Free radical addition polymerization chain transfer

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

When vinyl chloride is polymerized in solution there is no autoacceleration. Also, a major feature of vinyl chloride free-radical pohmierization is chain transferring to monomer. This is supported by experimental evidence. In addition, the growing radical chains can terminate by chain transferring to dead polymer molecules. The propagations then proceed from the polymer backbone. Such new growth radicals, however, are probably short-lived as they are destroyed by transfer to monomer. ... 

In addition to information concerning chain transfer ability, the data in Table 4 indicated a relationship between conversion and the complexing ability of the particular crown ether utilized as phase transfer catalyst. In fact, when percent conversion was plotted vs the log of the binding constants (log K) of the respective crowns for potassium cation, an apparently linear correlation was obtained. On the basis of some simple assumptions and free radical addition polymerization theory, however, it may be deduced that conversion should correlate with K rather than log K (Equation 1). This anomaly was resolved by using inhibitor-free monomer (Table 5), whereupon a good correlation (R=0.948) between the variables was obtained. 

In practice, another type of reaction sometimes occurs in free-radical addition polymerizations. These chain-transfer reactions kill a growing chain radical and can start a new one in its place (as long as the radical transfers to another monomer) ... 

FIGURE 9.4 Instantaneous number- and weight-fraction distributions of chain lengths in free-radical addition polymerization. Equations 9.47 and 9.48 are for termination by disproportionation and/or chain transfer Equations 9.58 and 9.59 are for termination by combination. 

A polymeric composition for reducing fluid loss in drilling muds and well cement compositions is obtained by the free radical-initiated polymerization of a water-soluble vinyl monomer in an aqueous suspension of lignin, modified lignins, lignite, brown coal, and modified brown coal [705,1847]. The vinyl monomers can be methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether, N-methylmethacrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, and additional AMPS. In this process a grafting process to the coals by chain transfer may occur. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Polymer Synthesis. General Procedure—All polymers were prepared by free-radical-initiated solution polymerization. Typical quantities utilized were as follows 5.0 g total monomer and 0.02 g AIBN or Vazo 33 in 30-60 mL solvent. More dilute solutions were employed in some cases to eliminate gel formation. In addition, a chain transfer agent, dodecanethiol, was used to control molecular weight in some polymerizations. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Chiefari J, Chong YK, Ercole F (1998) Living free radical polymerization by reversible addition-fragmentation chain transfer -the RAFT process. Macromolecules 31 5559-5562... 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

Pentadienyl-terminated poly(methyl methacrylate) (PMMA) as well as PSt, 12, have been prepared by radical polymerization via addition-fragmentation chain transfer mechanism, and radically copolymerized with St and MMA, respectively, to give PSt-g-PMMA and PMMA-g-PSt [17, 18]. Metal-free anionic polymerization of tert-butyl acrylate (TBA) initiated with a carbanion from diethyl 2-vinyloxyethylmalonate produced vinyl ether-functionalized PTBA macromonomer, 13 [19]. 

Reversible addition-fragmentation chain transfer (RAFT) polymerization using 2,2 -azobisisobutyronitrile and either A, A-dimethyl-5-thiobenzoylthiopropionamide or A-dimethyl-5-thiobenzoylthioacetamide as chain transfer agents has been used to prepare low polydispersity poly(A, A-dimethylacrylamide). The chain transfer agents were unusually effective in suppressing free radical termination reaction, thereby mimicking a living polymerization reaction. 

The free radical polymerization of pinenes and limonene is of little interest, because of the modest yields and DPs obtained with their homopolymerizations. However, their copolymerization with a variety of conventional monomers has been shown to produce some interesting materials, particularly in the case of controlled reversible addition fragmentation chain-transfer (RAFT) systems involving P-pinene and acrylic comonomers [5]. 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

There are four principal mechanisms that have been put forward to achieve living free-radical polymerization (1) Polymerization with reversible termination by coupling, the best example in this class being the alkoxyamine-initiated or nitroxide-mediated polymerization, as first described by Solomon et al. (1985) (2) polymerization with reversible termination by hgand transfer to a metal complex (usually abbreviated as ATRP),(Wang and Matyjaszewski, 1995) (3) polymerization with reversible chain transfer (also termed degenerative chain transfer)-, and (4) reversible addition/ffagmentation chain transfer (RAFT). 

---