymer chains to eliminate the unpaired electrons of both chains. There are two possibilities. [8]

1. Recombination is the reaction of the unpaired electrons of two chains to form a covalent bond between them. The product is a single polymer molecule with the combined length of the two reactant chains:

2. Disproportionation is the transfer of a hydrogen atom from one chain to the other, so that the two product chain molecules are unchanged in length but are no longer free radicals:

Initiation, propagation and termination steps also occur in chain reactions of smaller molecules. This is not true of the chain transfer and branching steps considered next.

Chain transfer

An example of chain transfer in styrene polymerization. Here X = Cl and Y = CCl3. Chain trasnfer.jpg
An example of chain transfer in styrene polymerization. Here X = Cl and Y = CCl3.

In some chain-growth polymerizations there is also a chain transfer step, in which the growing polymer chain RMn° takes an atom X from an inactive molecule XY, terminating the growth of the polymer chain: RMn° + XY → RMnX + Y°. The Y fragment ls a new active center which adds more monomer M to form a new growing chain YMn°. [9] This can happen in free radical polymerization for chains RMn°, in ionic polymerization for chains RMn+ or RMn–, or in coordination polymerization. In most cases chain transfer will generate a by-product and decrease the molar mass of the final polymer. [5]

Chain transfer to polymer: Branching

Another possibility is chain transfer to a second polymer molecule, result in the formation of a product macromolecule with a branched structure. In this case the growing chain takes an atom X from a second polymer chain whose growth had been completed. The growth of the first polymer chain is completed by the transfer of atom X. However the second molecule loses an atom X from the interior of its polymer chain to form a reactive radical (or ion) which can add more monomer molecules. This results in the addition of a branch or side chain and the formation of a product macromolecule with a branched structure. [10]

Classes of chain-growth polymerization

The International Union of Pure and Applied Chemistry (IUPAC) recommends definitions for several classes of chain-growth polymerization. [6]

Radical polymerization

Based on the IUPAC definition, [6] radical polymerization is a chain polymerization in which the kinetic-chain carriers are radicals. Usually, the growing chain end bears an unpaired electron. Free radicals can be initiated by many methods such as heating, redox reactions, ultraviolet radiation, high energy irradiation, electrolysis, sonication, and plasma. Free radical polymerization is very important in polymer chemistry. It is one of the most developed methods in chain-growth polymerization. Currently, most polymers in our daily life are synthesized by free radical polymerization, including polyethylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, styrene butadiene rubber, nitrile rubber, neoprene, etc.

Ionic polymerization

Ionic polymerization is a chain polymerization in which the kinetic-chain carriers are ions or ion pairs. [6] It can be further divided into anionic polymerization and cationic polymerization. Ionic polymerization generates many polymers used in daily life, such as butyl rubber, polyisobutylene, polyphenylene, polyoxymethylene, polysiloxane, polyethylene oxide, high density polyethylene, isotactic polypropylene, butadiene rubber, etc. Living anionic polymerization was developed in the 1950s. The chain will remain active indefinitely unless the reaction is transferred or terminated deliberately, which allows the control of molar weight and dispersity (or polydispersity index, PDI). [11]

Coordination polymerization

Coordination polymerization is a chain polymerization that involves the preliminary coordination of a monomer molecule with a chain carrier. [6] The monomer is first coordinated with the transition metal active center, and then the activated monomer is inserted into the transition metal-carbon bond for chain growth. In some cases, coordination polymerization is also called insertion polymerization or complexing polymerization. Advanced coordination polymerizations can control the tacticity, molecular weight and PDI of the polymer effectively. In addition, the racemic mixture of the chiral metallocene can be separated into its enantiomers. The oligomerization reaction produces an optically active branched olefin using an optically active catalyst. [12]

Living polymerization

Living polymerization was first described by Michael Szwarc in 1956. [13] It is defined as a chain polymerization from which chain transfer and chain termination are absent. [6] In the absence of chain-transfer and chain termination, the monomer in the system is consumed and the polymerization stops but the polymer chain remains active. If new monomer is added, the polymerization can proceed.

