Free-radical polymerisation termination

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Polymerisation does not continue until all of the monomer is used up because the free radicals involved are so reactive that they find a variety of ways of losing their radical activity. The two methods of termination in radical polymerisations are combination and disproportionation. The first of these occurs when two radical species react together to form a single bond and one reaction product as in Reaction 2.5. 

A certain free-radical polymerisation reaetion is described by the following sequence of initiation, addition and termination... 

Only very few results are available on the variation of DP with temperature, but they indicate that between -63.5° and -95.5° the DP does not vary significantly and hence EDP = 0 2 kcal/mole [76]. The obvious interpretation of the small EDP, and the large positive Er is that Er is essentially , which means that initiation is slow compared with propagation and termination, and that one is dealing here with a system which has kinetics resembling those of free-radical polymerisations. 

The normal dispersants used for both kaolin and calcium carbonate pigments are aqueous solutions of sodium polyacrylate. These are prepared by free radical polymerisation using various combinations of initiators and terminators which may be proprietary to the manufacturer. Number average molecular weights are... 

The chain length in free radical polymerisations is usually lower than would be expected from the mechanism of termination. The reason for this discrepancy is that the growing polymer chain can transfer the radical to other species, leading to termination of one chain, and thus generating a new radical that will react further. The following transfer mechanisms may occur ... 

The fabrication process of hydroxyl terminated polybutadiene is based on the free radical polymerisation of butadiene, initiated by hydrogen peroxide at 100-150 °C, in the presence of a solvent such as methanol [12], isopropanol [12], or in the presence of tricresyl phosphate [14]. The polymerisation in alcohols is used industrially. 

Chain-Growth Polymerisation with Termination A major exponent of this class of polymerisations are free radical polymerisations in the presence of a radical initiator. A classic example is the crosslinking (co)polymerisation of unsaturated polyester resins with styrene, initiated by the decomposition of a peroxide initiator. Some important reaction steps involved in free radical polymerisations are sketched in Reaction scheme 4. 

Figure 5.4 A tentative reaction mechanism for formation of carboxyl-terminated poly(2-ethyl hexyl acrylate) (CTPEHA) from free radical polymerisation of EHA in the presence of 4, 4 -azobis (4-cyanovaleric acid) (ABCVA) as a free radical initiator and dithiodiglycolic acid (DTDGA) as a chain transfer agent. (P = nM ) Reprinted with permission from D. Ratna, A.K. Banthia and P.C. Dtb, Journal of Applied Polymer Science, 2000, 78, 716. 2000, John Wiley and Sons Publishers...

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

In typical free radical polymerisation, polydispersity is above 2 and as high as 20. In anionic polymerisation, the absence of termination due to a living polymerisation, may cause very narrow molecular weight distribution with polydispersities as low as 1.06. 

The wide variation in the thermal degradation of PMMA can be explained in terms of the structure of the PMMA used and by the experimental conditions employed for preparing the polymer. A two-step degradation process results if the polymer has been prepared in the presence of air due to copolymerisation with oxygen but not to weak links formed by terminal combination since these would be present in all free-radical polymerisations. PMMA polymerised thermally is as stable as polymers initiated by free radicals in the absence of oxygen and peroxide impurities. It has a higher molecular weight and... 

However, it must be pointed out that values of os calculated using Eq. (10) will be overestimates of the weight fraction of polymer molecules produced by radical-radical termination reactions and which have undesired functionality (to,). This is because for certain polymerisations some of the bimolecular radical termination reactions contained in the k,[P fg term can actually give rise to the formation of polymer molecules possessing the desired functionality. This is best illustrated by listing the various combination and disproportionation reactions that can occur between two radicals in a matched chain transfer free-radical polymerisation. 

Low molecular weight polymers possessing a single, terminal functional-group can be prepared by free radical polymerisation. They compare favourably with those prepared by anionic polymerisation in terms of molecular weight distribution and degree of functionalisation. 

The propagation and termination reactions in free radical polymerisations can be represented by... 

In the case of ionic polymerisation, electrostatic interaction has a major influence, and occurs as a result of attraction between a polymeric ion and the permanent dipole on the monomer. In free radical polymerisation the effect of electrostatic forces are not as clearly defined. Since free radicals ate in effect neutral, attack on either side of the C=C bond is possible from an electrostatic point of view. However, it is suggested from experimental evidence that attack on a terminal olefine is almost exclusively at the CHj side of the C=C bond, as shown below ... 

The mechanism of free radical polymerisations belongs to the class of so-called chain reactions. Chain reactions are characterised by the fast, subsequent addition of monomers to an active centre at the chain end. The activity of the growing chain is transferred to the adding unit. The active centres are present in very low concentrations (10 -10 mol 1 ). The rate of addition is very high (10 -10 units per second) and the time of growth of a chain (time between initiation and termination of a chain) is quite short (0.1-10 s) relative to the total reaction time, which can be in the order of several hours. 

---