Polymerization of olefinsIndustrial Chemistry

PolyolefinsIn addition to the mentioned parameters (in particular molar ratios of monomers), the polymer properties depend quite critically on the catalyst used. This is because different structural elements can be formed even with simple monomers with practically equal free energies. The possible polymer enchainment patterns are:

Uses of polyolefins

which are obtained either by prolonged grinding of MgCl2 in a steel-ball mill or byits precipitation from a soluble precursor in the presence of some Lewis base, a so-called ‘‘inner donor’’, such as ethyl benzoate or a 1,3-diether. Subsequently asecond, so-called ‘‘outer donor’’, e.g. a silyl ether or a phthalic acid diester, is oftenadded. Upon being injected into the polymerization reactor these catalysts, whichcontain about 5–20% (w/w) of TiCl4, are activated by reaction with aluminiumalkyls, mainly triethylaluminium, to very high levels of productivity. Each gram ofcatalyst will typically produce 50–100 kg of polyethylene or polypropylene in thecourse of a few hours, its normal residence time in the reactor. The growth of thepolymer also takes place inside the catalyst grains; thus each grain is disintegratedinto several hundred minuscule particles with diameters of only 1–2 nm, whichremain imbedded in a polymer pellet with a diameter of about 1 mm, i.e. with avolume several thousand times greater than the catalyst grain from which it hadgrown (Figure 8). Because of their small mass and size, the catalyst fragments canremain in the final polymer product without interfering with its chemical, me-chanical or optical properties.

Box 3 Typical Polyolefin End Uses

Polyolefin materials have entered into so many varied applications in every-day life that a complete overview is not possible here. Table 1 summarizesthe sectors which consume the greatest proportions of ethylene- and pro-pylene-based polyolefin materials.

Table 1 Uses of ethylene- and propylene-based polyolefin materials(estimated percentage of total plastic application in Europe in2003, from www.plasticseurope.org; for list of abbreviations seeGlossary)

Packaging 37% Films for cooking (HDPE), food packaging (LDPE,PE-co-norbornene, PMP), paper laminating (PP, PIB),milk packaging (LDPE, PP), chewing gums (PIB) andcookery (PMP), containers (HDPE) and caps (LDPE)

Building andconstruction

18% Pipes (HDPE, LLDPE, PP, isotactic-PB, PMP, ABS),carpets (PP), storage tanks (HDPE, PB), asbestosreplacement (PP), hot-melt adhesive (atactic PB),insulating foils (PP, PIB)

Transportation 6% Transport tanks and containers (HDPE), lubricant(PIB), seals (EPD)

Electronic andelectricaldevices

9% Electrical home devices (PP, PS, ABS), cables (PMP),technical parts (PP), lamps (PS)

Agriculture 2% Crop protection (PE), twines, stripes and strings (PE,PP)

Medical 1% Blister packaging (PE-co-norbornene), medical devices(PMP)

Sports Sporting goods (ABS, PS)

227Polymerization Reactions

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Classes of olefin polymerization - A

1. Free Radical Polymerization (FRP) Monomers with unsaturated side groups (styrene, acrylate, 1,3-dienes, vinyl chloride, vinyl cyanide). Initiators are benzoyl peroxide (thermolysis) of azabisisobutyronitrile (AIBN) (photolysis). Emulsion polymerization / nanofabrication of semiconductor chips.

2. Anionic Polymerization (AP)A strongly nucleophilic anion X– (nBuLi, KNH2) reacts with H2C=CHR to form a new carbon-centered anion XCH2–CHR–, which attacks H2C=CHR → chain growth. As for free radical polymerization, the monomer has to afford stabilization by resonance (styrene, 1,3-dienes). In FRP, the radical disappears rapidly (e.g. by recombination), whereas resonance-stabilized anions are quite long-lived in aprotic media and give living polymerization, in which the anionic chain ends continue to grow upon addition of further monomer until they are quenched (e.g. by protonation). Atom-transfer and group-transfer polymerization are variants: Radical or anionic chain ends, instead of occurring freely, are temporarily released by breaking a labile bond (e.g. to Cu or Me3Si).

Classes of olefin polymerization - B3. Cationic Polymerization (CP)Initiation occurs by attachment of a proton or some other Lewis-acidic cation X+ to the H2C=CR2 vinyl monomer to form the XH2C–CR2+ carbocation, which then grows into a polymer chain by subsequent H2C=CR2 additions. Only 1,1-disubstitution (e.g. in isobutene) afford sufficient stabilization. Shorter chains are generally obtained, as proton release from the cationic chain end and other side reactions are easy.

