Activators Addition polymerisation

Addition polymerisation is effected by the activation of the double bond of a vinyl monomer, thus enabling it to link up to other molecules. It has been shown that this reaction occurs in the form of a chain addition process with initiation, propagation and termination steps. 

In addition, cyclodextrins incorporating a 2,6-bis(imino)pyridine unit have been used to support active iron polymerisation catalysts. Using the (i-cyclodextrin-based system 52, in the presence of a large excess of MAO, ethylene can be converted into HDPE (Fig. 16). Only low activities are, however, observed, which... 

The second common method of synthesising polymers (Fig. 5.3) is chain (addition) polymerisation. The most common type of addition polymer is based on ethene CH2 = CHj in which the monomer contains at least one double (tt) bond which on being activated, by free radical attack say, opens up to produce two single sigma bonds and the homopolymer poly(ethene). (Note in Fig. 5.3 the resultant polymer backbone is joined together by carbon-carbon bonds, unlike the condensation polymer systems (Fig. 5.1).)... 

Photoinitiated Addition Polymerisation Chromium (VI) ions undergo a photoredox process to give chromium (V) ions by a charge-transfer mechanism which involves the formation of active HCrO. ions that effectively initiate the photopolymerisation of acrylonitrile while 2,U,6-trimethylbenzoyldiphenylphosphine-... 

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

Fig. 22.1. (a) The ethylene molecule or monomer (b) the monomer in the activated state, ready to polymerise with others (<)-(f) the ethylene polymer ("polyethylene") the chain length is limited by the addition of terminators like —OH. The DP is the number of monomer units in the chain. 

Poly(vinyl alcohol) will function as a non-ionic surface active agent and is used in suspension polymerisation as a protective colloid. In many applications it serves as a binder and thickener is addition to an emulsifying agent. The polymer is also employed in adhesives, binders, paper sizing, paper coatings, textile sizing, ceramics, cosmetics and as a steel quenchant. 

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

Residues of fatty acids from emulsion polymerisation and from cure activation provide sites for bacterial attack when the rubber product is exposed to warm moist conditions. The addition of a biocide/fimgicide will give excellent fungal growth protection. 

Plesch, Polanyi, and Skinner [28] found that HC1, S02, C02, EtOH, and EtzO were not cocatalysts, and the last two substances were shown to be inhibitors in that the addition of moist air to a solution containing them did not induce polymerisation. The search for co-catalysts other than water led to the discovery that trichloroacetic acid, sulphuric acid, and 20 percent oleum would act as a co-catalyst to titanium tetrachloride in hexane at about -75°, though none of these acids alone showed any catalytic activity under these conditions [9, 71]. 

Since in many of the polymerisations with which we are concerned here the active species are carbenium ions, it is relevant to discuss briefly their structures. Carbenium ions can be formed from olefins by the addition of a cation, such as a proton or some other carbenium ion. 

Water present in the monomer solution before the addition of perchloric acid did not affect the rate of polymerisation (Experiment SGP6, Table 1), but if water was added to the catalyst solution before the polymerisation, with consequent formation of HC104, H20, which is insoluble in methylene dichloride, only the anhydrous acid was found to be an active catalyst (Experiment SGP7, Table 1). 

The observation that traces of water do not influence the rate of polymerisation if the water is present in the reaction medium before the acid is added (Experiment SGP6, Table 2), indicated that (a) The reaction leading to the formation of an ester is much faster than the addition of water to HC104 (b) the ester is fairly insensitive to quantities of water up to about 10 times its concentration [3], i.e., hydrolysis under these conditions is negligible. On the other hand, if H30+C104 is already present when the polymerisation is started, this is found to have no catalytic activity, most probably because it is insoluble in methylene dichloride (Experiment SGP7, Table 2). The destructive effect of water upon the carbonium ions formed at the end of the polymerisations will be discussed in a future paper. 

It now remains to place the concept of an ester as an active species into a wider chemical context, with special reference to polymerisation catalysts. Sinn and Patat [39] have emphasised the distinction between monofunctional and bifunctional catalytic systems and this distinction is obviously and necessarily related to the idea, explained above, that there is a difference in kind between polarised molecules and the ions which can be formed from them. Whereas the carbonium and other cations as reactive species are monofunctional, the esters evidently belong to the class of bifunctional catalysts their mode of action - the addition of their constituent parts across a double bond - is, in modern terminology, an insertion reaction. In this context, we must note the important... 

The well-known addition of tert-butyl chloride to ethylene by means of A1C13 without skeletal rearrangement is simply another example of the initiation step of a /-cat polymerisation, involving the insertion of the alkene into an ester, namely the C-Cl bond, which is activated by the A1C13, via a six-centred transition state (I), as shown in equation (ii) ... 

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