Introduction to Polymer Science and Technology Polymerisation

2.4.1 Ziegler-Natta catalysts

The discovery of Ziegler and Natta represents the first and most significant step in the synthesis of crystalline polyolefins. The German chemist Karl Ziegler (1898-1973) discovered in 1953 that when TiCl₃ and triethyl aluminium (TEA), (C₂H₅)₃A1, are combined together they produced an extremely active heterogeneous catalyst (the heterogeneous catalysis has the catalyst in a different phase from the reactants, but the homogeneous catalysis has the catalyst in the same phase as the reactants) for the polymerization of ethylene at atmospheric pressure.

Giulio Natta (1903-1979), an Italian chemist, developed variations of the Ziegler catalyst and extended the method to the production of stereoregular polypropylenes. Ziegler-Natta catalysts are, now, used worldwide to produce the following classes of polymers from a-olefins:

polyethylenes: HDPE, linear low density polyethylene LLDPE and ultra-high molecular weight polyethylene

(UHMWPE)

polypropylene: homopolymer, random copolymer and high impact copolymers

thermoplastic polyolefins (TPO)

ethylene propylene diene monomer polymers (EPDM)

polybutene (PB).

Classic Ziegler-Natta Catalyst is: TiCl₃ + A1(C₂H₅)₃

Introduction to Polymer Science and Technology

Polymerisation

Polymerization is believed to occur by the repeated insertion of a double bond from the monomer into a previously formed Ti-C bond. Efficiency of such a heterogeneous catalyst can be improved significantly by impregnating the catalyst on a solid support such as MgCl₂or MgO.

2.4.2 Metallocene catalysts

Metallocene catalysts are homogeneous and single site catalysts (SSC). Each catalyst molecule offers almost the same activity and accessibility to monomers. In a-olefins, this results in а very uniform product, and with the appropriate choice of catalysts, in a highly stereoregular product with a-olefins.

Metallocene compounds have two cyclic ligands, cyclopentadienides, bonded to a metal centre, see the following box:

Cyclopentadienide (cp) ions have a charge of -1, so with a cation such as Fe⁺², two of the anions will form an iron sandwich, known as ferrocene. If a metal with a bigger valency is involved, e.g., Zr⁺⁴, to balance the charge, the zirconium will also bond to two chloride ions to yield a neutral compound, bis-chlorozirconocene.

A derivative of bis-chlorozirconocene has aromatic rings fused to it. There is also an ethylene bridge, which links the top and bottom cp rings. These two features make this compound a great catalyst for making isotactic polymers.

Metallocenes by themselves are not active for polymerization. Usually, a co-catalyst is required to activate the metallocene. The activated metallocene catalysts can be used for olefin polymerization. Methylaluminoxane (MAO), (Al(CH₃)O)_n, is used to activate the metallocene. If the catalyst is chiral, stereoregular olefin polymerization becomes possible:

Introduction to Polymer Science and Technology

Polymerisation

H₂C-

ci

MAO

>99% isotacticPP

H,C

с

Cl

MAO

-> atacticPP

H₂C

MAO

-> syndiotactic PP

source: http://chem.rochester.edu/~chem234/lecture2.pdf

The concept of chirality is demonstrated with some ordinary items in the box below:

Chirality:

An object that cannot be superimposed on its mirror image is called chird

Mirrw

Mirrer

Chirol objects None hi та I objects

orts up er impossible Superimposable

fnirror linages mirror images

source: http://radaractive.bloaspot.com/2011/01/chiralitv-is-realitv-and-evolution.html

Introduction to Polymer Science and Technology Polymerisation

Metallocene polymerization is making abig impact in the plastics business. One of the exciting outcomes is that metallocene catalysis polymerization allows one to make polyethylenes with much higher molecular weight than possible with the Ziegler-Natta catalysis. This ultra-high molecular weight polyethylene (UHMWPE), e.g., Dyneema, exhibits molecular weights up to six or seven million, and are claimed to be better than Kevlar for making bullet proof vests/armours.

Metallocene catalysts are inherently soluble catalysts (homogeneous). Therefore, the solution-polymerisation process was the first commercial process to use metallocene catalyst to produce polyethylenes. Gas phase and slurry polymerisation processes require heterogeneous catalysts. Metallocene catalysts need to be supported so that they can be employed in gas phase or slurry phase olefin polymerization processes.

Metallocene catalysts have several advantages over classical heterogeneous Ziegler-Natta catalysts:

very high catalytic activities

able to polymerize a large variety of olefins which were not possible with classical Ziegler-Natta catalysts

the main chain termination mechanisms operating with metallocenes provide unsaturated chain ends that

introduce additional functionality

enable control of short as well as long chain branches with even spacings and uniform side-chain length

distribution, which affect the rheological properties, thus, processing

produce greater uniformity in micro-structural morphology, e.g., smaller spherulites of uniform size distribution

inPEs

their 'single site' nature enables these catalysts to produce extremely uniform homo and copolymers with

uniform comonomer distributions with narrow molecular weight distributions and a very small fraction of

extractable oligomers.

