**1. Introduction**

The consumption of polyolefins has been remaining growing with continuous catalyst technology innovation since the discovery of Ziegler-Natta catalysts in the 1950s [1, 2]. Numerous technologies are adopted to improve the performance of regular homopolypropylene (HPP), such as toughness, tensile strength, and transparency, and a series of PP-based polyolefins including isotacticity polypropylene (iPP), random copolypropylene (RPP), impact PP, PP-based block copolymer, functionalized PP, etc. are successfully commercialized by tailor-made catalysts and polymerization process [3–8]. Now these PP-based polyolefins are used in a

wide range of industries such as packaging, electrical and electronics, construction, automobile, medical, equipment, and facilities industries [9, 10].

According to the statistics from IHS Chemical (2018), the total production of polypropylene worldwide was about 56 million tons in 2016, and by 2022, about 75 million tons is predicted. The biggest increases have been taking place in Asia in recent years due to their dramatic expansion of economic share with a huge supply of cheap raw materials from Gulf Coast countries. China now possesses the largest market share in PP production of above 22 million tons.

Along with this massive production was the successful development of the catalyst technology and the polymerization process innovation [11]. The Z-N catalysts were at first immobilized in a carrier such as TiCl3 [9]. The two most significant improvements were the evolutionary use of MgCl2 as a reactive catalyst support, which can dramatically improve the catalyst performance with excellent shape control, and the discovery of electron donors (internal electron donors and external electron donors) in the catalyst system, which can improve the catalyst activity and control the stereoregularity, and led to the dramatic growth of iPP production.

Different from the multisite Z-N catalysts, single site metallocenes have not brought much attention to the polyolefin industry until the discovery of methylaluminoxane (MAO) by Sinn and Kaminsky due to the dramatic increase of polymerization activity as a cocatalyst. Typically, single-site metallocene catalysts make it possible to fine-tune the microstructure of the produced polymer chain by ligand design in the catalyst complexes, with excellent α-olefin incorporation ability. In 1990s, two successful samples of commercialization of metallocene catalysts were realized by ExxonMobil and Dow. In 1991, ExxonMobil was the first company to put metallocene catalysts into commercial use with the new Exxpol® Technology. Then in 1992, Dow launched the constrained geometry catalysts (CGC) with linked half-titanocenes containing amide ligands, which are still called metallocene catalysts for convenience, to produce metallocene-based polyolefins with INSITE™ technology [12].

Compared to the Exxpol® Technology developed by ExxonMobil, which is based on heterogeneous catalysts and gas-phase process, the INSITE® technology from Dow is based on the constrained geometry catalyst (CGC) in a solution process. The homogeneous catalyst system has the ability to control polymer microstructure with flexibility and simplicity from the homogeneous phase system. And also the relationship between the catalyst structure and the physical properties is easy to be characterized and modeled.

Polyolefin production from metallocene-based catalysts and a solution process were rapidly adopted for many applications; however, some drawbacks such as poor compression set and poor scratch resistance limited their applications. In 2004, a new post-metallocene catalyst with the pyridyl amine system was commercialized in the solution process by Dow to produce a family of propylene/ethylene copolymers called VERSIFY™ Plastomers and Elastomers [13, 14]. This pyridyl aminebased catalyst was developed through high throughput screening technology, and was suitable for production of propylene-based copolymers with high molecular weight over a wide range of chemical composition distributions.

In 2006, Dow announced olefin block copolymers which were produced by chain shuttling polymerization technology in a solution process. As shown in **Figure 1**, this technology employs two catalysts and a chain shuttling agent, and the two catalysts have totally different incorporation ability of α-olefin, thus producing different chain block-soft and hard PE segments-by chain shuttling agent (diethyl zinc), and the produced chains are composed of at least two alternating soft and hard segments [15]. The chain shuttling polymerization is illustrated in **Figure 3**. By combining the pyridyl-amine catalyst and CGC catalyst and alkylaluminum as the chain

**11**

**Figure 2.**

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

shuttling agent, the chain shuttling catalyst technology was also used to produce propylene-based stereoblock copolymer with a high molecular weight, at least some of which differ in irregular branching content, especially regio-irregular 2,1- and/or 3,1-monomer insertions [17], (**Figure 2**). Thus, the block copolymers obtain desirable properties due to the presence of alternating "soft" and "hard" blocks in the

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

*Schematic illustration of chain shuttling polymerization [3].*

*Regio-irregular 2,1- and/or 3,1-monomer insertions [11].*

same polymer chain [11].

**Figure 1.**

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

shuttling agent, the chain shuttling catalyst technology was also used to produce propylene-based stereoblock copolymer with a high molecular weight, at least some of which differ in irregular branching content, especially regio-irregular 2,1- and/or 3,1-monomer insertions [17], (**Figure 2**). Thus, the block copolymers obtain desirable properties due to the presence of alternating "soft" and "hard" blocks in the same polymer chain [11].

