Transfer-to-monomer

Transfer to monomer can occur in one of two ways. Both involve an abstraction process through which a hydrogen atom is transferred to the propagating chain. The free radical is transferred to the monomer to form a monomer radical. 

The monomer radical formed is free to initiate the propagation of another polymer chain. 

The newly propagated chain will have an unsaturated end group. This end group is available for re-initiation. 

Propagation will lead to the formation of a branched polymer chain. 

This type of reaction is typified by vinyl acetate, but may also occur with acrylic monomers with alkyl side groups. 

A transfer to monomer, solvent, etc., means that an individual polymer chain stops growing and a new one begins. The transfer thus lowers the degree of polymerization. 

For quantitative analysis, it is assumed that termination by initiator free radicals is negligibly small. In addition, termination should result solely from the recombination of two polymer free radicals. The number-average degree of polymerization is then equal to twice the kinetic chain length  

The degree of polymerization increases with increasing rate of propagation and falls with increasing rate of termination  

If the transfer reaction takes place only to monomer, then it follows that Z tr.x = with equation (20-115), after rearranging, this gives 

20 Free Radical Unipolymerization Table 20-6. Transfer Constants of Various Monomers 

Irrespective of the mechanism by which transfer to monomer occurs, the process will usually produce an unsaturated radical as a byproduct. This species initiates polymerization to afford a macromonomer that may be reaetive under typical polymerization conditions. 

Tabic 6.14 Selected Values for Transfer Constants to Monomer  

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The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

Chain transfer to monomer is the main reaction controlling molecular weight and molecular weight distribution. The chain-transfer constant to monomer, C, is the ratio of the rate coefficient for transfer to monomer to that of chain propagation. This constant has a value of 6.25 x lO " at 30°C and 2.38 x 10 at 70°C and a general expression of 5.78 30°C, chain transfer to monomer happens once in every 1600 monomer... 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

Polymerization of some vinyl monomers initiated by those colored aromatic complexes was described by Scott38 over twenty years ago, and recently the mechanism of this reaction has been elucidated in our laboratory43 where we demonstrated that polymerization initiation is due to an electron transfer to monomer, namely A - -M A-f-M . This system is useful, therefore, in... 

The relative propensity of radicals to abstract hydrogen or add to double bonds is extremely important. In radical polymerization, this factor determines the significance of transfer to monomer, solvent, etc. and hence the molecular weight and end group functionality (Chapter 6). It also provides one basis for initiator selection (Section 3.2.1). 

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small dial entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of n will usually be less than 0.4. In these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates or particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical termination, when it occurs in the particle phase, will usually be between a short species (one that lias just entered) and a long species. 

Early reports37 157 167 suggested that termination during VAc polymerization involved predominantly disproportionation. However, these investigations did not adequately allow for the occurrence of transfer to monomer and/or polymer, which are extremely important during VAc polymerization (Sections 6.2.6.2 and 6.2.7.4 respectively). These problems were addressed by Bamford et who used the gelation technique (Section 5.2.2,2) to show that the predominant radical-radical termination mechanism is combination (25 °C). 

Studies on VC polymerization are also complicated by the fact that only a small proportion of termination events may involve radical-radical reactions. Most termination is by transfer to monomer (Sections 4.3.1.2 and 6.2.63). Early studies on the termination mechanism which do not allow for this probably overestimate the importance of disproportionation.lb8 iw... 

In the case of S, it has been proposed that reinitiation may occur by hydrogen-atom transfer to monomer (Scheme 6.13).I2,6S... 

It has been proposed that transfer to monomer may not involve the monomer directly but rather the intermediate (110) formed by Diels-Alder dimerization (Scheme 6.28). 70 Since 110 is formed during the course of polymerization, its involvement could be confirmed by analysis of the polymerization kinetics. 

Stames el al.I7 have provided support for the above mechanism (Scheme 6.29) by determining the unsaturated chain ends (112) in low conversion PVAc by l3C NMR. They were able to distinguish (112) from chain ends that might have been formed if transfer involved abstraction of a vinylic hydrogen. The number of unsaturated chain ends (112) was found to equate with the number of -CH OAc ends suggesting that most chains arc formed by transfer to monomer. Stames et a . 13 also found an isotope effect k kD of 2.0 for the abstraction reaction with CTTpCHOiCCD as monomer. This result is consistent with the mechanism shown in Scheme 6.28 but is contrary to an earlier finding.174... 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

Diallyl monomers find significant use in cyclopolymerization (Section 4.4.1). Transfer to monomer is of greater importance in polymerizations of allyl than it is in diallyl monomers.184 This might, in part, reflect differences in the nature of the propagating species [e.g. a secondary alkyl (115) v.v a primary alkyl radical (116)]. Electronic factors may also play a role,185... 

PVAc is known to contain a significant number of long chain branches. Branches to the acetate methyl may arise by copolymerization of the VAe macromonomcr produced as a consequence of transfer to monomer (Section 6.2.6.2). Transfer to polymer may involve either the acetate methyl hydrogens (Scheme 6.34) or the methine (Scheme 6.35) or methylene hydrogens of the polymer backbone. 

PVC formed with diacyl peroxide or peroxydicarbonate initiators will contain a proportion of potentially labile a-haloester chain ends (6, Scheme 8.9). However, it is believed that most chain ends in PVC are formed by transfer to monomer as is discussed in Sections 4.3.1.2 and 6.2.6.3.47... 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

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