Polymerisation vinyl

The polymerisation of vinyl monomers can be initiated by radicals formed by a-cleavage or hydrogen abstraction of a carbonyl compound. 

The chain-propagation sequence can be stopped by a termination step. In radical systems, radical combination and disproportionation are important mechanisms by which this occurs. 

It is possible to improve the mechanical strength and weather-resistance of polymers by inducing a certain degree of cross-linking. 

---

When heated to 120°C, AIBN decomposes to form nitrogen and two 2-cyanoisopropyl radicals. The ease with which AIBN forms radicals, and the fact that the rate of information does not vary much in various solvents has resulted in wide use of AIBN as a free-radical initiator. AIBN is used commercially as a catalyst for vinyl polymerisation (see Initiators). 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

Additions to C=C are almost certainly the most important group of reactions involving radicals. This is due largely to the importance of addition (vinyl) polymerisation (p. 320), and the consequent extent to which its mechanism has been investigated but addition of halogens and of halogen hydracids is also of significance. 

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

The most important commercial polymers prepared by this method are obtained from monomers containing a carbon-carbon double bond. In such cases the reaction is called vinyl polymerisation. The addition polymerisation is also important for carbonyl compounds and 1, 2-epoxides. 

We have already come across a few examples in which oxidation-reduction reaction can initiate vinyl polymerisation. Such a polymerisation reaction is known as redox polymerisation. In such reactions the oxidant is generally referred to as initiator and the reductant as activator. 

Most initiators in typical vinyl polymerisations have efficiencies between 0.6 and 1 (i.e., between 60 and 10 per cent of all the radicals formed ultimately initiate Polymer chains). A fraction of the radicals may disappear under certain circumstances also through different reactions, i.e., through direct combination with atmospheric oxygen or other inhibiting substances present in the system. 

More recently certain transitional metal ions have been used for initiation of vinyl polymerisation. It has been explained by Santappa et al. and Nayak et al. that the termination in case of the metal ion initiation of polymerisation to be linear, may be shown as below ... 

The heuristic value of equation (3) is that we can see what is required for a good initiator D and I(R ) must be as large a possible, and AHS(R+) should be small compared to AHS(RI>1+) in other words, as an efficient initiator we need a large cation whose parent radical has a high ionisation potential, and which forms a strong bond to monomer. On the last two counts the trityl cation must be one of the least suitable. These considerations indicate that the first four ions listed in Table 1 should merit detailed study as initiators for vinyl polymerisations. 

Note that the product is a radical which can further react with PhCH=CH2, leading to the process of vinyl polymerisation. 

Ueda, M., Matsuda, H. and Matsumoto, Y. (1994). Chemical modification of wood by simultaneous ohgoesterification and vinyl polymerisation under hot pressing. Mokuzai Gakkaishi, 40(7), 725-732. 

An energy balance must be constructed also. In the case of vinyl polymerisation of monomer A, this can be written as... 

Radical addition, 312-323 carbon tetrachloride, 320 halogens, 313 hydrogen bromide, 316 sulphenyl halides, 320 vinyl polymerisation, 320 Radical anions, 218 Radical rearrangements, 335 Radicals, 20, 30,299-339 acyl, 306, 330, 335 addition to 0==C, 313-323 alkoxyl, 303... 

Van der Waals radii, 8 Vibrational modes, 342 Vinyl ethers, polymerisation, 189 Vinyl polymerisation, 320 branching in, 321 chain length in, 321 coordination, 322 induction period in, 321... 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

Direct evaluation of the degree of dissociation of the propagating ion pair in vinyl polymerisation is not possible because these carbocations are not living and, in particular, readily transfer a proton to monomer or residual water. 

Perhaps the most versatile cation for initiation of polymerisation is the tri-phenylmethyl species (Ph3C+) which is reactive towards both electron rich olefins and cyclic monomers. The more stable cycloheptatrienyl cation (C H,) has proved very useful in vinyl polymerisations but has been little used in ring opening reactions (/5). Not surprisingly, therefore, the dissociation of numerous salts of these two cations have been widely studied, and data for each is summarised in Tables 2 and 3 respectively. 

III. Application of Stable Organic Cations to Kinetic Studies of Vinyl Polymerisations... 

In general the initiation reaction for a vinyl polymerisation by any cation A+ may be represented by the simple equilibrium... 

Even more stable salts than those already mentioned can be prepared from oxonium, R30+, sulphonium, R3S+, and ammonium, R4N+ ions. Not surprisingly, however, these are in general too stable to initiate vinyl polymerisations though Et30+BF4 has been reported to polymerise alkyl vinyl ethers (96). Both oxonium and strained sulphonium ion salts are very efficient initiators of ring opening polymerisations as we shall see later (Section IV). 

The molecular weight limiting process in most cationic vinyl polymerisations is transfer to monomer and/or residual water. 

---