Propagation copolymerisation

Rea.CtlVltyRa.tlO Scheme. The composition of a copolymer at any point in time depends on the relative rates that each monomer can add to a chain end. If it is assumed that the chemical reactivity of a propagating chain depends only on the terminal unit and is not affected by any penultimate units, then four possible propagation steps in the copolymerisation of two monomers, and M2, with two growing chain ends, M and M2, can be written as follows ... 

The ratio describes the relative reactivity of polymer chain M toward monomer M and monomer M2. Likewise, describes the relative reactivity of polymer chain M2 toward M2 and M. With a steady-state assumption, the copolymerisation equation can be derived from the propagation steps in equations 3—6. 

The kinetics of copolymerisation are rather complex since four propagation reactions can take place if two monomers are present... 

Polyacrylonitrile is an excellent textile fibre but is difficult to dye. However, by its copolymerisation or by grafting on a second polymer, it is possible to maintain the desirable properties of the fibre, yet produce a textile which can be processed in the usual way. Among the various factors that govern the copolymerisation process, the concentration and reactivity of the monomer are quite important. At any given time, the chain may grow in four different ways as under. Here A and B are the radicals which are involved in propagating steps, whereas A and B are the respective monomers. 

Copolymerisation is the process in which a mixture of two or more monomers gets polymerised to yield a product. The product obtained is known as a copolymer. A copolymer product contains some units of each type of monomer and is different from a physical mixture of individual polymer molecules formed by different monomers. It is not always possible to make a copolymer with any two or more monomers. When two monomers A and B are copolymerised the rate of polymerisation is determined by concentration of monomers. Four different propagation reaction can occur for copolymerisation of A and B. AA, AB, BB, BA". 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

The discovery that the keto chelates (especially the p ones) are the species controlling the strict alternation of the monomers and the intrinsic copolymerisation rate in ethene/CO propagation has stimulated much research aimed at designing catalytic systems where the keto chelates can be readily opened by CO. These studies have allowed the development of a new generation of more efficient Pd" copolymerisation catalysts based on diphosphines with o-methoxyphenyl groups on the phosphorus atoms [13b, 31-34]. 

The chain transfer by protonolysis represents the predominant termination step in homogeneous ethene/CO copolymerisation, and involves the reaction between a propagating Pd-alkyl species and MeOH or adventitious water (Scheme 7.15a). As a result, the propagation is terminated with formation of a polymeric chain with a ketone-end group and Pd-OMe (or Pd-OH) species, which can re-enter the catalytic cycle by CO insertion. 

The general mechanistic features of the ethene/CO copolymerisation cycle (Scheme 7.2) are substantially valid also for styrene. In particular, the propagation steps are similar for both alkenes and consist of subsequent alternated migratory insertions of alkyl to CO and of acyl to olefin, with P-chelate and y-chelate resting states. The structures of the first intermediates in the syndiotactic copolymerisation of styrene derivatives with CO have been determined by an in situ NMR study using [(Pr DAB)Pd(Me)(NCMe)]BAr4 as precursor (Scheme 7.21) [38]. 

The mechanism of propene/CO copolymerisation by palladium catalysis is essentially analogous to those of ethene and styrene (i. e., chain propagation proceeds via alternating insertions of CO into Pd-alkyl and alkene into Pd-acyl controlled by p-chelates) [1]. 

Heterocyclic monomers containing both endocyclic and exocyclic heteroatoms such as cyclic esters (lactones, lactide, carbonates) and cyclic anhydrides undergo coordination polymerisation or copolymerisation involving complex formation between the metal atom and the exocyclic heteroatom [100,124]. Polymerisation of /1-lactones is representative of such coordination polymerisations with catalysts containing an Mt-X active bond the initiation and propagation steps are as follows ... 

Heterounsaturated monomers such as aldehydes or carbon dioxide polymerise and/or copolymerise with the participation of at least two metal atoms in multicentred transition states. Scheme (9) shows the initiation and propagation steps in the coordination polymerisation of carbonyl monomers with catalysts containing an Mt-X active bond [125] ... 

Furthermore, studies of the microstructure of copolymers formed by the low-temperature copolymerisation of cis-1 -(2 H)-propene (or trans isomer) and perdeuteropropene in the presence of soluble vanadium-based Ziegler-Natta catalysts showed syndiospecific propagation to involve a monomer insertion of the cis type [27]. 

In the copolymerisation, each monomer competes for the available catalytic active species, and the composition, structure and molecular weight of the copolymer produced reflect this competition. The mechanistic features of the copolymerisation are in principle similar to those of the homopolymerisation. In the copolymerisation, however, the effect of the kind of last inserted monomeric unit and the kind of comonomer undergoing insertion (Mi or M2) should be taken into consideration the propagation step can proceed in at least four ways [448] ... 

As in the case of homopolymerisation, no problem of isomerisation of propagating species exists in the copolymerisation of norbornene with carbon monoxide and hence 2,3-bicyclo[2.2.1]hept-2-ene units, as the only norbornene monomeric units appear in alternating copolymers with carbon monoxide [27] ... 

Copolymerisation, which is often carried out in order to gain a better insight into the nature of polymerisation initiating and/or propagating species and to modify the properties of the polymeric products formed, has also been satisfactorily carried out in the case of cycloolefins. The ring-opening metathesis... 

It seems that the initiation step of the copolymerisation most likely involves the oxirane reaction [according to scheme (3)]. Zinc alcoholate species formed in this reaction can easily propagate the copolymer chain, coordinating and enchaining both the oxirane [scheme (3)] and the cyclic carbonate [scheme (15)] comonomers. However, in the case of the cyclic carbonate, its enchainment may also proceed according to scheme (14), leading to decarboxylation. Thus, the obtained poly(ether-carbonate)s are characterised by a lower content of carbonate units with respect to the ether units [82,146]. 

Attempts to use the diethylzinc-pyrogallol (2 1) catalyst to copolymerise propylene sulphide and carbon dioxide failed, since the content of propylene thiocarbonate units in the copolymers formed was small and did not exceed 10 mol.-%. It has also been observed that the presence of carbon dioxide in this copolymerisation system causes a lowering of the molecular weight and yield of the copolymer formed. Thus, it has been suggested that propylene sulphide homopolymerisation was favoured over cross-propagation with carbon dioxide in the presence of a zinc-based coordination catalyst because of higher HSAB symmetry of the system in the former case. The zinc atom in the Zn-S unit of the catalyst is a rather soft acid and will prefer reaction with a soft base such as propylene sulphide rather than with hard carbon dioxide. The presence of a hard acid centre in the triethylaluminium-based catalyst should result in an increase in the affinity of the catalyst towards carbon dioxide [247],... 

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