Termination reactions, polymer systems

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. 

As the polymerization reaction proceeds, scosity of the system increases, retarding the translational and/ or segmental diffusion of propagating polymer radicals. Bimolecular termination reactions subsequently become diffusion controlled. A reduction in termination results in an increase in free radical population, thus providing more sites for monomer incorporation. The gel effect is assumed not to affect the propagation rate constant since a macroradical can continue to react with the smaller, more mobile monomer molecule. Thus, an increase in the overall rate of polymerization and average degree of polymerization results. 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

In this work, the kinetics of these reactions are closely examined by monitoring photopolymerizations initiated by a two-component system consisting of a conventional photoinitiator, such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and TED. By examining the polymerization kinetics in detail, further understanding of the complex initiation and termination reactions can be achieved. The monomers discussed in this manuscript are 2-hydroxyethyl methacrylate (HEMA), which forms a linear polymer upon polymerization, and diethylene glycol dimethacrylate (DEGDMA), which forms a crosslinked network upon polymerization. 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

The foregoing discussion has shown that termination reactions in cationic polymerization may be of many different kinds, that they may differ for apparently closely related systems, and that they may even be entirely absent. However, the polymers produced in many of these reactions are of low molecular weight and this means that transfer reactions are dominant. They may take on an even greater variety of forms than the termination reactions and their classification and discussion are still in an early stage of development. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

The need for well defined polymer species of low polydls-perelty and of known structure arises from the Increasing Interest In structure-properties relationship In dilute solution as well as In the bulk. A great variety of methods have been attempted, to synthesize so-called model macromolecules or tailor made polymers-over the past 20 years. The techniques based on anionic polymerization, when carried out In aprotic solvents, have proved best suited for such synthesis, because of the absence of spontaneous transfer and termination reactions that characterize such systems. The "living 1 polymers obtained are fitted at chain end with carbanionic sites, which can either Initiate further polymerization, or react with various electrophilic compounds, intentionally added to achieve functionalizations. Another advantage of anionic polymerizations is that di-functlonal Initiators are available, yielding linear polymers fitted at both chain ends with carbanionic sites. In this paper we shall review the various utility of anionic polymerization to the synthesis of tailor made well defined macromolecules of various types. 

In consideration of the kinetic law obtained, Rp i0 of magnitude range, one can conclude that the common polymerization mechanism, based on bimolecular termination reactions, is no longer valid for these multifunctional systems when irradiated in condensed phase. Indeed, for conventional radical-induced polymerizations, the termination step consists of the interaction of a growing polymer radical with another radical from the initiator (R), monomer (M) or polymer (P) through recombination or disproportionation reactions ... 

We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosin-triethanolamine system is real and not an artifact of the technique employed to determine the photospeed. 

A typical effect observed in the synthesis of linear polymers by a free-radical mechanism is the auto-acceleration process. At a particular conversion, when sufficient polymer has accumulated in the system for the viscosity to reach a certain level, the rate of the bimolecular termination reaction begins to fall because of diffusional restrictions to the encounter of two chain ends. However, the initiation and growth rates are hardly affected. 

For chainwise polymerizations, the analysis of model systems implies consideration of the homopolymerization or copolymerization of bifunctional monomers. Kinetic results cannot be directly extrapolated to the case of networks, because very important features such as intramolecular cycliza-tion reactions are not present in the case of linear polymers. However, the nature of initiation and termination reactions may be assessed. For example, using electron spin resonance (ESR), Brown and Sandreczki (1990) identified different types of radicals produced during the homopolymerization of a monomaleimide (a model compound of bismaleimides). 

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