**3. PP catalyst development and catalyst-defined polymer design**

## **3.1 Z-N catalysts**

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

catalysts or post-metallocene catalysts [19].

are directly used in a solution polymerization process.

**2. General structure and properties**

due to the tertiary C atom.

Furthermore, numerous methods such as using polymerizable chain shuttling agents and reactive comonomers were adopted to prepare functionalized PP polymer [18] in order to improve PP's interactive performance and broaden its applications to high value-added products, in which compatibility and adhesion with other materials are needed. Other propylene-based polymers, such as ethylene propylene diene monomer (EPDM) rubber, can also be produced by catalyst design of Z-N

Combined with the PP catalyst technology, a series of PP polymerization processes have been developed and commercialized successfully. And the polymerization processes are highly dependent on the PP catalyst system. Typically, in a gas phase, slurry processes, such as Spheripol (Basell), Hypol (Mitsui Chemicals), Unipol (Dow Chemical), Innovene (INEOS), Novelen (BASF), Spherizone (Basell),

and Borstar (Borealis), are required to meet the requirements for industrial equipment, such as shape control of products, avoiding reactor fouling with low investment and operating costs and without environmental impact, etc., and heterogenization of Z-N or metallocene catalysts. Isotacticity polypropylene (iPP), random copolypropylene (RPP), impact PP can be produced in these processes, while in contrast, molecular catalysts such as CGC and post-metallocene catalysts

PP is a semi-crystalline thermoplastic resin with a linear chain structure consisting of C and H elements. Similar to PE, PP has great chemical resistance toward solvents, acid, and alkali. The alignment of methyl groups attached on the chain backbone, however, may greatly influence the polymer's properties in several ways, including the introduction of a steric group and different stereostructures as shown in **Figure 3**. There are three main different stereoisomers of PP, isotactic PP (iPP), syndiotactic PP (sPP), and atactic PP (aPP) [16]. For iPP, all methyl groups are arranged on the same side of the polymer backbone, in sPP the methyl groups are located on alternating sides, while in aPP, the methyl groups are scattered randomly along the polymer chain. Compared to PE, iPP has higher melting point and modulus due to their stiffening chain and the helical crystal structure. In addition, chain scission rather than cross-linking happens in thermal and high-energy treatment

Stereoregularity of the methyl group branch separates crystallizable subspecies from amorphous subspecies; the melting point and modulus strength of sPP is

Similarly, copolymerization of propylene and α-olefin with various compositions can vary the crystallizing ability of the polymer chain. There are two general types of polypropylene copolymers: random copolymers and block copolymers. The

*Three types of stereoisomers of general PP chain: (a) isotactic, (b) syndiotactic, and (c) atactic [16].*

lower than iPP, and among them, aPP has the lowest melting point.

**12**

**Figure 3.**

The massive production of PP is predominated by the use of MgCl2-supported Z-N catalyst systems. These systems consist of a supported catalyst composition, formed by reaction of a transition metal halide (usually TiCl4) and an internal electron donor (ID), generally a Lewis base as a support (typically MgCl2), an alkylaluminum cocatalyst (e.g., triethylaluminum), and an external donor (ED), generally another Lewis base (e.g., alkoxysilanes). These catalyst compositions are independently added in the process of polymerization. A timeline overview on the historical progress in the commercial research and development of Z-N catalyst systems is presented in **Table 1**.

The first major improvement in the Z-N catalysts, which is based on research by Montedison (now LyondellBasell) in Italy and Mitsui in Japan, occurred in 1968 with the discovery of the milled MgCl2 support for ethylene polymerization. This technology was extended to the PP industry in 1970s by extra addition of internal and external electron donors to ameliorate the isotacticity of PP without inhibiting catalyst activity, which led to the third-generation of PP catalysts with high yield (15–30 kg PP/g cat), eliminating the need for catalyst residue removal (Ti ≤ 5 ppm), but the atactic component was still inconveniently too high.

The second breakthrough came out with "the reactor granule technology" (RGT) in the 4th generation catalysts in the 1980s. This heterogeneous catalysis was based on active MgCl2 and allowed for a real process simplification, eliminating the process for ash content and atactic removal and avoiding the occurrence of a large amount of fine powder. The particle morphology of the prepared catalyst could be replicated in the final polymer as the particles grow during the polymerization, which was called "replication phenomenon." The catalyst can have a granular or spherical form with a higher and longer activity (20–60 kg.PP/g.cat); also, it has high stereoregularity with isotactic index typically above 95%, tunable molecular weight distribution, and so on [21].

Several routes have been employed for the preparation of granular or spherical catalysts with controllable particle size and morphology. One feasible strategy is implemented by controlled preparation of a new support material, the adducts of MgCl2 and an alcohol, which is subsequently titanated and reacted with an internal donor to obtain supported catalyst. The archetypal example of controlled precipitation was disclosed in the work of Kashiwa and coworkers at Mitsui [22]. MgCl2 is contacted with 2-ethylhexanol in alkane solvent, forming a homogeneous solution. The formed solution then is reacted with phthalic anhydride and TiCl4. The mixture is subsequently contacted with diisobutyl phthalate (DIBP) as an internal donor to form precipitated solid particles with heating. Then the particles are treated


**Table 1.**

**15**

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made…*

with TiCl4 again to obtain the final catalyst. Another successful example using this precipitation method is the N series of catalysts (BRICI, Sinopec). According to this method, anhydrous MgCl2 is reacted with tributyl phosphate and epichlorohydrin in toluene to form a uniform solution. The solution is subsequently treated with phthalic anhydride and TiCl4. The resultant solid particles have regular spherical form; then the solid catalyst is contacted with DIBP and TiCl4 to obtain the final