4. Insertion Polymerization (IP)Insertion polymerization is the topic of this chapter. In processes 1 – 3, the initiator is – in principle – not attached to the carbon center by which the polymer chain grows. Therefore, it is not in position to control the stereochemistry of chain growth. Additionally, processes 1–3 are not suitable for unsubstituted olefins (ethene, propene, and other simple α-olefins), as these do not efficiently stabilize radicals, carbanions, or carbocations. These industrially abundant and cheap monomers are best polymerized by catalysts that operate by insertion polymerization, e.g., propene polymerization by successive olefin insertions into a metal-alkyl bond:

A positively charged metal catalyst, which is connected to a chain end bearing a partial negative charge, binds – and thus polarizes – a monomer molecule. The coordination sphere of the metal controls the stereochemistry of olefin coordination and insertion, which enhances the stereoregularity of the chain growth.

Insertion Polymerization (IP) Catalysts Heterogeneous

– Titanium-based catalysts. Discovered by in 1953 by Karl Ziegler and applied by Giulio Natta to the stereospecific polymerization of propene (isotactic polypropylene) in industrial production. Ziegler and Natta were Nobel laureates in 1963.

The Ziegler-Natta catalysts have continually improved since then (see below). They continue to dominate the processes. Despite (or because of) the degree of perfection attained, they permit only limited degrees of variability with regard to some relevant polymer properties.

– Chromium-containing catalysts (not discussed), discovered in 1951 by Paul Hogan and Robert Banks, usually called Phillips catalysts (from the homonymous company). The catalyst is prepared by adsorption of a chromium compound, mostly chromium trioxide, onto an amorphous silica support and subsequent reduction by exposure to ethene. They are used to produce high-density polyethylene (HDPE) with particularly high molar mass and do not polymerize propene.

Insertion Polymerization (IP) Catalysts

Homogeneous

– Zirconium(IV) and titanium(IV) complexes, mostly cyclopentadienyl complexes (either sandwich- or half-sandwich). They were discovered shortly after Ziegler's and Natta's reports in the 1950's.

– Nickel(II) and palladium(II) complexes with nitrogen (imine) ligands (not discussed here). These catalysts give access to polyolefins with a wider choice of properties and are thus likely to be increasingly used for the production of polymers for special-purpose applications, which require properties not easily accessible otherwise.

Ziegler Natta Catalysts

Preparation: TiCl4 + (ball milled MgCl2 + ethyl benzoate or 1,3-diether:”inner donor”) + Silyl ether or phthalic acid diesters:“outer donor”. While being injected into the reactor such catalyst is activated by AlEt3.

The growth of the polymer also takes place inside the catalyst grains. Each grain is disintegrated into several hundred minuscule particles with diameters of only 1–2 nm.

Box 4 Polyolefin Waste and Recycling

As plastic wastes are proliferating at increasing rates, infrastructures forwaste recycling are developing in almost all areas of major polymer con-sumption. Policies in this regard are based on four main steps:i) prevention/reduction of plastic waste, ii) material recycling (mechanical recycling,feedstock recycling), iii) energy recovery and iv) dumping.Feedstock recycling (chemical recycling) is the chemical reconversion of

polymers to raw materials, i.e. to monomeric olefins. Suitable pyrolysis andgasification processes are being tested but have not yet become commerciallycompetitive with the use of petrochemicals from crude oil. On the otherhand, energy recovery, the combustion of polyolefin wastes in incinerationplants to produce energy in the form of heat and electricity, appears to bethe most efficient and unproblematic polyolefin waste utilization, since thechemical composition of these materials (which are free of chlorine, sulfurand nitrogen) is practically indistinguishable from that of heavy oils, whichare burned in incineration plants, together with normal waste, to ensuretemperatures sufficiently high for the complete destruction of toxic effluents.

MgCl2 is ideal as a support for polymerization catalysts since it consists ofloosely aggregated, crystalline sub-particles, which are extensively fragmentedalready by initial polymer formation; hence high polymerization rates arereached immediately. Disadvantageous however, is the high fragility of theinitial MgCl2 support, which can lead to formation of polymer fines (dust) dueto the turbulence of gas-phase polymerizations. Good morphology control ofthe polymer grains is achieved however when a pre-polymerization step isapplied under mild conditions. Since the catalyst support fragments are now

catalystgrain

20-50 µm

ethene

polymer grain

500-1000 µm

primarycatalystparticle

primarypolymerparticle

1 µm

20-50 µm

Figure 8 Simplified model for the fragmentation of MgCl2 -supported Ziegler-Nattacatalyst grains to primary catalyst particles and shape conservation of growingpolymer grains.

