Functionalized polyolefins

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Functionalized polyolefins are olefin polymers with polar and nonpolar functionalities attached onto the polymer backbone.[ according to whom? ] There has been an increased interest in functionalizing polyolefins due to their increased usage in everyday life. Polyolefins are virtually ubiquitous in everyday life, from consumer food packaging to biomedical applications; therefore, efforts must be made to study catalytic pathways towards the attachment of various functional groups onto polyolefins in order to affect the material's physical properties.

Contents

Based on the polyolefin structure, functionalized polyolefin can be categorized into four main groups: randomly functionalized polyolefins, end-functionalized polyolefins, block polyolefins, and graft polyolefins. [1]

Randomly functionalized polyolefin

Randomly functionalized polyolefins have differing types, location, and amount of functionality on the polyolefin backbone. Randomly functionalized polyolefins can be synthesized through many familiar combination of polymerization techniques including: post-functionalization, ROMP/hydrogenation, ADMET/hydrogenation, radical polymerization, and catalytic copolymerization. [1]

Post-functionalization

Carbene insertion Carbene insertion polymerization.png
Carbene insertion

Post-functionalization of polyolefin occurs as the name suggests: functionalization occurs after a non-functionalized polyolefin is synthesized. One of the most common way to attach functionality onto a preexisting polymer backbone is through free radical reaction. Free radicals can be formed through plasma, peroxide initiation, etc. [3] When there is a free radical on the polyolefin chain, maleic anhydride [4] can be attached to promote further functionalization. Another approach is through direct insertion of carbenes onto the polyolefin backbone. [5] Though post-functionalization techniques are viable for the insertion of functional groups, harsh conditions must be used since regular non-functionalized polyolefins are highly unreactive.

Ring-opening metathesis polymerization (ROMP)

Tungsten complex catalyzed ring-opening metathesis polymerization ROMP example.png
Tungsten complex catalyzed ring-opening metathesis polymerization

Ring-opening metathesis polymerization (ROMP) must occur first followed by hydrogenation of the opened product. For example, functionalized cyclooctenes can result in functionalized polyolefins via ruthenium complex catalyzed ROMP. Copolymers of ethyl and vinyl acetate can be synthesized via this process. [6] First, a cycloctene functionalized with an ester functionality at the position 5 carbon reacts with a ruthenium complex. Next, the resulting open-ringed product is treated with hydrazine to hydrogenate the double bond resulting in ethane and vinyl acetate copolymer.

Acyclic diene metathesis (ADMET)

ADMET copolymerization with Ru catalyst ADMET Ru catalyst example.png
ADMET copolymerization with Ru catalyst

Acyclic diene metathesis (ADMET) is similar to ROMP in that subsequent hydrogenation is required. ADMET requires a certain type of diene in order for the polymerization to occur. Ruthenium complexes can once again be used for ADMET polymerization. [8] In this case, an α,ω-diene monomer with functionality is required.

Radical polymerization

Copolymer via radical polymerization Radicalpoly.png
Copolymer via radical polymerization

Radical polymerization occurs with an olefin and a vinyl monomer. Since olefins are not very reactive, harsh conditions must be met in order for the polymerization to occur. Ethylene and ethyl acrylate can react together to perform free radical polymerization. [10] In this process, boron trifluoride can be used as the protected group for the ethyl acrylate.

Catalytic polymerization

TMS functionalized polyolefin via Ti complex catalysis TMS functionalized polyolefin.png
TMS functionalized polyolefin via Ti complex catalysis

Catalytic polymerization appears to have the most control compared to other polymerization methods for randomly functionalized polyolefins. [12] Catalytic routes predominantly undergo a coordination/migratory insertion pathway. The functionality of the olefin highly affects the reactivity of the olefin, and hence its relative rate of coordination. Examples of early transition metal catalysts includes titanium and zirconium complexes. Early transition metals can easily form oxides; therefore, protection groups, like the use of methylaluminoxane (MAO) due to its Lewis acidity, can be used to prevent side reactions from happening. [13] As a cocatalyst, MAO is well known for their use in metallocene chemistry as they activate metallocene complexes for olefin polymerization. [14] To remove the MAO protecting group, the reaction can be treated with acid. Instead of MAO, trimethylsilyl (TMS) have also been used to protection functional groups such as amine, since the amine functionality can easily react with other olefins to form branched polymer chains. [15]