The spherical MgCl2∙nEtOH support is also a commercially successful example by Basell/Avant ZN range, Sinopec/BRICI DQ catalyst [24], and Brorealis [25]. Spray-drying or controlled precipitation and emulsion process can be adopted to produce the spherical MgCl2∙nEtOH support. As exemplified in Avant ZN range, molten emulsions of the MgCl2∙nEtOH (n ≈ 2–3) adduct in paraffin oil are cooled rapidly to obtain spherical particles with a narrow particle size distribution. Similarly, the spherical support of MgCl2∙nEtOH (n ≈ 3) is used in the DQ catalyst [26–28]. Instead of contacting a solid support with catalyst components, the emulsion-based catalyst of Borealis is based on a liquid/liquid phase system, and the

Furthermore, Mg(OEt)2 has been successfully used as a starting material in SHAC and Toho Titanium THC catalyst system [29–35]. Mg(OEt)2 support plays a significant role in the development of the super high activity catalyst (SHAC) system, with ethylbenzoate as the internal electron donor in the early stage. The internal electron donor ethylbenzoate can be replaced easily by other donors; for example, benzoate is used in commercial SHAC 310 catalyst and phthalate in SHAC 320 catalyst. As a starting material, Mg(OEt)2 also could be converted to a carboxylate by contacting with CO2, to form a soluble Mg medium. The soluble carboxylate Mg can be reacted directly with TiCl4, which shapes the basis of the Amoco CD catalyst [36–38]. After reacting with a Grignard solution such as nBuMgCl, the Mg(OEt)2 could be converted to a Mg(OR)Cl support with controlled morphology. A catalyst based on this Mg(OEt)Cl support has also been mentioned by SABIC

The 4th RGT catalysts have also promoted the revolutionary development of PP-based production processes, such as Hypol, Unipol, Spheripol, Novelen, Spherizone, Catalloy, etc., and made it possible to generate multiphase alloys and blends directly in reactors, producing high-performance materials not available from conventional technologies. First, the catalyst is the only active center, propylene polymerization takes place in the catalyst, as the polymerization goes on, the catalyst grows into a polymer particle with active site within it, so both the catalyst

For Z-N catalysts, the internal and external electron donors are very critical in tuning the chain structure, MWD of the polymer, and hydrogen response; their effect is mainly determined by the binding energy and mobility on the MgCl2 surface, controlling the stereoregularity of the PP chain. As mentioned above, internal electron donors are added during catalyst preparation while external electron donors are used in the process of polymerization. For a given system, changing the internal donor/Ti ratio and/or Al/external electron donor might lead to a dramatic

The search for further catalyst improvements brought to light various novel internal donors which were not readily extracted from the support by the alkylaluminum cocatalyst. The diether compounds used as an internal donor in the 5th generation Z-N catalyst systems, in particular 2,2-disubstituted-1,3-dimethoxypropanes with an O–O distance in the range of 2.8–3.2Ǻ, which are not extracted easily due to the relatively strong interaction with the catalyst surface, when contacting the AlEt3 cocatalyst, show high stereospecificity even without using an external

and the polymer particle can act as reactor during the polymerization.

*DOI: http://dx.doi.org/10.5772/intechopen.85963*

catalyst components are contained in a liquid phase.

[39, 40] and Basell/Akzo Nobel [40, 41].

difference in the polymer structure.

catalyst [23].

*Performance development of Ziegler-Natta catalysts for polypropylene.*

### *Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made… DOI: http://dx.doi.org/10.5772/intechopen.85963*

with TiCl4 again to obtain the final catalyst. Another successful example using this precipitation method is the N series of catalysts (BRICI, Sinopec). According to this method, anhydrous MgCl2 is reacted with tributyl phosphate and epichlorohydrin in toluene to form a uniform solution. The solution is subsequently treated with phthalic anhydride and TiCl4. The resultant solid particles have regular spherical form; then the solid catalyst is contacted with DIBP and TiCl4 to obtain the final catalyst [23].

The spherical MgCl2∙nEtOH support is also a commercially successful example by Basell/Avant ZN range, Sinopec/BRICI DQ catalyst [24], and Brorealis [25]. Spray-drying or controlled precipitation and emulsion process can be adopted to produce the spherical MgCl2∙nEtOH support. As exemplified in Avant ZN range, molten emulsions of the MgCl2∙nEtOH (n ≈ 2–3) adduct in paraffin oil are cooled rapidly to obtain spherical particles with a narrow particle size distribution. Similarly, the spherical support of MgCl2∙nEtOH (n ≈ 3) is used in the DQ catalyst [26–28]. Instead of contacting a solid support with catalyst components, the emulsion-based catalyst of Borealis is based on a liquid/liquid phase system, and the catalyst components are contained in a liquid phase.

Furthermore, Mg(OEt)2 has been successfully used as a starting material in SHAC and Toho Titanium THC catalyst system [29–35]. Mg(OEt)2 support plays a significant role in the development of the super high activity catalyst (SHAC) system, with ethylbenzoate as the internal electron donor in the early stage. The internal electron donor ethylbenzoate can be replaced easily by other donors; for example, benzoate is used in commercial SHAC 310 catalyst and phthalate in SHAC 320 catalyst. As a starting material, Mg(OEt)2 also could be converted to a carboxylate by contacting with CO2, to form a soluble Mg medium. The soluble carboxylate Mg can be reacted directly with TiCl4, which shapes the basis of the Amoco CD catalyst [36–38]. After reacting with a Grignard solution such as nBuMgCl, the Mg(OEt)2 could be converted to a Mg(OR)Cl support with controlled morphology. A catalyst based on this Mg(OEt)Cl support has also been mentioned by SABIC [39, 40] and Basell/Akzo Nobel [40, 41].