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Mechanism

highly active and do not require further activation. These catalysts, while notable to generate polypropylene or higher polyolefins, are used extensively forthe production of the strictly linear, so-called high-density polyethylene (HDPE)with particularly high molar mass and, hence, optimal mechanical and thermalstrength. Their active sites are generally considered to consist of Cr(II) centres,but the ligand environment of these centres and the pathways by which theyinduce the insertion of ethylene molecules into growing polymer chains are stillvery incompletely understood [E. Groppo, C. Lamberti, S. Bordiga, G. Spoto,A. Zecchina, Chem. Rev. 2005, 105, 115].

7.4 Soluble Olefin Polymerization Catalysts

Polymerization catalysis with soluble complexes of group IV transition metals,in particular with hydrocarbon-soluble titanocene complexes, was discovered inthe 1950’s, shortly after the appearance of Ziegler’s and Natta’s reports onsolid-state catalysts, and rather thoroughly studied from then on. Alkylalu-minium compounds, such as AlEt2Cl, are required to activate also these solublecatalysts. In distinction to their solid-state counterparts, however, early solublecatalysts were able to polymerize only ethylene, and not any of its higherhomologues. After their activation by methylalumoxanes had been discovered(Section 7.4.1), soluble catalysts became as efficient as solid-state catalysts – in

Figure 9 Simplified model for the formation of active centres of Ziegler-Natta catalystsby adsorption of TiCl4 on solid MgCl2, reduction of Ti(IV) to Ti(III) chloride-alkyl exchange with aluminium alkyls and polyethylene (PE) formation bycoordination and insertion of ethylene molecules.

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Polypropylene chain growth: Stereoregularity

Isotactic

Active site in Ziegler-Natta catalysts

Homogeneous catalysts

Stereospecific Olefin Polymerization Catalyzed by ansa-Zirconocenes / MAOSimple zirconocene catalysts, such as [ZrMe2(Cp)2], produce relatively short-chain PP (several hundred to a few thousand monomer units). Ansa zirconocenes give chain lengths of some ten to hundred thousand monomer units. (See below why this difference).

The most important difference is the tacticity of the polymer:

Correlation of Polypropylene Microstructures with Metallocene Structures

Complex A + MAO gives isotactic polyproylene (The nature of the chain errors suggests sterocontrol by the chiral catalyst, that is enantiomorphic site control)

Complex B + MAO gives atactic polyproylene

General mechanism

How about stereocontrol?

Chain termination

PN ≅rinsertionrtermination

Chain lengths can be expressed as PN = mean degree of polymerization. For polyolefins,

Therefore, in addition to high rates of chain growth, reduced termination rates are required to produce long-chain polymers. Release of the unsaturated chain end can occur by β-H transfer to the metal (bottom) or to a monomer molecule (top):

Union Carbide Fluidized Bed ProcessThe reactor, about 30 m high, has a characteristic shape with a lower cylindrical reaction section, and an upper expanded section in which the gas velocity is reduced to allow entrained particles to fall back into the bed. The feed gas enters the reactor from the bottom through a distributor plate which provides an even upward flow of gas and prevents polymer powder from falling. Depending on the product being made, reaction temperatures range between 80 and 100°C and pressures between 7 and 20 bar. Most often ethylene conversion is only ca. 2% per pass; the unreacted monomer is then recycled. The process operates close to the melting point of the polymer; accurate temperature control is thus necessary to avoid particle agglomeration. The final reaction mixture is fed into a powder cyclone from which residual monomers are recovered and recompressed.

Union Carbide fluidized-bed process (Figure 6): The reactor, about 30 m high,has a characteristic shape with a lower cylindrical reaction section, and anupper expanded section in which the gas velocity is reduced to allow entrainedparticles to fall back into the bed. The feed gas enters the reactor from thebottom through a distributor plate which provides an even upward flow of gasand prevents polymer powder from falling. Depending on the product beingmade, reaction temperatures range between 80 and 1001C and pressuresbetween 7 and 20 bar. Most often ethylene conversion is only ca. 2% per pass;the unreacted monomer is then recycled. The process operates close to themelting point of the polymer; accurate temperature control is thus necessary toavoid particle agglomeration. The final reaction mixture is fed into a powdercyclone from which residual monomers are recovered and recompressed.