Another useful reaction is the use of zirconium metallocene complexes to copolymerize olefin with borane monomer. After reaction with a borane monomer, such as 9-borabicyclonoane (9-BBN), subsequent functionalization can result in hydroxyl functionalities. [16]

Turning from early transition metals to late transition metals, palladium and nickel catalysts have been used to copolymerize ethylene and methylacrylate. [13] [17]

End functionalized polyolefin

End functionalized BBN oxidation End functionalization BBN oxidation.png
End functionalized BBN oxidation

End functionalized polyolefins are polyolefin with functionality either at one end or at both ends of the chain. One example of end functionalization is through living polymerization. Using a vanadium terminated polypropene chain, subsequent reaction with carbon monoxide and acid can result in an aldehyde terminated polypropene chain. [18] This reaction moves forward under low temperature conditions (~-78 °C). Through metallocene supported polymerization, chain transfer can occur with the use of a borane chain transfer agent, which results in an end functionalized polymer chain. [19] One advantage of this chain transfer process is the limited use of metals, which decreases cost.

Block and graft polyolefin

Graft vs block chain Graft block chain.png
Graft vs block chain

Block and graft polyolefin can provide high amount of functional groups onto the polymer chain. Synthesis of both block and graft functionalized polyolefin proceed through a combination of polymerization reactions, most notably via coordination/insertion mechanism and radical polymerization. [20] Some disadvantages of this method include the lack of controlled polymerization and the requirement of multi-step mechanisms.

Related Research Articles

A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:

<span class="mw-page-title-main">Polyethylene</span> Most common thermoplastic polymer

Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

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.

<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.

A post-metallocene catalyst is a kind of catalyst for the polymerization of olefins, i.e., the industrial production of some of the most common plastics. "Post-metallocene" refers to a class of homogeneous catalysts that are not metallocenes. This area has attracted much attention because the market for polyethylene, polypropylene, and related copolymers is large. There is a corresponding intense market for new processes as indicated by the fact that, in the US alone, 50,000 patents were issued between 1991-2007 on polyethylene and polypropylene.

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

Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often creates fewer undesired by-products and hazardous wastes than alternative organic reactions. For their elucidation of the reaction mechanism and their discovery of a variety of highly active catalysts, Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock were collectively awarded the 2005 Nobel Prize in Chemistry.

Polyketones are a family of high-performance thermoplastic polymers. The polar ketone groups in the polymer backbone of these materials gives rise to a strong attraction between polymer chains, which increases the material's melting point (255 °C for copolymer, 220 °C for terpolymer. Trade names include Poketone, Carilon, Karilon, Akrotek, and Schulaketon. Such materials also tend to resist solvents and have good mechanical properties. Unlike many other engineering plastics, aliphatic polyketones such as Shell Chemicals' Carilon are relatively easy to synthesize and can be derived from inexpensive monomers. Carilon is made with a palladium catalyst from ethylene and carbon monoxide. A small fraction of the ethylene is generally replaced with propylene to reduce the melting point somewhat. Shell Chemical commercially launched Carilon thermoplastic polymer in the U.S. in 1996, but discontinued it in 2000. SRI International offers Carilon thermoplastic polymers. Hyosung announced that they would launch production in 2015.

Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes.

A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More specialized polyolefins include polyisobutylene and polymethylpentene. They are all colorless or white oils or solids. Many copolymers are known, such as polybutene, which derives from a mixture of different butene isomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived from ethylene, and polymethylpentene is derived from 4-methyl-1-pentene. Polyolefins are not olefins themselves because the double bond of each olefin monomer is opened in order to form the polymer. Monomers having more than one double bond such as butadiene and isoprene yield polymers that contain double bonds (polybutadiene and polyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries.