The 4th RGT catalysts have also promoted the revolutionary development of PP-based production processes, such as Hypol, Unipol, Spheripol, Novelen, Spherizone, Catalloy, etc., and made it possible to generate multiphase alloys and blends directly in reactors, producing high-performance materials not available from conventional technologies. First, the catalyst is the only active center, propylene polymerization takes place in the catalyst, as the polymerization goes on, the catalyst grows into a polymer particle with active site within it, so both the catalyst and the polymer particle can act as reactor during the polymerization.

For Z-N catalysts, the internal and external electron donors are very critical in tuning the chain structure, MWD of the polymer, and hydrogen response; their effect is mainly determined by the binding energy and mobility on the MgCl2 surface, controlling the stereoregularity of the PP chain. As mentioned above, internal electron donors are added during catalyst preparation while external electron donors are used in the process of polymerization. For a given system, changing the internal donor/Ti ratio and/or Al/external electron donor might lead to a dramatic difference in the polymer structure.

The search for further catalyst improvements brought to light various novel internal donors which were not readily extracted from the support by the alkylaluminum cocatalyst. The diether compounds used as an internal donor in the 5th generation Z-N catalyst systems, in particular 2,2-disubstituted-1,3-dimethoxypropanes with an O–O distance in the range of 2.8–3.2Ǻ, which are not extracted easily due to the relatively strong interaction with the catalyst surface, when contacting the AlEt3 cocatalyst, show high stereospecificity even without using an external

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

**14**

**Generation**

1st

δ-TiCl3.0.33AlCl3 + AlEt

 Cl 2

(1957–1970)

2nd

δ-TiCl3 + AlEt Cl 2

10–15

94–97

Low

Irregular powder

(1970–1978)

3rd

TiCl4/benzoate/MgCl2 + AlEt3 + benzoate

15–30

90–95

8–10

Low

Regular/irregular powder

(1978–1980)

4th (1980)

MgCl2/TiCl4/phthalate + AlEt3/silane

20–60

95–99

6–8

Medium

Particles with regular shapes and

adjustable size and PSD. Designed

distribution of the different products

inside each particles

three-dimensional catalyst granule

architecture (RGT)

MgCl2/TiCl4/diester + AlEt3

50–130

95–99

4–6

Very high

MgCl2/TiCl4/diester + AlEt3/silane

MgCl2/TiCl4/succinate + AlEt3/silane

MgCl2/TiCl4/phthalate-free donor +

AlEt3/silane

40–70

95–99

10–15

Medium

Very high

Designed distribution of the different

No purification, no

atactic removal

products inside each particles

RGT

5th (1988)

RGT

(1999) RGT 6th Phthalate

replacement

*aPolymerization conditions: liquid propene, 70°C, 2 h.*

**Table 1.**

*Performance development of Ziegler-Natta catalysts for polypropylene.*

**Composition and structure**

**Productivitya**

**II** 

**Mw/**

**H2**

**Technology control**

**Process** 

**requirements**

Need of purification

and atactic removal

Need of purification

and atactic removal

No purification,

need of atactic

removal

No purification, no

atactic removal

**(wt%)**

**Mn**

**response**

Low

Irregular powder

**(kg PP/g cat)**

0.8–1.2

88–91 donor [42–47]. The diether compounds exhibit particularly high polymerization activity, typically giving yields exceeding 100 kg PP/g cat, good high hydrogen sensitivity, and relatively narrow MWD. Aliphatic dicarboxylic ester-based internal donor and, in particular, succinates and polyol esters have been employed by Basell [48, 49] and BRICI/Sinopec. Different from the 4th generation phthalate-based catalysts, the succinate internal donors developed by Basell produce PP with much broader MWD 10–15 using an alkoxysilane as an external donor. In addition, the succinate-based catalysts also generate ethylene-propylene copolymers with lower glass transition temperature, which enables the production of heterophasic copolymers having great balance of stiffness and toughness. Similarly, the polyol ester family developed by BRICI/Sinopec as an internal donor has a similar polymerization performance to succinate-based systems. However, unlike the succinates, this catalyst system yields high stereoselectivity even without alkoxysilane external donors [50]. In addition to this, mixed donors are also employed in many cases, for example, the mixture donor system of succinate and diether [51] or succinate and dimethoxytoluene [52].

In recent decades, numerous researches focus on finding more potent electron donors for the 6th generation Z-N PP catalysts. Despite the fact that the phthalatebased catalysts produce PP with far lower phthalate content below the 0.3 wt% (3000 mg/kg) concentration limit in the REACH Regulation (EC) 1907/2006, a totally phthalate-free solution is highly motivated and becomes a competitive advantage. As shown in **Figure 4**, there are several types of nonphthalate electron donors that are commercially used or proposed [53]. The new nonphthalate solution (Consista donors) of Dow (Now Grace), as a exemplified example 1,2-phenylene dibenzoate donors (**Figure 5**) [54–57], undoubtedly takes the vanguard in the developments of nonphthalate replacement. It is worth noting that a more complicated catalyst preparation process may be required in contact with the internal donors and TiCl4. In order to improve the final polymer particle morphology and catalyst performance, the process of use of ethylbenzoate or 2-methoxy ethylbenzoate as buffering to the procatalyst and TiCl4 may be needed before adding the phenylene dibenzoate [58].

Typically, alkoxysilane compounds are used as the external donors and added in the polymerization process. The stereospecificity of the Z-N catalysts could be controllable by changing the substituents of the alkoxysilanes containing relatively bulky groups [53]. The correlations between the structure of the silanes' external donor and their polymerization performance have been discussed in detail by Härkönen and Seppala [59–65]. Silanes containing hydrocarbon substituents and oxygen atom with the appropriate size and electron density are expected to obtain PP with high isotacticity index. An industrial silane external donor typically contains at least one secondary or tertiary carbon linked to the silicon atom. It is reported that this bulky group could protect the silane against removal from the catalyst surface when contacting with aluminum alkyl [66]. By far, cyclohexyl(methyl)dimethoxysilane (donor C) and dicyclopentyldimethoxysilane (donor D) have been most commonly used [66]. Some commercial external donors are listed in **Figure 6**. When compared to donor D of the polymerization performance, donor C gives high hydrogen sensitivity and the latter gives particularly high stereospecificity [67] and broader MWD [68].

