J.C. Bevington

An appraisal of NMR methods for study of end-groups derived from initiators in radical polymerizations

Abstract Various studies by NMR spectroscopy of end-groups in polymers and copolymers are reviewed, particular attention being paid to groups formed during radical polymerization from initiators enriched with carbon-13. The sensitivity of the end-group analysis is not as high as that achievable when the initiator is labelled with a radioactive isotope but the specificity is incomparably better. It is possible to identify the incorporated initiator fragments and also to make deductions about the nature and stereochemistry of the monomeric units close to the end-groups. For quantitative work requiring comparison of the numbers of initiator fragments and monomeric units, the conditions for acquisition of the NMR spectrum of the polymer must be selected with care.

Multinuclear NMR studies of end-groups in polymers—III. Search for abnormal end-groups derived from azoisobutyronitrile☆

Abstract Samples of azoisobutyronitrile (AIBN), labelled with deuterium and nitrogen-15, have been used to initiate polymerizations and copolymerizations of styrene and methyl methacrylate. As in the case of [13C]-AIBN, the chemical shift associated with an end-group labelled with a particular isotope depends upon the nature of the attached monomer unit but there is no evidence of more than one type of end-group.

Carbon-13 n.m.r. study of end-groups in polymers prepared using azoisobutyronitrile as radical initiator

Abstract 13 C-azoisobutyronitrile has been used to initiate polymerizations of methyl methacrylate and styrene. The n.m.r. spectra of the resulting polymers show peaks arising from the combined initiator fragments with chemical shifts dependent on the nature of the adjacent monomer unit.

End-groups in polymers and copolymers of ethyl acrylate prepared using azoisobutyronitrile as initiator

Abstract Samples of azoisobutyronitrile enriched with carbon-13 or deuterium have been used for NMR studies of the initiator fragments incorporated in polymers of ethyl acrylate and copolymers of that monomer with styrene. It has been deduced that at 60° styrene is 3.3 times as reactive as ethyl acrylate towards the 2-cyano-2-propyl radical.

The reactivities of diarylbutadienes towards typical initiating radicals: the cases of the 1,4-and 2,3-isomers of diphenyl-1,3-butadiene

Abstract Studies have been made of the reactivities of 1,4-diphenyl-1,3-butadiene and 2,3-diphenyl-1,3-butadiene towards the benzoyloxy, p -fluorobenzoyloxy and 1-cyano-1-methylethyl radicals. The 1,4-isomer has very high reactivity towards the aroyloxy radicals but very low reactivity towards the substituted alkyl radical. The 2,3-isomer is moderately reactive towards all three initiating radicals. The results have been obtained by analyses for end-groups in samples of poly(methyl methacrylate) produced in the presence of one of the dienes with the appropriate source of initiating radicals.

Comparisons of the reactivities of methacrylonitrile and acrylonitrile and of the corresponding polymer radicals

SummaryIt is confirmed that the polymethacrylonitrile radical is much less reactive than the polyacrylonitrile radical towardstrans-stilbene. Methacrylonitrile and acrylonitrile are similar in their reactivities towards the 2-cyano-2-propyl radical and the benzoyloxy radical.

Further study of fluorinated derivatives of benzoyl peroxide as initiators of radical polymerizations: kinetic chain lengths and related quantities

Abstract Styrene and methyl methacrylate have been polymerized at 60°C using 2-, 3- and 4-fluorobenzoyl peroxides as initiators. Analyses for the fluorine-containing end-groups were performed by means of 19 F NMR. Kinetic chain lengths and rates of initiation have been calculated for the polymerizations and the average number of initiator fragments in the polymer molecules has been found in each case. The rates of dissociation of the peroxides are comparable with that of azobisisobutyronitrile. The results are consistent with the view that termination for styrene is largely by combination and for methyl methacrylate is almost entirely by disproportionation.

Iodoform as a transfer agent in radical polymerizations

Abstract Iodoform has been examined as a reactive transfer agent in the radical polymerizations at 60°C of styrene and methyl methacrylate using azobisisobutyronitrile as initiator. The transfer constants have been determined by a procedure depending upon the determination of the initiator fragments incorporated as end-groups in the polymers. The polymerizations are retarded by iodoform; this effect is largely caused by an impurity, most probably elemental iodine, which is generated in the polymerizing systems by decomposition of the transfer agent. It is deduced that the transfer constants for iodoform with styrene and methyl methacrylate are ca. 10 and 3, respectively.

The effect of multifunctional initiators on molecular weight in free radical polymerization

Abstract There has been determination of the molecular weight distributions which are to be expected when symmetrical multifunctional initiators of the structure R1-R1-R2-R2-R1-R1 are used to initiate free radical polymerizations in two stages. Such initiators are often used to make block copolymers. Cases considered include those where R1-R1 is more labile than R2-R2 and so initiates the first stage of polymerization, and those where R2-R2 is the more labile and so initiates the first stage. For cases where disproportionation occurs, analytical expressions are given for molecular weight distributions; where combination is the termination mode, Monte Carlo simulations have been done because analytical expressions could not be obtained.

Application of a new approach to the determination of transfer constants for systems showing retardation

Degradative transfer reactions for diphenylpicrylhydrazine (DPPH-H), 1,3-diphenylpropene (DPP), and fluorene (FLU) have been examined using styrene and methyl methacrylate as monomers. A new procedure has been followed, depending upon analyses of polymers for end groups derived from azobisisobutyronitrile used as initiator for the polymerizations. The method requires information or assumptions about the efficiency of re-initiation during transfer particularly for a monomer, such as methyl methacrylate, for which disproportionation is prominent in the termination process. For DPPH-H, the efficiency of re-initiation is zero; for DPP and FLU with styrene, the efficiencies are close to unity but they are smaller when methyl methacrylate is used.