Introduction to Polymer Science and Technology Polymerisation

In conclusion, synthesis of vinyl polymers with stereoregular molecules was not possible until the advent of stereospecific polymerisation catalysts such as Ziegler-Natta and more recently metallocenes. Furthermore with these catalysts it is possible to produce polymer backbone chains that consist of blocks of different tacticity, e.g., a polypropylene with atactic and isotactic segments in its molecules can be polymerised by using zirconocene. The resultant polymer can be described as a thermoplastic elastomer (TPE) because of the presence of hard (crystalline) and soft (amorphous) domains. Various outcomes can be tailored by controlling the ratios of these domains.

Polymers consist of macromolecules, the size of which depends on the degree of polymerisation.The subject of the molecular weight of polymeric molecules is covered in the next section.

2.5 Molecular weight and molecular weight distribution

During polymerisation not all polymer chains grow to the same length and this results in a distribution of molecular weights. Accordingly, molecular weight measurements based on, for instance, viscosity and osmotic pressure produce average values. The average molecular weight of the polymer (M) is related to the average degree of polymerisation (DP):

M = M_o x DP, where, M_o is the molecular weight of the repeat unit/monomer.

The unit for molecular weight is normally g/mol, but often it is convenient to omit the unit by expressing it as the ratio of the mass of the molecule to 1/12* of the mass of an atom of ¹²C.

There are several ways of expressing average molecular weight, including number average (M_n) and weight average (M_w) molecular weights:

M_n is based on the number fractions of molecules with a given mass M_;(i.e., doing a weighting based on the number of molecules of a given mass), and M_w is based on the weight fraction of molecules with mass M. (i.e., doing a weighting based on the mass of molecules of a given mass).

where, N is the total number of chains, N.is the number of chains with molecular mass M..

M.=iS,(m,M,) or M_w

where, M is the total mass, m. is the mass of molecules with mass M..

The other definitions for molecular weight are viscosity average (M_y) and z-average (M_z). The values obtained depend on the type of averaging used and they correspond to each other as follows M_n < M_y < M_w < M_z. Laboratory techniques for measuring molecular weight are listed in Table 2.2, which also shows the suitability of these methods at different molecular weight values. The viscosity of the polymer solution is a physical property that is closely linked to the molecular weight. The relation between the intrinsic viscosity r\ and the viscosity-average molecular weight:

Introduction to Polymer Science and Technology

Polymerisation

where, К and a are polymer and solvent specific empirical constants that are obtained by calibration experiments using samples of known molecular weight and determining their [r\].

A simple alternative to viscosity measurement is the measurement of the melt flow index (MFI)or melt flow rate(MFR), which are convenient parameters used just for comparing polymer melts and polymers that are difficult to dissolve. These measure the amount of molten material that flows through a defined orifice under a certain weight. The material is extruded at a given temperature for 10 minutes and the amount of extrudate recorded in grammes. There is an inverse correlation between MFI and viscosity and/or the molecular weight of the polymer. MFR is also, confusingly, used to indicate "melt flow ratio", the ratio between two melt flow indices under two different load levels. For clarity, this should be reported as flow rate ratio (FRR), or simply flow ratio. It is an index that can mislead, since the ratio of the totally different MFI values can produce the same ratio, commonly used as an indication of the way in which rheological behaviour is influenced by the molecular mass distribution of the material.

Table 2.2Analytical techniques for measuring molecular weights of various ranges

M_w/ M_nis the polydispersity indexand indicates how uniform or otherwise the molecular weight distribution(MWD) is. In general, a narrowmolecular weight distribution leads to more uniform property values, a narrower softening/ melting temperature range, a lower stress cracking sensitivity, and better chemical resistance. A broadmolecular weight distribution has advantages for processing because the low molecular weight fractions behave like lubricants. The polymer is less brittle because the low molecular weight fractions can act as plasticisers.

2.5.1 Influence of molecular weight on properties

Molecular weight influences microstructure, and both rheological/processing and end-use properties of polymers. The Polymerisation process has to proceed to a significant level in order to yield high enough molecular weight for the product to be deemed commercially viable. Polymers used as plastics, fibres, paints/adhesives and rubbers are expected to have number-average molecular weights over 10,000. Molecular weight in the range of 10,000 to 1,000,000 enables polymers to be

Introduction to Polymer Science and Technology

Polymerisation

used in so many different applications. As molecular weight increases mechanical properties improve, but melt processing becomes difficult as illustrated in Figure 2.6. As can be seen the mechanical properties increase rapidly initially with the increasing molecular weight and then slow down reaching a steady value. It is important to keep the molecular weight at a level so that the increase in viscosity does not make processing difficult. Specific examples of the influence of molecular weight on properties for polyethylenes and polypropylene are presented in Ehrenstein (2001, p54) and Strong (1996, pl62).

Figure 2.6 Ageneralised plot of property against molecular weight for polymeric materials

Date: 2015-12-11; view: 1241