The Mw and MWD of a PP are critical to the end-use application of the PP product. For fiber spinning applications, relatively low MW and narrow MWD are favored. In contrast, high melt-strength is required for extrusion of pipes and thick sheets; therefore, broad MWD and relatively high MW are needed. For heat-resistant PP, generally with high isotactic stereoregularity, broadening MWD is beneficial for balance between high rigidity and toughness. And choosing

**17**

**Figure 5.**

**Figure 4.**

*Recent disclosure of internal donors [53].*

*1,2-Phenylene dibenzoate donors from Dow.*

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made…*

*DOI: http://dx.doi.org/10.5772/intechopen.85963*

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made… DOI: http://dx.doi.org/10.5772/intechopen.85963*

**Figure 4.** *Recent disclosure of internal donors [53].*

**Figure 5.** *1,2-Phenylene dibenzoate donors from Dow.*

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

donor [42–47]. The diether compounds exhibit particularly high polymerization activity, typically giving yields exceeding 100 kg PP/g cat, good high hydrogen sensitivity, and relatively narrow MWD. Aliphatic dicarboxylic ester-based internal donor and, in particular, succinates and polyol esters have been employed by Basell [48, 49] and BRICI/Sinopec. Different from the 4th generation phthalate-based catalysts, the succinate internal donors developed by Basell produce PP with much broader MWD 10–15 using an alkoxysilane as an external donor. In addition, the succinate-based catalysts also generate ethylene-propylene copolymers with lower glass transition temperature, which enables the production of heterophasic copolymers having great balance of stiffness and toughness. Similarly, the polyol ester family developed by BRICI/Sinopec as an internal donor has a similar polymerization performance to succinate-based systems. However, unlike the succinates, this catalyst system yields high stereoselectivity even without alkoxysilane external donors [50]. In addition to this, mixed donors are also employed in many cases, for example, the mixture donor system of succinate and diether [51] or succinate and

In recent decades, numerous researches focus on finding more potent electron donors for the 6th generation Z-N PP catalysts. Despite the fact that the phthalatebased catalysts produce PP with far lower phthalate content below the 0.3 wt% (3000 mg/kg) concentration limit in the REACH Regulation (EC) 1907/2006, a totally phthalate-free solution is highly motivated and becomes a competitive advantage. As shown in **Figure 4**, there are several types of nonphthalate electron donors that are commercially used or proposed [53]. The new nonphthalate solution (Consista donors) of Dow (Now Grace), as a exemplified example 1,2-phenylene dibenzoate donors (**Figure 5**) [54–57], undoubtedly takes the vanguard in the developments of nonphthalate replacement. It is worth noting that a more complicated catalyst preparation process may be required in contact with the internal donors and TiCl4. In order to improve the final polymer particle morphology and catalyst performance, the process of use of ethylbenzoate or 2-methoxy ethylbenzoate as buffering to the procatalyst and TiCl4 may be needed before adding the

Typically, alkoxysilane compounds are used as the external donors and added in the polymerization process. The stereospecificity of the Z-N catalysts could be controllable by changing the substituents of the alkoxysilanes containing relatively bulky groups [53]. The correlations between the structure of the silanes' external donor and their polymerization performance have been discussed in detail by Härkönen and Seppala [59–65]. Silanes containing hydrocarbon substituents and oxygen atom with the appropriate size and electron density are expected to obtain PP with high isotacticity index. An industrial silane external donor typically contains at least one secondary or tertiary carbon linked to the silicon atom. It is reported that this bulky group could protect the silane against removal from the catalyst surface when contacting with aluminum alkyl [66]. By far, cyclohexyl(methyl)dimethoxysilane (donor C) and dicyclopentyldimethoxysilane (donor D) have been most commonly used [66]. Some commercial external donors are listed in **Figure 6**. When compared to donor D of the polymerization performance, donor C gives high hydrogen sensitivity and the latter gives particularly high

The Mw and MWD of a PP are critical to the end-use application of the PP product. For fiber spinning applications, relatively low MW and narrow MWD are favored. In contrast, high melt-strength is required for extrusion of pipes and thick sheets; therefore, broad MWD and relatively high MW are needed. For heat-resistant PP, generally with high isotactic stereoregularity, broadening MWD is beneficial for balance between high rigidity and toughness. And choosing

**16**

dimethoxytoluene [52].

phenylene dibenzoate [58].

stereospecificity [67] and broader MWD [68].

**Figure 6.**

*Typical industrial external donors in use.*

alkoxysilane external donor in the polymerization process is a convenient approach to control stereoregularity, MWD, and the H2 response and ethylene incorporation. However, it is often the fact that a single external donor is difficult to yield desirable control over multiple properties of the final polymer. To overcome this problem, multiple external donors could be taken into consideration to tune polymer properties by mixed external donors or separate addition in different reactors. Take for example that Exxon have exploited combination of tetraethoxysilane (TEOS) and donor D in a two-stage polymerization process [69]. TEOS alone was utilized at the first stage, producing high melting index polymer; then, donor D was added at the second stage, forming resin with high isotacticity and low melt flow rate, similar to those produced by D donor alone. However, the combination of multiple donors and a two-stage process obtained a final polymer with high isotacticity, high melting index, and broader MWD. In addition, the mixed donor D and TEOS systems showed higher incorporation of ethylene and high hydrogen sensitivity in continuous copolymerization of propylene with ethylene [70]. Another case in point of combination of multiple external donors was revealed in the "self-extinguishing" catalysts concept of Dow's SHAC™ catalyst system in the Unipol process [71]. In the process of polymerization, a mixed external donor consisting of alkyl benzoates and alkoxysilanes is used; the catalyst systems are very active at the normal operating conditions but dramatically lose their activity at higher temperatures, and therefore prevent reactor-fouling. Further development in the "self-extinguishing" catalysts is also mentioned by combinations of NPTMS with aliphatic esters such as di-n-butyl sebacate or isopropyl myristate [72].