**Figure 1.** *Schematic illustration of chain shuttling polymerization [3].*

**Figure 2.** *Regio-irregular 2,1- and/or 3,1-monomer insertions [11].*

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

automobile, medical, equipment, and facilities industries [9, 10].

market share in PP production of above 22 million tons.

wide range of industries such as packaging, electrical and electronics, construction,

According to the statistics from IHS Chemical (2018), the total production of polypropylene worldwide was about 56 million tons in 2016, and by 2022, about 75 million tons is predicted. The biggest increases have been taking place in Asia in recent years due to their dramatic expansion of economic share with a huge supply of cheap raw materials from Gulf Coast countries. China now possesses the largest

Along with this massive production was the successful development of the catalyst technology and the polymerization process innovation [11]. The Z-N catalysts were at first immobilized in a carrier such as TiCl3 [9]. The two most significant improvements were the evolutionary use of MgCl2 as a reactive catalyst support, which can dramatically improve the catalyst performance with excellent shape control, and the discovery of electron donors (internal electron donors and external electron donors) in the catalyst system, which can improve the catalyst activity and control the stereoregularity, and led to the dramatic growth of iPP production. Different from the multisite Z-N catalysts, single site metallocenes have not brought much attention to the polyolefin industry until the discovery of methylaluminoxane (MAO) by Sinn and Kaminsky due to the dramatic increase of polymerization activity as a cocatalyst. Typically, single-site metallocene catalysts make it possible to fine-tune the microstructure of the produced polymer chain by ligand design in the catalyst complexes, with excellent α-olefin incorporation ability. In 1990s, two successful samples of commercialization of metallocene catalysts were realized by ExxonMobil and Dow. In 1991, ExxonMobil was the first company to put metallocene catalysts into commercial use with the new Exxpol® Technology. Then in 1992, Dow launched the constrained geometry catalysts (CGC) with linked half-titanocenes containing amide ligands, which are still called metallocene catalysts for convenience, to produce metallocene-based polyolefins with INSITE™

Compared to the Exxpol® Technology developed by ExxonMobil, which is based on heterogeneous catalysts and gas-phase process, the INSITE® technology from Dow is based on the constrained geometry catalyst (CGC) in a solution process. The homogeneous catalyst system has the ability to control polymer microstructure with flexibility and simplicity from the homogeneous phase system. And also the relationship between the catalyst structure and the physical properties is easy to be

Polyolefin production from metallocene-based catalysts and a solution process were rapidly adopted for many applications; however, some drawbacks such as poor compression set and poor scratch resistance limited their applications. In 2004, a new post-metallocene catalyst with the pyridyl amine system was commercialized in the solution process by Dow to produce a family of propylene/ethylene copolymers called VERSIFY™ Plastomers and Elastomers [13, 14]. This pyridyl aminebased catalyst was developed through high throughput screening technology, and was suitable for production of propylene-based copolymers with high molecular

In 2006, Dow announced olefin block copolymers which were produced by chain shuttling polymerization technology in a solution process. As shown in **Figure 1**, this technology employs two catalysts and a chain shuttling agent, and the two catalysts have totally different incorporation ability of α-olefin, thus producing different chain block-soft and hard PE segments-by chain shuttling agent (diethyl zinc), and the produced chains are composed of at least two alternating soft and hard segments [15]. The chain shuttling polymerization is illustrated in **Figure 3**. By combining the pyridyl-amine catalyst and CGC catalyst and alkylaluminum as the chain

weight over a wide range of chemical composition distributions.

**10**

technology [12].

characterized and modeled.

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 catalysts or post-metallocene catalysts [19].

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 are directly used in a solution polymerization process.

## **2. General structure and properties**

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 due to the tertiary C atom.

Stereoregularity of the methyl group branch separates crystallizable subspecies from amorphous subspecies; the melting point and modulus strength of sPP is lower than iPP, and among them, aPP has the lowest melting point.

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

**13**

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

comonomer used is typically ethylene, and in the random copolymers, the ethylene content is usually less than 7% [20]. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity and makes the polymer more transparent. The impact copolymers of propylene-ethylene are virtually a blend of block EP rubber, HPP, and random copolymer, based on the granule reactor technology, so the impact copolymers can obtain great impact strength due to EP rubber phase spread in the PP matrix. A third comonomer can also be used as in ethylene-propylene rubber or EPDM increasing its low

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

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

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 inconve-

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

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

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

temperature impact strength.

systems is presented in **Table 1**.

weight distribution, and so on [21].

**3.1 Z-N catalysts**

niently too high.

**Figure 3.**

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

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

comonomer used is typically ethylene, and in the random copolymers, the ethylene content is usually less than 7% [20]. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity and makes the polymer more transparent. The impact copolymers of propylene-ethylene are virtually a blend of block EP rubber, HPP, and random copolymer, based on the granule reactor technology, so the impact copolymers can obtain great impact strength due to EP rubber phase spread in the PP matrix. A third comonomer can also be used as in ethylene-propylene rubber or EPDM increasing its low temperature impact strength.
