Solvents chains

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. 

Solution Polymerization. This method is not commercially important, although it is convenient and practical, because it provides viscous cements that are difficult to handle. Also, the choice of the solvent is a key parameter due to the high solvent chain-transfer constants for acrylates. 

Let us consider a simple self-avoiding walk (SAW) on a lattice. The net interaction of solvent-solvent, chain-solvent and chain-chain is summarized in the excluded volume between the monomers. The empty lattice sites then represent the solvent. In order to fulfill the excluded volume requirement each lattice site can be occupied only once. Since this is the only requirement, each available conformation of an A-step walk has the same probability. If we fix the first step, then each new step is taken with probability q— 1), where q is the coordination number of the lattice ( = 4 for a square lattice, = 6 for a simple cubic lattice, etc.). 

In solution the molecules of a polymer undergo various segmental motions, changing rapidly from one conformation to another, so that the molecule itself effectively takes up more space than the volume of its segments alone. As we have seen, the size of the individual molecules depends on the thermodynamic quality of the solvent in good solvents chains are relatively extended, whereas in poor solvents they are contracted. 

In aprotic solvents, chain transfer occurs exclusively by fl-H elimination, unless a protic acid or water is present in the reaction mixture, in which case protonolysis may occur. Indirect evidence (for example, M, and M measurements) proves that P-H chain transfer in aprotic solvents is slower than methanolysis in protic solvents with comparable structures of the Pd" catalyst [5f, 17, 20, 21]. This effect and the possibility of using well-defined catalysts have remarkably favored the use of in situ NMR spectroscopy for the detection of intermediates during CO/copolymerisation in organic solvents. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

TheF term describes the hydrophobic effects of solutes on the hydrogen-bonded solvent chain. The source of this term is the decrease in mixing entropy owing to the hydrogen bond formed among solvent molecules ... 

Transfer reactions to small molecules (T) including monomer, solvent, chain transfer agent (CTA), etc., are all very similar and are generalized as follows ... 

This result may be compared with that deriving from the affine mean-field approach, originally suggested by Flory to treat good-solvent chain expansion, that is [17],... 

Also because of the unusual nature of this solvent, chain transfer to solvent is important in this system (see later). 

Let us consider the states of a chain made of Nx monomers, in solution successively in its own monomer, in its dimer, in its trimer, etc. In the first case, the repulsive interaction between monomers of the chain produces a gyration swelling 3 G which obeys the laws already observed. In the other extreme case where the test chain finds itself in a melt of other identical chains, the repulsive interaction is screened and the test chain is in a quasi-Brownian state. In intermediate situations the swelling 3E0 of the test chain of Nx monomers varies with the number N of monomers (per chain) of the solvent chains. Observation of the variation law of XG with N clearly reveals the structure of polymer solutions. Such observation requires the labelling of the chain made of iV, monomers therefore it is appropriate to use neutron scattering. Kirste and Lehneh2 made the experiment with blends of polydimethylsiloxane (PDMS)... 

Hence, the equivalent Brownian area S (per chain) for the solvent chains may be calculated with the help of the relation S 2 °AZ WA/W — 12.54 x 10 4Mwnm2. 

The albove kinetic analysis has been simplified for purposes of illustration by omitting the fact that chain-transfer reactions may occur. In the absence of solvent, chain transfer may occur between the radical E and the monomer M or the polymer P. 

The results of the dilute solution polymerization suggested that solvent chain transfer was a governing factor In the termination step. By the same token, the mechanism of solvent chain transfer termination forms free radicals which do not carry carboxyl groups, and serve as potent initiator species In the polymerization media. It was concluded that in order to obtain higher functionality of the PnBA the solvent had to be eliminated from the polymerization reactor. 

---