Radical Homopolymerization

In transfer reactions the growth of a chain is ended, for example, by transfer of a hydrogen atom from the molecule ZH, but at the same time a new polymer chain is started by the radical Z that is formed simultaneously. Thus, several macromolecules result from one primary radical therefore, the kinetic chain length, i.e., the total number of monomer molecules induced to be polymerized by one primary radical, is much larger than the degree of polymerization of the macromolecules formed. A general scheme is as follows  

Let us for the moment disregard chain transfer reactions. Radical polymerization then consists of three component reactions initiation, propagation of the polymer chains, and termination of chain growth. The rate of primary radical formation, v, by decomposition of the initiator I, may be written  

The rate constant /c, contains a factor that allows for the efficiency of initiation not all the radicals generated by the initiator are capable of starting polymer chains, some are lost by combination or other reactions. The initiator efficiency is defined as the ratio of the number of initiator molecules that start polymer chains to the number of initiator molecules decomposed under the given conditions of the polymerization. With most radical initiators the efficiency lies between 0.6 and 0.9 it also depends on the nature of the monomer. 

Here it is assumed that kp is independent of the number of monomer molecules already added [R] denotes the concentration of radicals in the system. The rate of the termination reaction v, is given by  

According to Bodenstein, for a chain reaction in the steady state, the number of radicals formed and disappearing in a given time must be the same. This applies to most addition polymerizations, at least in the region of low conversion. Under these conditions v, and v, may be equated  

Inorganic and organic peroxo compounds, e.g., peroxodisulfates, peroxides, hydroperoxides, peresters Protic acids Proton acceptors Organometallic compounds 

Aliphatic azo compounds Lewis acids with or without coinitiators Lewis bases Mixed catalysts (Ziegler-Natta catalysts) 

Substituted ethanes, e.g., benzpinacol Carbonium ions Organometallic compounds p-Complexes with transition metals, e.g., metallocenes 

Redox systems with inorganic and organic components lodonium ions Electron-transfer agents, e.g., alkali metals, alkali-aromatic complexes, alkali metal ketyls Activated transition metal oxides 

Controlled radical polymerization stable nltroxy radicals, R-X/Cu(I)/ligand, thlocarbamates (transferters)  

---

We saw in the last chapter that the stationary-state approximation is apphc-able to free-radical homopolymerizations, and the same is true of copolymerizations. Of course, it takes a brief time for the stationary-state radical concentration to be reached, but this period is insignificant compared to the total duration of a polymerization reaction. If the total concentration of radicals is constant, this means that the rate of crossover between the different types of terminal units is also equal, or that R... 

Note that this inquiry into copolymer propagation rates also increases our understanding of the differences in free-radical homopolymerization rates. It will be recalled that in Sec. 6.1 a discussion of this aspect of homopolymerization was deferred until copolymerization was introduced. The trends under consideration enable us to make some sense out of the rate constants for propagation in free-radical homopolymerization as well. For example, in Table 6.4 we see that kp values at 60°C for vinyl acetate and styrene are 2300 and 165 liter mol sec respectively. The relative magnitude of these constants can be understod in terms of the sequence above. 

This interpretation has been challenged by Kamo and co-workers53 who independently studied the same systems and arrived at the conclusion that the polymer arises from the radical homopolymerization of the Diels-AIder adduct through its residual double bond, to give structure 16. 

This chapter is primarily concerned with the chemical microstructure of the products of radical homopolymerization. Variations on the general structure (CHr CXY) are described and the mechanisms for their formation and the associated Tate parameters are examined. With this background established, aspects of the kinetics and thermodynamics of propagation are also considered (Section 4.5). 

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). The reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerizalion (Chapter 6) arc considered elsewhere. 

A new rate model for free radical homopolymerization which accounts for diffusion-controlled termination and propagation, and which gives a limiting conversion, has been developed based on ft ee-volume theory concepts. The model gives excellent agreement with measured rate data for bulk and solution polymerization of MMA over wide ranges of temperature and initiator and solvent concentrations. 

PVPA was prepared by the free-radical homopolymerization of vinyl-phosphonyl dichloride using azobisisobutyronitrile as initiator in a chlorinated solvent. The poly(vinylphosphonyl chloride) formed was then hydrolysed to PVPA (Ellis, 1989). No values are available for the apparent pA s of PVPA, but unpolymerized dibasic phosphonic acids have and values similar to those of orthophosphoric acid, i.e. 2 and 8 (Van Wazer, 1958). They are thus stronger acids than acrylic acid, which as a pK of 4-25, and it is to be expected that PVPA will be a stronger and more reactive acid than poly(acrylic acid). 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

F. Vargas, J. Alvarez, and R. Suarez. Nonlinear study of the periodic operation for free-radical homopolymerization reactors. In IEEE Int. Conf. Control Applications, volume 1, pages 84-89, 1989. 

The rates and degrees of polymerizations in radical copolymerizations conform essentially to the same laws as for radical homopolymerization (see Sect. 3.1). Raising the initiator concentration causes an increase in the rate of polymerization and at the same time a decrease in the molecular weight a temperature rise has the same effect. However, these assertions are valid only for a given... 

Tanaka, H. Yoshida, S. Kinetic study of the radical homopolymerization of captodative substituted methyl a-(acyloxy)acrylates. Macromolecules 1995, 28, 8117-8121. 

Radical homopolymerization and copolymerization with MMA initiated by AIBN in benzene solution or in bulk led to high MW graft (co)polymers. 

So far, a great number of well-defined macromonomers as branch candidates have been prepared as will be described in Sect. 3. Then a problem is how to control their polymerization and copolymerization, that is how to design the backbone length, the backbone/branch composition, and their distribution. This will be discussed in Sect. 4. In brief, radical homopolymerization and copolymerization of macromonomers to poly(macromonomers) and statistical graft copolymers, respectively, have been fairly well understood in comparison with those of conventional monomers. However, a more precise control over the backbone length and distribution by, e.g., a living (co)polymerization is still an unsolved challenge. 

Radical homopolymerization kinetics of some typical macromonomers, such as those from PSt, 23,24 [30,31], and PMMA, 25 [32,33], have been studied in detail by means of ESR methods. 

Radical homopolymerization and copolymerization of macromonomers are fairly well understood and reveal their characteristic behaviors that have to be compared with those of conventional monomers. A detailed mechanism of the polymer-polymer reactions involved, however, appears still to be an issue. Ionic or, desirably, living polymerization and copolymerization are still an important... 

Fig. 27. Nadimide radical homopolymerization initiated by maleimide [84] and cyclopen-tadiene [41]...

Table 5. Initiated radical homopolymerization of captodative acrylonitriles CH2 = C(d)CN...

For the strong donor monomer VCZ, the photoreaction depends on the solvent basicity and the molar ratio of the donor and the acceptor. In strongly basic solvents such as dimethyl formamide (DMF), the radical homopolymerization of VCZ occurs in the presence of catalytic amounts of FN or diethyl fumarate (DEF), but it is replaced by radical copolymerization in an equimolar amount of the monomers. The cationic homopolymerization of VCZ, which proceeds in less basic solvents, e.g., benzene, and the cyclodimerization of VCZ, which proceeds in moderately basic solvents, e.g., acetone, is accompanied by the radical copolymerization of VCZ with FN or DEF [6],... 

---