**19**

**Figure 7.**

*Hoechst (left) and BASF metallocenes [9].*

of MAO.

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made…*

Since the discovery of MAO by Sinn and Kaminsky, single site catalysts, and metallocenes particularly, have presented significant and meaningful innovation in olefin polymerization catalysis. For commercialization of metallocene catalysts, these metallocene systems have to be compatible with the advanced process tech-

From a commercial perspective, iPP seems a reasonable starting point, and metallocenes suitable for iPP production generally are based on supported-zirconocenes. So far, over 900 applications since 1984 have been patented on the iPP-based metallocenes by Hoechst, Exxon, Fina, Mitsui, BASF, and so on. The typical chemi-

The single-site metallocenes allow microstructure tailoring in the molecular

Along with the development of the tailor-made metallocenes, the homogeneous

stereospecific catalysis has allowed the industrial preparation of atactic polypropylene elastomers with high molecular weight and longer iPP blocks. Several metallocene complexes have been employed for producing PP-based elastomer by designing the architectures of the metallocene catalyst. A "molecular switch" approach was reported by Waymouth and Coates, the unbridged metallocene catalyst can be changed into a dual-site catalyst by intramolecular oscillation to generate stereoblock PP without extra metallocene [85–89]. A schematic illustration is shown in **Figure 9**, and these complexes were thought to oscillate stereocontrol between their rac-like (isospecific) and meso-like (nonstereospecific) configurations during polymerization, thus producing stereoblock PP containing alternating isotactic and atactic blocks. This intriguing idea was further developed by other groups to discover other metallocenes having similar behavior of oscillating stereocontrol, such

level of the produced PP with narrow molecular weight distribution (Mw/ Mn = 2). The chance to control the polymer molecular structure by metallocene complex design helped to shape a better knowledge of the basic structure-performance relations in PP. As illustrated in **Figure 8**, different molecular chains of PP with various stereoregularity can be produced by design of the metallocene catalyst structure [73–76], such as isotactic polypropylene (iPP) [77, 78], hemiisotactic polypropylene (hiPP) [79], syndiotactic polypropylene (sPP) [80], and stereoblock polypropylene [81–83]. The correlation between the microstructure of PP and the symmetry of the metallocene complexes has been established via Ewen's stereocontrol rules [84]. These tailor-made metallocene catalysts indicated the 2nd breakthrough in olefin polymerization followed by the discovery

cal structure of the employed zirconocenes is illustrated in **Figure 7**.

*DOI: http://dx.doi.org/10.5772/intechopen.85963*

nologies, which are referred to as "drop-in catalysts."

**3.2 Metallocene catalysts**

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made… DOI: http://dx.doi.org/10.5772/intechopen.85963*

## **3.2 Metallocene catalysts**

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

alkoxysilane external donor in the polymerization process is a convenient approach to control stereoregularity, MWD, and the H2 response and ethylene incorporation. However, it is often the fact that a single external donor is difficult to yield desirable control over multiple properties of the final polymer. To overcome this problem, multiple external donors could be taken into consideration to tune polymer properties by mixed external donors or separate addition in different reactors. Take for example that Exxon have exploited combination of tetraethoxysilane (TEOS) and donor D in a two-stage polymerization process [69]. TEOS alone was utilized at the first stage, producing high melting index polymer; then, donor D was added at the second stage, forming resin with high isotacticity and low melt flow rate, similar to those produced by D donor alone. However, the combination of multiple donors and a two-stage process obtained a final polymer with high isotacticity, high melting index, and broader MWD. In addition, the mixed donor D and TEOS systems showed higher incorporation of ethylene and high hydrogen sensitivity in continuous copolymerization of propylene with ethylene [70]. Another case in point of combination of multiple external

donors was revealed in the "self-extinguishing" catalysts concept of Dow's SHAC™ catalyst system in the Unipol process [71]. In the process of polymerization, a mixed external donor consisting of alkyl benzoates and alkoxysilanes is used; the catalyst systems are very active at the normal operating conditions but dramatically lose their activity at higher temperatures, and therefore prevent reactor-fouling. Further development in the "self-extinguishing" catalysts is also mentioned by combinations of NPTMS with aliphatic esters such as di-n-butyl

**18**

**Figure 6.**

*Typical industrial external donors in use.*

sebacate or isopropyl myristate [72].

Since the discovery of MAO by Sinn and Kaminsky, single site catalysts, and metallocenes particularly, have presented significant and meaningful innovation in olefin polymerization catalysis. For commercialization of metallocene catalysts, these metallocene systems have to be compatible with the advanced process technologies, which are referred to as "drop-in catalysts."

From a commercial perspective, iPP seems a reasonable starting point, and metallocenes suitable for iPP production generally are based on supported-zirconocenes. So far, over 900 applications since 1984 have been patented on the iPP-based metallocenes by Hoechst, Exxon, Fina, Mitsui, BASF, and so on. The typical chemical structure of the employed zirconocenes is illustrated in **Figure 7**.

The single-site metallocenes allow microstructure tailoring in the molecular level of the produced PP with narrow molecular weight distribution (Mw/ Mn = 2). The chance to control the polymer molecular structure by metallocene complex design helped to shape a better knowledge of the basic structure-performance relations in PP. As illustrated in **Figure 8**, different molecular chains of PP with various stereoregularity can be produced by design of the metallocene catalyst structure [73–76], such as isotactic polypropylene (iPP) [77, 78], hemiisotactic polypropylene (hiPP) [79], syndiotactic polypropylene (sPP) [80], and stereoblock polypropylene [81–83]. The correlation between the microstructure of PP and the symmetry of the metallocene complexes has been established via Ewen's stereocontrol rules [84]. These tailor-made metallocene catalysts indicated the 2nd breakthrough in olefin polymerization followed by the discovery of MAO.