Ring-closing metathesis (RCM) is a widely used variation of olefin metathesis in organic chemistry for the synthesis of various unsaturated rings via the intramolecular metathesis of two terminal alkenes, which forms the cycloalkene as the E- or Z- isomers and volatile ethylene.

Ring-opening metathesis polymerization (ROMP) is a type of olefin metathesis chain-growth polymerization. The driving force of the reaction is relief of ring strain in cyclic olefins. A variety of heterogeneous and homogeneous catalysts have been developed. Most large-scale commercial processes rely on the former while some fine chemical syntheses rely on the homogeneous catalysts. Catalysts are based on transition metals such as W, Mo, Re, Ru, and Ti.

Acyclic diene metathesis or 'ADMET' is a special type of olefin metathesis used to polymerize terminal dienes to polyenes:

<span class="mw-page-title-main">Concurrent tandem catalysis</span>

Concurrent tandem catalysis (CTC) is a technique in chemistry where multiple catalysts produce a product otherwise not accessible by a single catalyst. It is usually practiced as homogeneous catalysis. Scheme 1 illustrates this process. Molecule A enters this catalytic system to produce the comonomer, B, which along with A enters the next catalytic process to produce the final product, P. This one-pot approach can decrease product loss from isolation or purification of intermediates. Reactions with relatively unstable products can be generated as intermediates because they are only transient species and are immediately used in a consecutive reaction.

Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.

In polymer chemistry, chain walking (CW) or chain running or chain migration is a mechanism that operates during some alkene polymerization reactions. CW can be also considered as a specific case of intermolecular chain transfer. This reaction gives rise to branched and hyperbranched/dendritic hydrocarbon polymers. This process is also characterized by accurate control of polymer architecture and topology. The extent of CW, displayed in the number of branches formed and positions of branches on the polymers are controlled by the choice of a catalyst. The potential applications of polymers formed by this reaction are diverse, from drug delivery to phase transfer agents, nanomaterials, and catalysis.

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:

<span class="mw-page-title-main">Sequence-controlled polymer</span>

A sequence-controlled polymer is a macromolecule, in which the sequence of monomers is controlled to some degree. This control can be absolute but not necessarily. In other words, a sequence-controlled polymer can be uniform or non-uniform (Ð>1). For example, an alternating copolymer synthesized by radical polymerization is a sequence-controlled polymer, even if it is also a non-uniform polymer, in which chains have different chain-lengths and slightly different compositions. A biopolymer with a perfectly-defined primary structure is also a sequence-controlled polymer. However, in the case of uniform macromolecules, the term sequence-defined polymer can also be used.

<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.

<span class="mw-page-title-main">Allyl glycidyl ether</span> Chemical compound

Allyl glycidyl ether is an organic compound used in adhesives and sealants and as a monomer for polymerization reactions. It is formally the condensation product of allyl alcohol and glycidol via an ether linkage. Because it contains both an alkene and an epoxide group, either group can be reacted selectively to yield a product where the other functional group remains intact for future reactions.

β-Carbon elimination is a type of reaction in organometallic chemistry wherein an allyl ligand bonded to a metal center is broken into the corresponding metal-bonded alkyl (aryl) ligand and an alkene. It is a subgroup of elimination reactions. Though less common and less understood than β-hydride elimination, it is an important step involved in some olefin polymerization processes and transition-metal-catalyzed organic reactions.

References

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  17. ↑ Carlini, Carlo; De Luise, Valentina; Martinelli, Marco; Galletti, Anna Maria Raspolli; Sbrana, Glauco (2006-01-01). "Homo- and copolymerization of methyl methacrylate with ethylene by novel Ziegler-Natta-Type nickel catalysts based on N,O-nitro-substituted chelate ligands". Journal of Polymer Science Part A: Polymer Chemistry. 44 (1): 620–633. Bibcode:2006JPoSA..44..620C. doi:10.1002/pola.21161. ISSN   1099-0518.
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