7.2.2 Polypropylene Production

This is often conducted to make impact-resistant polyolefin blends. For this pur-pose, isotactic polypropylene, which is tough but somewhat brittle, is producedon the catalyst pellet in a first reaction step, using highly active stereospecific

Figure 6 Union-Carbide process. a) catalyst hopper; b) fluidized-bed reactor; c) cyclone;d) filter; e) polymer take-off system; f) product recovery cyclone; g) monomerrecovery compressor; h) purge hopper; i) recycling compressor; j) recycle gascooler.

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Novolen ProcessPP homopolymer, impact, and random copolymers are produced in two vertical stirred gas-phase reactors in series. In the first, which operates at 80°C and 20–35 bar monomer pressure and produces PP, propene is injected as liquid to cools the exothermic polymerization by evaporating. The product is continuously transferred to cyclone and either deactivated with steam or transferred to the second reactor, which operates at 60°C and 10–25 bar. There, ethene and propene are copolymerized to cover the tough, but brittle particles of isotactic PP with a softer copolymer.

Ring opening metathesis polymerization (ROMP)

Polycyclooctene (Vestenamer®) cyclooctene 12 kt/a WCl6 / 25 – 100 °C / hexane

Degussa – Hüls, since 1980

Polynorbornene (Norsorex®)norbornene RuCl3 / HCl / BuOH / 25 – 100 °C

CdF / Atofina, since 1976

Polydicyclopentadiene dicyclopentadiene

Telene® (Goodrich) [R3NH]2x–6xMoxMoxOy /

Pentem® (Nippon Zeon) Et2AlCl / ROH / 25 – 100 °C

Metton® (Hercules Inc.) > 136. kt/a WCl6/WOCl4/Et2AlCl/ROH/25–100°C

Materia, Hitachi ruthenium-based catalysts / 25–100°C

The Vestenamer® ProcessSince 1980, Degussa-Hüls has been producing the metathetical polymer of cyclooctene under the commercial name of Vestenamer® 8012, also known as TOR (trans-polyoctenamer). Vestenamer is used as as a blending material to impart greater elasticity and durability to other rubbers (see http://www.degussa-hpp.com/ger/produkte/kautschuk/index.shtml). The catalyst is WCl6-based in hexane as solvent

Two polymer fractions: – high-molecular weight fraction (> 105 Da), true polymer

– low-molecular weight fraction (25 %), different cyclic oligomers

Explanation: Competition between propagation and intramolecular backbiting metathesis:

Different commercial products depending on the trans isomer content (controlled by polymerization conditions): Vestenamer 8012 → 80% trans, high crystallinity, rigid

Vestenamer 6013 → 60% trans, lower crystallinity, softer, suitable for lower temperature applications.

PolynorbornenePolynorbornene is produced by ROMP of 2-norbornene:

The catalyst is based on RuCl3 / HCl in butanol and operates in air.

Norsorex® is commercial name of the 90% trans polymer of norbornene with very high molecular weight (> 3 × 106 g/mol). It is the first commercial metathesis polymer (1976, CdF-Chimie, France), now produced by Elf Atochem.

It is sold as a molding powder and the final vulcanized product is used in various vehicle fittings such as bumpers, arm-rests and engine mounting.

Other uses: oil spill recovery (see http://www.freepatentsonline.com/y2007/0137528.html):

Several grades from Norsorex are available (Norsorex NS or Norsorex APX1 for instance). The behavior in oil may vary from simple gelling effect without expansion to gelling and expansion. Norsorex® is a white polymer powder, it is hydrophobic and oleophilic and has a low density (0.96 g/cm 3 ). It is insoluble and inert in water. It has been developed by ATOFINA to absorb high quantities of various hydrocarbons including for instance naphtenic oil, kerosene aromatic oil.

General Considerations on Homogeneous ROMP

Although ROMP is generally extremely rapid and high yielding du to the release of ring strain, the chemistry of ROMP is not trivial.

Catalysts have extremely high productivity, as the rate of chain propagation far exceeds that of any decomposition reactions that might occur. However, due to the presence of repeating olefin units in the polymer product, it is also important to ensure that propagation is more rapid than:

i) intermolecular chain transfer (leading to break up of existing polymer chains:

ii) intramolecular chain transfer (leading to macrocyclic products:

The intrinsic reactivity of strained cycloalkanes (norbornene, cyclobutene) ensures that they react as desired, and simple homogeneous metal halide catalysts are often effective for this transformation.