Phosphinimide ligands

Structure and Bonding

R3PN- (4,1)

(5)

Properties and Reactivity

Synthesis

(6,1)

Applications

Polyethylene Catalysis

History

Phosphinimide ligands have shown recent promise in the area of ethylene polymerization. Since 1955 this field has been dominated by metallocene based catalysts starting with the Ziegler catalyst, followed shortly by the kaminsky catalyst in 1976(10,11). The initial design bases for phosphinimide ligands was suggested due to the fact they have similar steric and electronic properties to metallocene catalysts(8).

Phosphinimide Ligands Compared to Cylclopentadienyl Ligands in Ethylene Polymerization

In most respects the sterics of phosphinimide ligands and electronic properties are considered analogous to cyclopentadienyl ligands(7,8,9). Stericly the metal bound phosphinimide ligand t-Bu3PN produces a cone angles of 87°, quite similar to 83° for cylclopentadienyl suggesting they occupy similar area(8). However, thi does not take into account that the bulkyness of the phosphinimide ligand is partly removed from the metal, by the distance of the nitrogen metal bond. This removal of bulkyness can be considered a beneficial to catalysis as one of the strategies to increase polymerization is to increase the exposure of the metal centre(8). The consequence to a less stericly crowded metal centre appears to be deactivation of the catalysis by lewis acids(7,8).

Diagram of cyclopent. Lign and phophino ligand

The phosphinimide ligands provide an alternative to metallocene catalysts due to their capability to be easily altered during synthesis, allowing for immense variation in the sterics and electronics properties of the system. Therefore, increased specificity of the catalytic properties may be obtained. Also mechanistic and catalytic studies of PE synthesis may be obtained easier through substituent variation and 31P nmr analysis.

Table of catalysis.-9 pg 5

General Catalysis Scheme

Activation

The catalyst must be activated in order for polymerization to occur. MAO is a common cocatalyst (activator) for PE polymerization. The discrete structure of MAO is unknown as is the mechanism of this is reaction however, a general activation scheme has been purposed(10,11,12).

Scheme1 scheme 2

Another scheme uses a strong organo-lewis acids such as B(C6F5)3 as a cocatalyst to activate the polymerization through methyl abstraction(9,11) producing a highly reactive zwitterion form(9). Different methods of activation have shown to produce different ethylene polymerization activity(8,9) making it a very important concept to analyse. In general it is observed that phosphinimide catalysts show less activity when activated with MAO(9).

Mechanism of Polymerization

The catalyst is thought to be a homogenous, single sited and therefore produces pdi comparable to metallocene catalysts(8,9) which are also believed to be homogenous, single sited catalysts. The catalytic process is assumed to proceed in much of the same way as metallocene based catalysts as ethylene is thought to interact primarily with the metal centre and not through the bulky ligands(7,12).

Mechanism

Deactivation

Catalytic degredation most commonly occurs from cationic dimerization, lewis acid interactions (often with AlMe3 cocatalyst (MAO)) activating a C-H bond or by excess boron (from another cocatalysts B(C6F5)3) reacting with the active zwitterion(8,9). Catalytic degradation has been shown to occur slower with bulkier phosphinimide ligands(8,9) which prevent such interactions.

-Activation 11 -olefin polymerization (1,3,7,8 -industrial 9 -general catalysis(2,10)