Formation 4 Copolymerisation

Recently Sergeev et al. 90 91> have developed a low temperature condensation method for the formation of inclusion compounds of thiourea with reactive and volatile guests, avoiding the use of solvents. The two guests in the joint inclusion compound of thiourea with 1,3-cyclopentadiene and maleic anhydride underwent Diels-Alder addition at 170 K. These two substances do not react at this low temperature unless they are present in the thiourea complex the usual endo isomer of the product is formed. Apart from copolymerisation reactions this appears to be the first use of the thiourea canal to study reactions between different materials. 

A further development [27] is the formation of so-called sugar-acrylate copolymers in which acrylic acid is copolymerised with glucose or other saccharides. Unlike other sequestering agents these polymers are said to be readily biodegradable, this being the main reason for their development. 

Copolymerisation is also possible (Fig. 4). Dimethyldisilane reacts with diphenylsilane with formation of a copolymer with the composition H[(MeSiHx)(PhSiHy)]nH. This copolymer is a viscous liquid and is spinnable. By heating to 180° C the polymerization continues and a solid results [23]. The presence of branched structures, which were not found with the polymerization of monosilanes, the very rapid polymerization rate achievable, and the observable SiSi cleavage points to another mechanism, as was postulated for monosilanes. 

In the second stage of the reaction, the free radical produced on the backbone of the base polymer initiates polymerisation which results in the formation of graft copolymerisation as under ... 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

However, this is not always the case. Excess of K - K has been found to occur during the initial stage of the copolymerisation when the cooligomer chain bound to the metal is still soluble and catalysis occurs in the homogeneous phase [36]. This may also occur when protonolysis involves H20 in place of MeOH, with formation of a Pd - OH+ species, which regenerates Pd - H+ by insertion of CO to Pd - COOH+ followed by C02 evolution. Thus in each catalytic cycle one molecule of CO is not incorporated into the polymer chain, but is consumed as C02 ... 

Excess of polymer E-E has also been found and in some cases only E-E forms, for instance during the initial stage of catalysis by Pd(dapp)2+ in the presence of an oxidant, usually benzoquinone or naphtoquinone (BQ, NQ) [37]. The oxidant favours the formation of Pd - OCH3+ at the expense of Pd - H+ [15] and in the copolymerisation process one molecule of oxidant is... 

Efficient copolymerisation can also be achieved in solvents other than the alcohols (Table VI). Thus the order of effectiveness for the present copolymerisation of these additional solvents is DMSO>DMF>dioxan>acetone>>chloroform>hexane. Acid enhancement is also observed in the first of these four solvents (Table VI). Characteristically (5), acid increases the intensity of the Tromms-dorff peak if it is already present in the system (dioxan) or alternatively induces the formation of the gel peak if it is not present in the solutions prior to acid addition (DMSO). 

Another way to recover the catalyst from the dormant site is the copolymerisation of ethene, but this is slower and less attractive than the use of hydrogen. Furthermore the use of ethylene inevitably results in the formation of propylene-ethylene copolymers with all the consequent effects on polymer properties. 

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

Formation of Active Pd" Sites and Initiation of Ethene/CO Copolymerisation... 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

Tertiary alcohols are rather inefficient in alkene/CO copolymerisation as the formation of alkoxy palladium complexes is less favored for them than for primary and secondary alcohols, while the lack of p hydrogens does not allow the formation of Pd-H. Some primary alcohols activated by electron-withdrawing substituents, e. g., CF3CH2OH, are equally unable to form Pd-H due to their low propensity to oxidation [6c]. 

Only with less efficient catalysts and at low temperature, have p-chelate intermediates been intercepted by P H HP NMR spectroscopy in the course of copolymerisations in MeOH-d4 [5g]. The unambiguous detection of p-chelates has been observed in a reaction catalysed by the l,r-bis(diphenylphosphino)ferro-cene complex [Pd(H20)2(dppf)](0Ts)2 (3) at room temperature (Scheme 7.7) [5g]. As shown in the sequence of P H NMR spectra reported in Figure 7.8, the P-chelate intermediates 4- disappeared already at 50 °C. A parallel model study confirmed the formation and the structure of the dppf P-chelates and also provided information of more elusive intermediates (see Section 7.2.1.8) [19]. 

Notably, this HP NMR investigation showed the formation of a transient binuc-lear p-H-p-CO complex, [Pd2(p-H)(p-CO)(dppf)2]OTs (5), and of the termination product [Pd(p-OH)(dppf)]2(OTs)2 (6) (Chart 7.1). Based on the in situ study, these compounds could be isolated, characterised and used to catalyse copolymerisation reactions. Both complexes proved to be active in batch copolymerisation reactions. However, the productivities in polyketone were significantly lower than those... 

A cis-coordinating ligand is apparently required to bind and activate MeOH so that a methoxy group is transferred to the polyketone chain and a hydride remains on palladium. Two mechanisms are possible for this reaction (i) nucleophilic attack by the oxygen at the acyl carbonyl with concerted formation of Pd-H (ii) formation of a Pd(acyl) (methoxy) complex and H, followed by reductive elimination and subsequent proton attack on a Pd center. No experimental evidence favoring either mechanism in ethene/CO copolymerisation has been provided so far. 

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. 

In anhydrous organic solvents, ethene/CO copolymerisation termination occurs exclusively by P-H transfer to give vinyl terminated polyketone and Pd-H (Scheme 7.15c). On the other hand, traces of water are very difficult to eliminate and consequently chain transfer by protonolysis is often observed, together with p-H transfer. Experimental evidence in this sense has been straightforwardly obtained by an in situ NMR study of the chemical stability of the p-chelate [Pd(CH7CH7C(0)-Me)(dppe)]PF5 (7) in wet and anhydrous CD2CI2 [5ej. Figure 7.13 reports a sequence of P H NMR spectra taken after dissolution of the p-chelate in the wet solvent already the first spectrum at room temperature showed the formation of the p-hydroxo binuclear complex [Pd(OH)(dppe)]2(PF )2 (8), that was the only detectable species after 15 h. 

The formation of p-hydroxo diphosphine complexes by protonolysis of the (5-keto chelates with water (Figure 7.9) is another factor that contributes to a decrease in the copolymerisation activity (Scheme 7.28) [5f]. In general, the p-hydroxo complexes can re-enter the copolymerisation cycle by reaction with CO that breaks the binuclear structure to give Pd-H via Pd-C(0)0H [5a, 13, 36]. The contribution... 

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