Due to the low PDI and predictable molecular weight, living polymerization is at the forefront of polymer research. It can be further divided into living free radical polymerization, living ionic polymerization and living ring-opening metathesis polymerization, etc.

Ring-opening polymerization

Ring-opening polymerization is defined [6] as a polymerization in which a cyclic monomer yields a monomeric unit which is acyclic or contains fewer cycles than the monomer. Generally, the ring-opening polymerization is carried out under mild conditions, and the by-product is less than in the polycondensation reaction. A high molecular weight polymer is easily obtained. Common ring-opening polymerization products includes polypropylene oxide, polytetrahydrofuran, polyepichlorohydrin, polyoxymethylene, polycaprolactam and polysiloxane. [14]

Reversible-deactivation polymerization

Reversible-deactivation polymerization is defined as a chain polymerization propagated by chain carriers that are deactivated reversibly, bringing them into one or more active-dormant equilibria. [6] An example of a reversible-deactivation polymerization is group-transfer polymerization.

Comparison with step-growth polymerization

Polymers were first classified according to polymerization method by Wallace Carothers in 1929, who introduced the terms addition polymer and condensation polymer to describe polymers made by addition reactions and condensation reactions respectively. [15] However this classification is inadequate to describe a polymer which can be made by either type of reaction, for example nylon 6 which can be made either by addition of a cyclic monomer or by condensation of a linear monomer. [15]

Flory revised the classification to chain-growth polymerization and step-growth polymerization, based on polymerization mechanisms rather than polymer structures. [15] IUPAC now recommends that the names of step-growth polymerization and chain-growth polymerization be further simplified to polycondensation (or polyaddition if no low-molar-mass by-product is formed when a monomer is added) and chain polymerization. [6]

Most polymerizations are either chain-growth or step-growth reactions. [16] Chain-growth includes both initiation and propagation steps (at least), and the propagation of chain-growth polymers proceeds by the addition of monomers to a growing polymer with an active centre. In contrast step-growth polymerization involves only one type of step, and macromolecules can grow by reaction steps between any two molecular species: two monomers, a monomer and a growing chain, or two growing chains. [17] In step growth, the monomers will initially form dimers, trimers, etc. which later react to form long chain polymers.

In chain-growth polymerization, a growing macromolecule increases in size rapidly once its growth is initiated. When a macromolecule stops growing it generally will add no more monomers. In step-growth polymerization on the other hand, a single polymer molecule can grow over the course of the whole reaction. [16]

In chain-growth polymerization, long macromolecules with high molecular weight are formed when only a small fraction of monomer has reacted. Monomers are consumed steadily over the course of the whole reaction, [17] but the degree of polymerization can increase very quickly after chain initiation. [17] However in step-growth polymerization the monomer is consumed very quickly to dimer, trimer and oligomer. The degree of polymerization increases steadily during the whole polymerization process.

The type of polymerization of a given monomer usually depends on the functional groups present, and sometimes also on whether the monomer is linear or cyclic. Chain-growth polymers are usually addition polymers by Carothers' definition. They are typically formed by addition reactions of C=C bonds in the monomer backbone, which contains only carbon-carbon bonds. [16] Another possibility is ring-opening polymerization, as for the chain-growth polymerization of tetrahydrofuran [16] or of polycaprolactone (see Introduction above).

Step-growth polymers are typically condensation polymers in which an elimination product as such as H2O are formed. Examples are polyamides, polycarbonates, polyesters, polyimides, polysiloxanes and polysulfones. [18] If no elimination product is formed, then the polymer is an addition polymer, such as a polyurethane or a poly(phenylene oxide). [18] Chain-growth polymerization with a low-molar-mass by-product during chain growth is described by IUPAC as "condensative chain polymerization". [19]

Compared to step-growth polymerization, living chain-growth polymerization shows low molar mass dispersity (or PDI), predictable molar mass distribution and controllable conformation. Generally, polycondensation proceeds in a step-growth polymerization mode.


