David R. Burfield

Analysis of epoxidized natural rubber. A comparative study of d.s.c., n.m.r., elemental analysis and direct titration methods

A comparative analysis of epoxidized natural rubber samples by 1H and 13C n.m.r., titrimetric, elemental and d.s.c. techniques has been made. Whereas the titrimetric method is only applicable at low epoxy contents (< 15 mol%) both n.m.r. methods give reasonable precision over the compositional range of 20–75 mol%. Elemental analysis appears less reliable. D.s.c. analysis through measurement of Tg provides the highest precision of measurement but requires independent calibration by one or more of the primary methods. The epoxy content may also be related to the polymer density.

Cold crystallization of natural rubber and its synthetic analogues: The influence of chain microstructure☆

Abstract The crystalizabilities of eight synthetic high cis-1,4-polyisoprene rubbers have been compared with that of purified natural rubber by differential scanning calorimetry measurements. The detailed microstructure of the rubbers as determined by 13C and 1H nuclear magnetic resonance have also been studied. Whereas natural rubber is characterized by an essentially sterically pure main chain of 100% cis-1,4 content, the structure of the synthetic rubbers spans the range 90.9 to 99.1 mol%cis-1,4 content with varying amounts of trans-1,4 and 3,4 units. The study shows that the purified natural rubber samples undergo cold crystallization more readily than the synthetic analogues and this is attributable to the more perfect microstructure of the natural product. Generally the crystallizability of the rubbers correlates with the average cis sequence lengths of the polymer main chain.

D.s.c. studies of the reaction between epoxidized natural rubber and benzoic acid

Abstract The reactions between epoxidized natural rubber (ENR) and benzoic acid have been investigated at elevated temperatures, in the range 398–433 K, by differential scanning calorimetry (d.s.c.). The reaction is characterized by a significant increase in the glass transition temperature ( T g ) of the material, the increase being proportional to the amount of added acid. The kinetics of the reaction have been elucidated by monitoring the rate of change of T g with respect to the reaction time as well as by direct measurements of the enthalpy change of reaction. The reaction is found to be first order with respect to benzoic acid and to have an overall activation energy of 70 kJ mol −1 . 13 C nuclear magnetic resonance studies employing 13 C-enriched benzoic acid showed that the benzoic acid was rapidly incorporated into the polymer main chain with the formation of ester linkages. Secondary, slower reactions were also in evidence but have not been positively identified. The incorporation of benzoic acid leads to a T g elevation of about 3.7 K mol% −1 modification and suggests that such reactions might prove useful in changing the physical properties of such rubbers. The significant elevation of T g associated with ENR vulcanized by dibasic acids is probably predominantly due to the modified main chain structure rather than the crosslinking network.

Ziegler-Natta catalysis: 6. Effect of electron donors on the course of polymerization

Abstract The activating effect of triethylamine in the VCl 3 AlR 3 system has been explained in terms of a previously proposed kinetic scheme. Activation is observed at low amine concentrations and is thought to occur by removal of adsorbed aluminium alkyl species from the catalyst surface. Deactivation occurs at higher donor concentrations and is believed to be due to competitive adsorption of donor with monomer. These proposals are consistent with the rate, molecular weight and polymer tacticity findings of other workers in Ziegler-Natta systems where the donor is capable of interaction with both the transition metal and metal alkyl components.

Determination of epoxy groups in natural rubber by degradation methods

Abstract The degradation of acid hydrolysed natural and epoxidized synthetic rubbers with lead tetraacetate and periodic acid have been studied, viscometrically. The epoxy content calculated from the degradation experiments is very much lower than that determined by a direct titration method. The discrepany is attributed mainly to incomplete hydrolysis of the rubber epoxides. The results demonstrate that the epoxy content of natural rubber is predominantly located on the rubber backbone.

Distribution of abnormal groups in natural rubber

Abstract Fresh samples of natural rubber were fractionated and the distribution of epoxy and amine groups over the molecular weight range was investigated. The origin of these abnormal groups is suggested. A scheme is proposed to explain the formation of lactone groups from the reaction products of the rubber epoxide and simple amino-acids. The presence of other functional groups is discussed.

Glass transition and crystallization phenomena in epoxidized trans-polyisoprene: a differential scanning calorimetry study

Abstract The effect of epoxidation of trans -1,4-polyisoprene (TPI) on the glass transition temperature T g and crystallization of the polymer has been investigated by differential scanning calorimetry. The value of T g is shown to be linearly dependent on the extent of chain modification, and increases from 201K for the unmodified polymer to 274K for complete epoxidation. The modification inhibits the extent of crystallization, and above 25 mol% epoxidation little or no cold crystallization occurs at 263K. The extent of crystallization is found to fit a previously proposed relationship: log( ΔH m ) = log k + n log x A , where ΔH m is the heat of fusion, k a constant, n the critical sequence length of repeating units for crystallization to occur and x A the mole fraction of unmodified repeat units. For TPI, values of n = 6–7 are found for samples crystallized at 263K.

The synthesis of low molecular weight hydroxy-tipped polyethylene and polypropylene by the intermediacy of Ziegler—Natta catalysts

Abstract The use of diethylzinc in conjunction with conventional and supported Ziegler—Natta catalysts permits the synthesis of polypropylene and polyethylene samples with g.p.c. M n values as low as 4000 and 3000 respectively. At high diethylzinc concentrations the number of metal—polymer bonds appears to be 40% for polypropylene and 50% for polyethylene respectively, as measured by a tritium-quench radiotracer method. The metal-bound macromolecules can be efficiently oxidized with molecular oxygen at 60°C, and subsequently hydrolysed to give hydroxy-tipped polymers. The oxidation efficiency appears to be variable and to be lower at high diethylzinc concentrations.

Comparative d.s.c. studies of the crystallization of natural rubber and its synthetic analogues

Abstract A comparative d.s.c. crystallization study has shown that deproteinized natural rubber (DPNR) undergoes crystallization much more readily than synthetic analogues of 99% cis -1,4 purity. Acetone extracted DPNR is closer in behaviour to the synthetic materials whereas, the latter, when doped with 1% w/w stearic acid or rubber extract approach the crystallization character of DPNR. The differences in crystallization behaviour are thus largely, but not entirely, due to the presence of nucleating impurities in the natural product. Secondary rate effects may also be caused by slight differences in polymer microstructure or variation in molecular weight distribution. The maximum extent of crystallinity observed under the most favourable conditions approaches 40% for the DPNR samples as determined from the observed heat of fusion of the crystalline regions.

Reactions of a mono- and a tri-substituted epoxide with some simple and β-substituted primary amines; novel examples of electrophilic anchimeric assistance

The reactions of a number of primary amines with a mono- and a tri-substituted epoxide have been examined. The results indicate that the presence of water has a strong reaction-accelerating effect for both heterocycles, and product studies have led to the isolation of secondary and tertiary amino-alcohols. Reactions of some amines possessing a weakly acidic group in the β-position proceeded anomalously rapidly, even under dry conditions, and it is suggested that these represent new examples of electrophilic anchimeric assistance.