Along with the development of the tailor-made metallocenes, the homogeneous stereospecific catalysis has allowed the industrial preparation of atactic polypropylene elastomers with high molecular weight and longer iPP blocks. Several metallocene complexes have been employed for producing PP-based elastomer by designing the architectures of the metallocene catalyst. A "molecular switch" approach was reported by Waymouth and Coates, the unbridged metallocene catalyst can be changed into a dual-site catalyst by intramolecular oscillation to generate stereoblock PP without extra metallocene [85–89]. A schematic illustration is shown in **Figure 9**, and these complexes were thought to oscillate stereocontrol between their rac-like (isospecific) and meso-like (nonstereospecific) configurations during polymerization, thus producing stereoblock PP containing alternating isotactic and atactic blocks. This intriguing idea was further developed by other groups to discover other metallocenes having similar behavior of oscillating stereocontrol, such

**Figure 7.** *Hoechst (left) and BASF metallocenes [9].*

#### **Figure 8.**

*Correlation between metallocene structures and polypropylene architectures.*

#### **Figure 9.**

*Molecular switching between the two site configurations during polymerization of the unbridged metallocene catalyst.*

as bis(2-aryl-indenyl) zirconocenes and hafnocenes with diverse aryl substitutions [90–92]. Moreover, the types of solvents and counterions can unfavorably limit ligand rotation; therefore, PP with complex microstructures might be produced.

**21**

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made…*

By combining different stereospecific metallocenes, thermoplastic PP elastomers could also be prepared. In 1996, Chien employed a mixture of two different metallocenes, which consists of isospecific C2v-symmetric metallocenes such as rac-ethylenebis(1-indenyl) zirconium dichloride or rac-dimethyl silylbis(1-indenyl) zirconium dichloride and nonstereospecific C2-symmetric metallocene such as ethylenebis(9-fluorenyl)zirconium dichloride, to produce PP-based elastomers by propylene homogeneous polymerization [93–95]. The combined dual-site metallocene catalysts generate a reactor blend comprising isotactic PP, atactic PP, and stereoblock PP containing alternating isotactic and atactic blocks. In the produced blends, the stereoblock PP contains immiscible isotactic and atactic PP blocks due to the chain transfer between stereospecific and nonstereospecific sites. Different from the early multisite catalysts for generating PP-based elastomer, this dual-site catalyst system, which combines two single-site metallocene catalysts, allows flexible tailoring of the elastomer properties by simply changing the molar ratio of added C2/C2v metallocenes. Surprisingly, Fink successfully synthesized iPP-b-sPP stereoblock PP using silica-supported MAO/dual-site metallocene catalysts that combined isospecific rac-Me2Si[Ind]2ZrCl2 with syndiospecific iPr(FluCp)ZrCl2. In another example of producing iPP-b-sPP stereoblock PP, Rytter and coworkers used AlMe3 together with the MAO as an activator to the homogeneous dual-site catalyst system consisting of isospecific rac-Me2Si(4-t-Bu-2-MeCp)2ZrCl2 and syndiospecific Ph2C(FluCp)ZrCl2 [96]. Many novel polymers can be produced by post-modification reactivity of metallocene-generated polymers. For example, vinyl-terminated PE could be incorporated onto a polypropylene backbone by a metallocene catalyst [97]. However, due to poisoning effects, polar groups should be avoided in the process of olefin copolymerizations, but protecting routes such as polar grafted PP can be employed in their incorporation [98]. Furthermore, 1,4-divinylbenzene comonomer is often used in propylene polymerization to generate styryl-capped chains that can be further functionalized at the chain ends [99]. Chain ends by chain transferring to

alkyl aluminum can also be functionalized by a variety of reactions [100].

their brilliant toughness, the optical properties, and filler compatibility.

Like the single-site metallocene, the post-metallocene catalysts also possess the ability to tune the structure and properties of the prepared PP with precision, through tailoring the substitution patterns of the ligand [101–103]. Syndiotactic polypropylene (sPP) was produced by a phenoxy-imine ligand system discovered by scientists at Mitsui and simultaneously at Cornell University in New York. Subsequently, a series of researches and development focusing on post-metallocenes resulted in highly isotactic PP with high activity (**Figure 10**) [104–106]. In 2004, based on the Symyx Technologies (Symyx) using high throughput technology by Dow, VERSIFY™ Plastomers and Elastomers which are based on propylene/ethylene copolymers were developed with a pyridyl amine catalyst system (**Figure 13**, the 2nd complex). The produced VERSIFY™ Plastomers and Elastomers possess narrow molecular weight distribution, a broad chemical composition distribution compared with polymers from other single-site catalysts, and unique regio defects, which are very critical to their properties. These novel materials are the first PP-based polyolefins made in Dow's solution process, and they have excellent optical properties with high clarity and great processing performance compared to their ethylene-based counterparts. Although high temperature performance, elastic recovery, and anti-scratching and mar resistance performance are inadequate in many applications in high value fields due to their high ethylene-propylene rubber phase, they have been expanded to many new applications as new materials owing to

**3.3 Post-metallocene catalyst**

*DOI: http://dx.doi.org/10.5772/intechopen.85963*

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made… DOI: http://dx.doi.org/10.5772/intechopen.85963*

