Highly branched polyethene

Palladium catalysts with diimine ligands based on 2,6-diisopropyl aniline polymerize ethene to a rubbery, highly branched polyethene with low glass transition temperatures [12 a]. The interest in these materials results from their possible appheation as a rubber modifier in engineering plastics [3j,k]. However, the activity of the palladium catalysts is not satisfactory for technical use in a world scale plant We intended to improve the activity by increasing the Lewis acidity of the metal center by using relatively electron-deficient bromo phenyl diimine hgands. 

Exercise 29-5 High-pressure polyethene (Section 10-8C) differs from polyethene made with the aid of Ziegler catalysts (Section 10-8D) in having a lower density and lower Tm. It has been suggested that this is due to branches in the chains of the high-pressure material. Explain how such branches may arise in the polymerization process and how they would affect the density and Tm. 

A nickel-based catalyst system, which produces, in the absence of comonomers, highly short-chain branched polyethene was developed by Brookhart et al. [23]. Independently, the groups of Brookhart [24, 25, 26] and Gibson [27, 28, 29, 30] developed efficient iron- and cobalt-based catalyst systems. Nickel or palladium is typically sandwiched between two a-di-imine ligands, while iron and cobalt are tridentate complexed with imino and pyridyl ligands. 

Technical ethylene polymerization leads to three major classes of PE materials low-density polyethene (LDPE), linear low-density polyethene (LLDPE), and high-density polyethene (HOPE). The three classes of PE material differ in the degree and type of branching in the polymer. These differences lead to different physicochemical properties of the polymer, resulting in different application areas of the PE material. 

The first polymerizations were free radical reactions. In 1933 researchers at ICI discovered that ethene polymerizes into a branched structure that is now known as low density polyethene (LDPE). In the mid- 50s a series of patents were issued for new processes in which solid catalysts were used to produce polyethene at relatively low pressures. The first was granted to scientists at Standard Oil (Indiana) who applied nickel oxide on activated carbon and molybdenum oxide on alumina. Their research did not lead to commercial processes. In the late 40s Hogan and Banks of Phillips were assigned to study the di- and trimerization of lower olefins. The objective was to produce high octane motor fuels. When they tried a chromium salt as promoter of a certain catalyst (Cr was a known reforming... 

Radically created polyethene typically contains a total number of 10 to 50 branches per 1000 C atoms. Of these, 10% are ethyl, 50% are butyl, and 40% are longer side chains. With the simplified formulars (6) and (7), not all branches observed could be explained [33,34]. A high-pressure stainless steal autoclave (0.1 to 0.51 MPa) equipped with an inlet and outlet valve, temperature conductor, stirrer, and bursting disk is used for the synthesis. Best performance is obtained with an electrically heated autoclave [35-41]. 

On a laboratory scale, single site catalysts based on metallocene/MAO are highly useful for the copolymerization of ethene with other olefins. Propene, 1-butene, 1-pentene, 1-hexene, and 1-octene have been studied in their use as comonomers, forming linear low-density polyethene (LLDPE) [188,189]. These copolymers have a great industrial potential and show a higher growth rate than the homopolymer. Due to thee short branching from... 

---