Application

Chain polymerization products are widely used in many aspects of life, including electronic devices, food packaging, catalyst carriers, medical materials, etc. At present, the world's highest yielding polymers such as polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), etc. can be obtained by chain polymerization. In addition, some carbon nanotube polymer is used for electronical devices. Controlled living chain-growth conjugated polymerization will also enable the synthesis of well-defined advanced structures, including block copolymers. Their industrial applications extend to water purification, biomedical devices and sensors. [11]

Related Research Articles

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events.

<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, 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">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.

<span class="mw-page-title-main">Radical polymerization</span> Polymerization process involving free radicals as repeating units

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> Chemical reaction that produces polymers

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.

<span class="mw-page-title-main">Branching (polymer chemistry)</span> Attachment of side chains to the backbone chain of a polymer

In polymer chemistry, branching is the regular or irregular attachment of side chains to a polymer's backbone chain. It occurs by the replacement of a substituent on a monomer subunit by another covalently-bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type. Branched polymers have more compact and symmetrical molecular conformations, and exhibit intra-heterogeneous dynamical behavior with respect to the unbranched polymers. In crosslinking rubber by vulcanization, short sulfur branches link polyisoprene chains into a multiple-branched thermosetting elastomer. Rubber can also be so completely vulcanized that it becomes a rigid solid, so hard it can be used as the bit in a smoking pipe. Polycarbonate chains can be crosslinked to form the hardest, most impact-resistant thermosetting plastic, used in safety glasses.

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">Robert Gilbert (chemist)</span>

Robert Goulston Gilbert is a polymer chemist whose most significant contributions have been in the field of emulsion polymerisation. In 1970, he gained his PhD from the Australian National University, and worked at the University of Sydney from then until 2006. In 1982, he was elected a fellow of the Royal Australian Chemical Institute; in 1994, he was elected a fellow of the Australian Academy of Science. In 1992, he was appointed full professor, and in 1999 he started the Key Centre for Polymer Colloids, funded by the Australian Research Council, the University and industry. He has served in leadership roles in the International Union of Pure and Applied Chemistry (IUPAC), the world ‘governing body’ of chemistry. He was founding chair (1987–98) of the IUPAC Working Party on the Modelling of Kinetics Processes of Polymerisation, of which he remains a member, and is a member of the IUPAC scientific task groups on starch molecular weight measurements, and terminology. He was vice-president (1996–97) and president (1998–2001) of the IUPAC Macromolecular Division, and secretary of the International Polymer Colloids Group (1997–2001). As of 2007, he is Research Professor at the Centre of Nutrition and Food Science, University of Queensland, where his research program concentrates on the relations between starch structure and nutrition.

In polymer chemistry, chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule:

In polymer chemistry, the kinetic chain length of a polymer is the average number of units called monomers added to a growing chain during chain-growth polymerization. During this process, a polymer chain is formed when monomers are bonded together to form long chains known as polymers. Kinetic chain length is defined as the average number of monomers that react with an active center such as a radical from initiation to termination.

<span class="mw-page-title-main">Living free-radical polymerization</span>

Living free radical polymerization is a type of living polymerization where the active polymer chain end is a free radical. Several methods exist. IUPAC recommends to use the term "reversible-deactivation radical polymerization" instead of "living free radical polymerization", though the two terms are not synonymous.

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).

Radical disproportionation encompasses a group of reactions in organic chemistry in which two radicals react to form two different non-radical products. Radicals in chemistry are defined as reactive atoms or molecules that contain an unpaired electron or electrons in an open shell. The unpaired electrons can cause radicals to be unstable and reactive. Reactions in radical chemistry can generate both radical and non-radical products. Radical disproportionation reactions can occur with many radicals in solution and in the gas phase. Due to the reactive nature of radical molecules, disproportionation proceeds rapidly and requires little to no activation energy. The most thoroughly studied radical disproportionation reactions have been conducted with alkyl radicals, but there are many organic molecules that can exhibit more complex, multi-step disproportionation reactions.

