Polyolefin Chain Growth

A crucial property of a polyolefin is its regioregularity. It is governed by the catalyst used for its production, i.e. by the regioselectivity with which it controls the direction of olefin insertion into a metal-alkyl bond. For example, propylene can insert either with its CFI2 or its CHMe end toward the metal. The first 

Due to this chain-migration process ethylene is polymerized to macromolecules containing multiple branches - rather than to the linearly enchained polymer obtained with classical solid-state catalysts. In propylene polymerization with these catalysts 1,2-insertions give the normal methyl-substituted polymer chains, but after each 2,1-insertion the metal centre is blocked by the bulky secondary alkyl unit and can apparently not insert a further propylene. Instead the metal must then first migrate to the terminal, primary C atom before chain growth can continue by further propylene insertions. By this process, also called 1,CO-enchainment or polymer straightening, some of the methyl or (in the case of higher olefins) alkyl substituents are incorporated into the chain. 

Some Ni(II)-catalysts polymerize higher olefins by 2,co-enchainment. Here even the primary insertion product appears too bulky for further chain growth due to its adjacent alkyl branch the metal thus has to migrate to the unencumbered end of the alkyl side chain before another insertion can occur [V. Mohring, G. Fink Angew. Chem. Intern. Ed. Engl. 1985, 24, 1001]. 

In zirconocene-catalyzed olefin polymerizations similar processes are involved. Here polymerization rates depend at least linearly on olefin concentrations an 

Due to their narrow aperture, zirconocene-based catalysts insert olefins almost exclusively in the 1,2- or primary direction. Small proportions of 2,1-inserted propylene units and, for some catalysts, 1,3-inserted units derived from them by chain straightening, are a cause of melting-point lowering in some metallocene-produced polypropylenes. 

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We encounter homogeneous catalysts in both step-growth and chain-growth polymerization processes. We saw several examples of these types of reactions in Chapter 2. For example, the acid catalyzed polymerization of polyesters occurs via a homogeneous process as do some metallocene catalyzed polymerization of polyolefins. 

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

Different from analytical dynamic solutions, instantaneous distributions are derived to describe the polymer microstructure made at a particular instant in time during the polymerization. These distributions are snapshots of the polymer microstructure at a given moment during the polymerization. A classic instantaneous distribution used to describe the CLD of linear polyolefins and other polymers that follow analogous chain growth kinetics was developed independently by Schulz [79] and Flory [80, 81]... 

Olefins (from the French olefiant, oil-forming ), or alkenes, are hydrocarbon molecules with at least one double carbon-carbon bond. Alpha (a-)olefins are alkenes with a double bond at the first (alpha-) carbon. Polyolefins are polymer molecules made using free radical or ionic initiators to open these reactive double bonds in an addition (chain-growth) polymerization, producing essentially linear high molecular weight thermoplastic polymers. 

The addition of reagents containing X-H bonds in which X is more electronegative than H typically lead to addition across the M-C bond in the direction opposite to the addition of silane or borane to the early metal catalysts. Polymerization of etiiylene with lanthanide catalysts in the presence of phosphines generates phosphine-terminated polymers (Scheme 22.12) - by a mechanism in which the alkyl chain is protonated, and a metal-phosphido complex is generated. This phosphido complex then inserts olefin to start the growth of a phosphine-functionalized polyolefin. Marks subsequently showed that a similar process can be conducted witii amines. In this case, the bulky dicyclohexylamine was needed to sufficiently retard the rate of protonation to allow chain growth. The steric bulk also makes the olefin insertion more favorable thermodynamically. 

For the growth of isotactic polypropylene chains and higher polyolefin chains at the chiral coordination sites of ansa-metallocene catalysts, the following explanation is now firmly established [11,12] Formation of the new C-C bond requires that the a-olefin substituent and the C(a)-C(P) bond of the metal-bound polymeryl chain are oriented anti to each other along the incipient C-C bond, while the C(a)-C(P) chain segment must reside in an open quadrant of the chiral metallocene coordination site. The latter is thus considered to control the enantiofacial orientation of the a-olefin in the insertion transition state TS (Fig. 4) by way of the C(a)-C(P) chain-segment lever . 

The most important chain-growth polymers are polyolefins and other vinyl polymers. Examples of the former are polyethylene, and polypropylene, and examples of the latter are poly(vinyl chloride), polystyrene, poly(vinyl alcohol), polyacrylonitrile, and poly(methyl acrylates). The most common stepwise reactions are condensation polymerizations. Polyamides, such as nylon 6-6, which is poly(hexamethylene adipamide), and polyesters, such as poly(ethylene terephthalate), are the most important commercial condensation polymers. These polymers were originally developed for use in fiber manufacture because of their high melting points but are now used also as thermoplastics. Polycarbonate is an engineering plastic that is made from bisphenol A and phosgene by a stepwise reaction. 

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