By combining different stereospecific metallocenes, thermoplastic PP elastomers could also be prepared. In 1996, Chien employed a mixture of two different metallocenes, which consists of isospecific C2v-symmetric metallocenes such as rac-ethylenebis(1-indenyl) zirconium dichloride or rac-dimethyl silylbis(1-indenyl) zirconium dichloride and nonstereospecific C2-symmetric metallocene such as ethylenebis(9-fluorenyl)zirconium dichloride, to produce PP-based elastomers by propylene homogeneous polymerization [93–95]. The combined dual-site metallocene catalysts generate a reactor blend comprising isotactic PP, atactic PP, and stereoblock PP containing alternating isotactic and atactic blocks. In the produced blends, the stereoblock PP contains immiscible isotactic and atactic PP blocks due to the chain transfer between stereospecific and nonstereospecific sites. Different from the early multisite catalysts for generating PP-based elastomer, this dual-site catalyst system, which combines two single-site metallocene catalysts, allows flexible tailoring of the elastomer properties by simply changing the molar ratio of added C2/C2v metallocenes. Surprisingly, Fink successfully synthesized iPP-b-sPP stereoblock PP using silica-supported MAO/dual-site metallocene catalysts that combined isospecific rac-Me2Si[Ind]2ZrCl2 with syndiospecific iPr(FluCp)ZrCl2. In another example of producing iPP-b-sPP stereoblock PP, Rytter and coworkers used AlMe3 together with the MAO as an activator to the homogeneous dual-site catalyst system consisting of isospecific rac-Me2Si(4-t-Bu-2-MeCp)2ZrCl2 and syndiospecific Ph2C(FluCp)ZrCl2 [96].

Many novel polymers can be produced by post-modification reactivity of metallocene-generated polymers. For example, vinyl-terminated PE could be incorporated onto a polypropylene backbone by a metallocene catalyst [97]. However, due to poisoning effects, polar groups should be avoided in the process of olefin copolymerizations, but protecting routes such as polar grafted PP can be employed in their incorporation [98]. Furthermore, 1,4-divinylbenzene comonomer is often used in propylene polymerization to generate styryl-capped chains that can be further functionalized at the chain ends [99]. Chain ends by chain transferring to alkyl aluminum can also be functionalized by a variety of reactions [100].

#### **3.3 Post-metallocene catalyst**

Like the single-site metallocene, the post-metallocene catalysts also possess the ability to tune the structure and properties of the prepared PP with precision, through tailoring the substitution patterns of the ligand [101–103]. Syndiotactic polypropylene (sPP) was produced by a phenoxy-imine ligand system discovered by scientists at Mitsui and simultaneously at Cornell University in New York. Subsequently, a series of researches and development focusing on post-metallocenes resulted in highly isotactic PP with high activity (**Figure 10**) [104–106].

In 2004, based on the Symyx Technologies (Symyx) using high throughput technology by Dow, VERSIFY™ Plastomers and Elastomers which are based on propylene/ethylene copolymers were developed with a pyridyl amine catalyst system (**Figure 13**, the 2nd complex). The produced VERSIFY™ Plastomers and Elastomers possess narrow molecular weight distribution, a broad chemical composition distribution compared with polymers from other single-site catalysts, and unique regio defects, which are very critical to their properties. These novel materials are the first PP-based polyolefins made in Dow's solution process, and they have excellent optical properties with high clarity and great processing performance compared to their ethylene-based counterparts. Although high temperature performance, elastic recovery, and anti-scratching and mar resistance performance are inadequate in many applications in high value fields due to their high ethylene-propylene rubber phase, they have been expanded to many new applications as new materials owing to their brilliant toughness, the optical properties, and filler compatibility.

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

as bis(2-aryl-indenyl) zirconocenes and hafnocenes with diverse aryl substitutions [90–92]. Moreover, the types of solvents and counterions can unfavorably limit ligand rotation; therefore, PP with complex microstructures might be produced.

*Molecular switching between the two site configurations during polymerization of the unbridged metallocene* 

*Correlation between metallocene structures and polypropylene architectures.*

**20**

**Figure 8.**

**Figure 9.**

*catalyst.*

**Figure 10.** *Post-metallocenes for isotactic PP.*

#### **Figure 11.**

*Polypropylene with different stereoisomers: isotactic polypropylene with isolated stereodefects (a), isotactic stereoblock polypropylene (b), gradient (c), and stereoblock polypropylenes (d), which contain hard isotactic and flexible atactic segments [3].*

**23**

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made…*

Inspired by the chain shuttling technology, Dow then using the pyridyl amine catalyst system combined with alkylaluminum as chain shuttling agent provided an alternative way to make stereo-block polypropylene with narrow MWD and high molecular weight [108–110]. As illustrated in **Figure 11**, single-site racemic catalysts are used to prepare stereoblock PP, and the catalyst system actually is a dual-site catalyst containing a 50/50 mixture of the two enantiopure sites, yielding isotactic stereoblock PP. In the solution polymerization, polar solvents such as 1,2-difluorobenzene are preferred, in order to improve alkylaluminum-mediated polymeryl shuttling between the two sites, giving rise to alternating isotactic blocks with either (R) or (S) configuration [111, 112]. There are also other combined pyridyl amine catalysts with CGC or metallocene catalysts employed in the chain shuttling

Ketimide and amidinate complexes can also be used for ethylene propylene diene monomer (EPDM) in a high temperature solution polymerization process, and the highest activities in ethylene polymerization were achieved with bis(tert-butyl)ketimide (N═C-t-Bu2)-ligated titanium complexes [19]. This type of ketimide catalysts is licensed from Nova Chemicals by DSM Elastomers for the production of a new EPDM rubber product, with the trademark of Keltan ACE (referred to as "advanced catalysis elastomers"). The catalyst structure was further modified to amidinate complex 9 with the general structure [(C5R5)Ti─{N═C(Ar)NR'2}X2] (X = Me or Cl) [113–116].