<span class="mw-page-title-main">Reversible-deactivation radical polymerization</span> Type of chain polymerization

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

Catalyst transfer polymerization (CTP), or catalyst transfer polycondensation, is a type of living chain-growth polymerization that is used for synthesizing conjugated polymers. Benefits to using CTP over other methods are low polydispersity and control over number average molecular weight in the resulting polymers. Very few monomers have been demonstrated to undergo CTP.

References

  1. ↑ Young, R.J. (1987). Introduction to Polymers. Chapman & Hall. ISBN   0-412-22170-5.
  2. ↑ "chain polymerization". Gold Book. IUPAC. doi:10.1351/goldbook.C00958 . Retrieved 1 April 2024.
  3. ↑ R.J.Young (1983). Introduction to polymers . Chapman and Hall. ISBN   0-412-22170-5.
  4. ↑ Plastics packaging : Properties, processing, applications, and regulations (2nd ed.). Hanser Pub. 2004. ISBN   1-56990-372-7.
  5. 1 2 Flory, Paul (1953). Principles of polymer chemistry. Cornell University Press. ISBN   0-8014-0134-8.
  6. 1 2 3 4 5 6 7 8 9 Penczek, Stanisław; Moad, Graeme (2008). "Glossary of terms related to kinetics, thermodynamics, and mechanisms of polymerization (IUPAC Recommendations 2008)" (PDF). Pure and Applied Chemistry . 80 (10): 2163–2193. doi:10.1351/pac200880102163. S2CID   97698630.
  7. ↑ Allcock, Harry R.; Lampe, Frederick W.; Mark, James E. (2003). Contemporary Polymer Chemistry (3rd ed.). Pearson Prentice Hall. p. 60. ISBN   0-13-065056-0.
  8. 1 2 3 Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. pp. 57–58. ISBN   0-216-92980-6.
  9. ↑ Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. pp. 63–64. ISBN   0-216-92980-6.
  10. ↑ Rudin, Alfred (1982). The Elements of Polymer Science and Engineering. Academic Press. pp. 220–221. ISBN   0-12-601680-1.
  11. 1 2 Sawamoto, Mitsuo (January 1991). "Modern cationic vinyl polymerization". Progress in Polymer Science. 16 (1): 111–172. doi:10.1016/0079-6700(91)90008-9.
  12. ↑ Kaminsky, Walter (1 January 1998). "Highly active metallocene catalysts for olefin polymerization". Journal of the Chemical Society, Dalton Transactions (9): 1413–1418. doi:10.1039/A800056E. ISSN   1364-5447.
  13. ↑ Szwarc, M. (1956). "'Living' Polymers". Nature. 178 (4543): 1168–1169. Bibcode:1956Natur.178.1168S. doi:10.1038/1781168a0.
  14. ↑ Hofsten, E. "Population growth-a menace to what?". Polymer Journal. ISSN   1349-0540.
  15. 1 2 3 Rudin, Alfred (1982). The Elements of Polymer Science and Engineering. Academic Press. pp. 155–161. ISBN   0-12-601680-1.
  16. 1 2 3 4 Rudin, Alfred (1982). The Elements of Polymer Science and Engineering. Academic Press. pp. 158–160. ISBN   0-12-601680-1.
  17. 1 2 3 Aplan, Melissa P.; Gomez, Enrique D. (3 July 2017). "Recent Developments in Chain-Growth Polymerizations of Conjugated Polymers". Industrial & Engineering Chemistry Research. 56 (28): 7888–7901. doi:10.1021/acs.iecr.7b01030.
  18. 1 2 Fried, Joel R. (2003). Polymer Science & Technology (2nd ed.). Prentice Hall. p. 24. ISBN   0-13-018168-4.
  19. ↑ Herzog, Ben; Kohan, Melvin I.; Mestemacher, Steve A.; Pagilagan, Rolando U.; Redmond, Kate (2013). "Polyamides". Ullmann's Encyclopedia of Industrial Chemistry. American Cancer Society. doi:10.1002/14356007.a21_179.pub3. ISBN   978-3527306732. S2CID   241272519.
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