Chain-growth polymerization

From Wikipedia, the free encyclopedia

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.[1] There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.

Introduction[]

IUPAC definition

Chain polymerization: Chain reaction in which the growth of a polymer chain
proceeds exclusively by reaction(s) between monomer(s) and active site(s)
on the polymer chain with regeneration of the active site(s) at the end of
each growth step.[2]

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.

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°).[6]

  • I → 2 R°
  • R° + M → RM1°

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[7]

  • RM1° + M → RM2°
  • ...............
  • RMn° + M → RMn+1°

For some polymers, chains of over 1000 monomer units can be formed in milliseconds.[7]

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.[7]

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:

  • RMn° + RMn° → Pn+m

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:

  • RMn° + RMn° → Pn + Pm

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

In some chain-growth polymerizations there is also a chain transfer step, in which the polymer A takes an atom from a B molecule, terminating the growth of polymer chain A. The B molecule produces a new active center and a new growing chain instead. This can happen in free radical polymerization, in ionic polymerization or in coordination polymerization. In most cases chain transfer will generate a by-product and decrease the molar mass of the final polymer.[5]

Branching[]

A branching step is the attachment of a side chain to the main chain of the growing polymer molecule. This will result in the formation of a product macromolecule with a branched structure.

Classes of chain-growth polymerization[]

The International Union of Pure and Applied Chemistry (IUPAC) recommends definitions for several classes of chain-growth polymerization.[2]

Radical polymerization[]

Based on the IUPAC definition,[2] 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.[2] 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).[8]

Coordination polymerization[]

Coordination polymerization is a chain polymerization that involves the preliminary coordination of a monomer molecule with a chain carrier.[2] 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.[9]

Living polymerization[]

Living polymerization was first described by Michael Szwarc in 1956.[10] It is defined as a chain polymerization from which chain transfer and chain termination are absent.[2] 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[2] 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.[11]

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.[2] An example of a reversible-deactivation polymerization is group-transfer polymerization.

Comparison to other polymerization methods[]

Previously, based on the difference between condensation reaction and addition reaction, Wallace Carothers classified polymerization as condensation polymerizations and addition polymerizations in 1929. However, Carothers' classification is not good enough in mechanism aspect, as in some case, addition polymerizations shows condensation features while condensation polymerization shows addition features. Then the classification was optimized as step-growth polymerization and chain-growth polymerization. Based on IUPAC recommendation, the names of step-growth polymerization and chain-growth polymerization were further simplified as polyaddition and chain polymerization.

Step-growth polymerization[]

A step-growth reaction could happen between any two molecules with same or different degree of polymerization, usually monomers will form dimers, trimers, in matrix and finally react to long chain polymers. The mechanism of step-growth reaction is based on their functional group. Step-growth polymerization includes polycondensation and polyaddition. Polycondensation is a kind of polymerization whose chain growth is based on condensation reaction between two molecules with various degree of polymerization. The typical example are polyesters, polyamides and polyethers. It is sometimes confused by condensation previous definition of condensation polymerization. Polyaddition is a type of step-growth polymerization of which chain growth is based on addition reaction between two molecules of various degree of polymerization. The typical example for polyaddition is the synthesis of polyurethane. Compared to chain-growth polymerization, where the production of the growing chaingrowth is based on the reaction between polymer with active center and monomer, step-growth polymerization doesn't have initiator or termination. The monomer in step-growth polymerization will be consumed very quickly to dimer, trimer or oligomer. The degree of polymerization will increase steadily during the whole polymerization process. On the other hand, in chain-growth polymerization, the monomer is consumed steadily but the degree of polymerization can increase very quickly after chain initiation.[12] Compared to step-growth polymerization, living chain-growth polymerization shows low PDI, predictable molecular mass and controllable conformation. Some researchers are working on the transformation of two polymerization methods. Generally, polycondensation proceeds in a step-growth polymerization mode. Substituent effect, catalyst transfer and biphasic system could be used for inhibiting the activity of monomer, and further prevent monomers from reacting with each other. It could make polycondensation process go in a chain-growth polymerization mode.

Polycondensation[]

The chain growth of polycondensation is based on condensation reaction. Low-molar-mass by-product will be formed during polymerization. It is a previous way to classify polymerization, which was introduced by Carothers in 1929. It is still used currently in some case. The step-growth polymerization with low-molar-mass by-product during chain growth is defined as polycondensation. The chain-growth polymerization with a low-molar-mass by-product during chain growth is recommended by IUPAC as "condensative chain polymerization".[13]

Addition polymerization[]

Addition polymerization is also a type of previous definition. The chain growth of addition polymerization is based addition reactions. There is no low-molar-mass by-product formed during polymerization. The step-growth polymerization based on addition reaction during chain growth is defined as polyaddition. Based on that definition, the addition polymerization contains both polyaddition and chain polymerization except condensative chain polymerization we used now.

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.[8]

References[]

  1. ^ Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5
  2. ^ a b c d e f g h 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.
  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. ^ a b Flory, Paul (1953). Principles of polymer chemistry. Cornell University Press. ISBN 0-8014-0134-8.
  6. ^ Allcock, Harry R.; Lampe, Frederick W.; Mark, James E. Contemporary Polymer Chemistry (3rd ed.). Pearson Prentice Hall. p. 60. ISBN 0-13-065056-0.
  7. ^ a b c Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. pp. 57–58. ISBN 0-216-92980-6.
  8. ^ a b Sawamoto, Mitsuo (January 1991). "Modern cationic vinyl polymerization". Progress in Polymer Science. 16 (1): 111–172. doi:10.1016/0079-6700(91)90008-9.
  9. ^ 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.
  10. ^ Szwarc, M. (1956). "'Living' Polymers". Nature. 178 (4543): 1168. doi:10.1038/1781168a0.
  11. ^ Hofsten, E. "Population growth-a menace to what?". Polymer Journal. ISSN 1349-0540.
  12. ^ 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.
  13. ^ 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.

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