The production of PP and PP-based polymers in commercial scale is highly related to catalyst and process technology. Typically the processes could be divided into four

For homopolymer and random copolymers, bulk, slurry, and gas-phase processes can be employed. By connecting an additional gas-phase reactor to the polymerization equipment, in which the EPR is generated by ethylene-propylene copolymerization, impact PP can be produced. For the production of a polypropylene-based elastomer and most of the commercial EPDM rubbers, a solution process is required with a homogeneous catalyst system. In a slurry, bulk, or a gas-phase reactor, the polymer is generated around the heterogeneous catalyst particles. The slurry process and bulk process typically employ autoclaves or loop reactors. Gas-phase reactors generally adopt the form of fluidized-bed or stirred-bed. In fluidized-bed reactors, a gaseous stream of nitrogen and monomer is responsible for fluidizing the polymer particles and transferring the reaction heat, while in a stirred-bed reactor, mechanical stirring is employed to distribute the polymer particles and transfer heat. In a specific gas-phase stirred-bed reactor, horizontal or vertical layout can be taken.

The polymerization can be conducted in a single reactor or multiple reactors. Single reactors are typically employed to produce uniform composition, while multiple reactors in series can be adopted for PP with more complex microstructure and composition distribution expanding the properties of homopolymers and copolymers. Impact PP copolymers are typical examples produced in multiple reactors. The target product can be achieved in the two different steps diversifying the polymer microstructures. Isotactic polypropylene particles are formed in the first reactor by propylene homopolymerization, while a stream of mixture of propylene/ethylene is fed to the second reactor to make propylene-ethylene rubber phase copolymers, which is dispersed within the same catalyst and homopolymer particles. A schematic illustration of impact PP production is shown in **Figure 13**.

categories: gas-phase, bulk, slurry, and solution polymerization technologies.

Different reactor configurations are illustrated in **Figure 12**.

*DOI: http://dx.doi.org/10.5772/intechopen.85963*

**4. PP polymerization process**

polymerization, to produce stereo-block polypropylene [3].

*Versatile Propylene-Based Polyolefins with Tunable Molecular Structure through Tailor-Made… DOI: http://dx.doi.org/10.5772/intechopen.85963*

Inspired by the chain shuttling technology, Dow then using the pyridyl amine catalyst system combined with alkylaluminum as chain shuttling agent provided an alternative way to make stereo-block polypropylene with narrow MWD and high molecular weight [108–110]. As illustrated in **Figure 11**, single-site racemic catalysts are used to prepare stereoblock PP, and the catalyst system actually is a dual-site catalyst containing a 50/50 mixture of the two enantiopure sites, yielding isotactic stereoblock PP. In the solution polymerization, polar solvents such as 1,2-difluorobenzene are preferred, in order to improve alkylaluminum-mediated polymeryl shuttling between the two sites, giving rise to alternating isotactic blocks with either (R) or (S) configuration [111, 112]. There are also other combined pyridyl amine catalysts with CGC or metallocene catalysts employed in the chain shuttling polymerization, to produce stereo-block polypropylene [3].

Ketimide and amidinate complexes can also be used for ethylene propylene diene monomer (EPDM) in a high temperature solution polymerization process, and the highest activities in ethylene polymerization were achieved with bis(tert-butyl)ketimide (N═C-t-Bu2)-ligated titanium complexes [19]. This type of ketimide catalysts is licensed from Nova Chemicals by DSM Elastomers for the production of a new EPDM rubber product, with the trademark of Keltan ACE (referred to as "advanced catalysis elastomers"). The catalyst structure was further modified to amidinate complex 9 with the general structure [(C5R5)Ti─{N═C(Ar)NR'2}X2] (X = Me or Cl) [113–116].

## **4. PP polymerization process**

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

*Polypropylene with different stereoisomers: isotactic polypropylene with isolated stereodefects (a), isotactic stereoblock polypropylene (b), gradient (c), and stereoblock polypropylenes (d), which contain hard isotactic* 

**22**

**Figure 11.**

**Figure 10.**

*Post-metallocenes for isotactic PP.*

*and flexible atactic segments [3].*

The production of PP and PP-based polymers in commercial scale is highly related to catalyst and process technology. Typically the processes could be divided into four categories: gas-phase, bulk, slurry, and solution polymerization technologies.

For homopolymer and random copolymers, bulk, slurry, and gas-phase processes can be employed. By connecting an additional gas-phase reactor to the polymerization equipment, in which the EPR is generated by ethylene-propylene copolymerization, impact PP can be produced. For the production of a polypropylene-based elastomer and most of the commercial EPDM rubbers, a solution process is required with a homogeneous catalyst system. In a slurry, bulk, or a gas-phase reactor, the polymer is generated around the heterogeneous catalyst particles. The slurry process and bulk process typically employ autoclaves or loop reactors. Gas-phase reactors generally adopt the form of fluidized-bed or stirred-bed. In fluidized-bed reactors, a gaseous stream of nitrogen and monomer is responsible for fluidizing the polymer particles and transferring the reaction heat, while in a stirred-bed reactor, mechanical stirring is employed to distribute the polymer particles and transfer heat. In a specific gas-phase stirred-bed reactor, horizontal or vertical layout can be taken. Different reactor configurations are illustrated in **Figure 12**.

The polymerization can be conducted in a single reactor or multiple reactors. Single reactors are typically employed to produce uniform composition, while multiple reactors in series can be adopted for PP with more complex microstructure and composition distribution expanding the properties of homopolymers and copolymers. Impact PP copolymers are typical examples produced in multiple reactors. The target product can be achieved in the two different steps diversifying the polymer microstructures. Isotactic polypropylene particles are formed in the first reactor by propylene homopolymerization, while a stream of mixture of propylene/ethylene is fed to the second reactor to make propylene-ethylene rubber phase copolymers, which is dispersed within the same catalyst and homopolymer particles. A schematic illustration of impact PP production is shown in **Figure 13**.
