Synthetic Polyisoprene Rubber as a Mimic of Natural Rubber: Recent Advances on Synthesis, Nanocomposites, and Applications.

Up to now, rubber materials have been used in a wide range of applications, from automotive parts to special-design engineering pieces, as well as in the pharmaceutical, food, electronics, and military industries, among others. Since the discovery of the vulcanization of natural rubber (NR) in 1838, the continuous demand for this material has intensified the quest for a synthetic substitute with similar properties. In this regard, synthetic polyisoprene rubber (IR) emerged as an attractive alternative. However, despite the efforts made, some properties of natural rubber have been difficult to match (i.e., superior mechanical properties) due not only to its high content of cis-1,4-polyisoprene but also because its structure is considered a naturally occurring nanocomposite. In this sense, cutting-edge research has proposed the synthesis of nanocomposites with synthetic rubber, obtaining the same properties as natural rubber. This review focuses on the synthesis, structure, and properties of natural and synthetic rubber, with a special interest in the synthesis of IR nanocomposites, giving the reader a comprehensive reference on how to achieve a mimic of NR.

Synthesis of Biobased Hydroxyl-Terminated Oligomers by Metathesis Degradation of Industrial Rubbers SBS and PB: Tailor-Made Unsaturated Diols and Polyols.

Biobased hydroxyl-terminated polybutadiene (HTPB) was successfully synthesized in a one-pot reaction via metathesis degradation of industrial rubbers. Thus, polybutadiene (PB) and poly(styrene-butadiene-styrene) (SBS) were degraded via metathesis with high yields (>94%), using the fatty alcohol 10-undecen-1-ol as a chain transfer agent (CTA) and the second-generation Grubbs−Hoveyda catalyst. The identification of the hydroxyl groups (-OH) and the formation of biobased HTPB were verified by FT-IR and NMR. Likewise, the molecular weight and properties of the HTPB were controlled by changing the molar ratio of rubber to CTA ([C=C]/CTA) from 1:1 to 100:1, considering a constant molar ratio of the catalyst ([C=C]/Ru = 500:1). The number average molecular weight (Mn) ranged between 583 and 6580 g/mol and the decomposition temperatures between 134 and 220 °C. Moreover, the catalyst optimization study showed that at catalyst loadings as low as [C=C]/Ru = 5000:1, the theoretical molecular weight is in good agreement with the experimental molecular weight and the expected diols and polyols are formed. At higher ratios than those, the difference between theoretical and experimental molecular weight is wide, and there is no control over HTPB. Therefore, the rubber/CTA molar ratio and the amount of catalyst play an important role in PB degradation and HTPB synthesis. Biobased HTPB can be used to synthesize engineering design polymers, intermediates, fine chemicals, and in the polyurethane industry, and contribute to the development of environmentally friendly raw materials.

Synthesis, characterization and biodegradation studies of polyurethanes: Effect of unsaturation on biodegradability.

The presence of unsaturation in the main chain of the polymer promotes the biodegradation process. To elucidate this hypothesis, one unsaturated polyurethane (PUU) and another saturated polyurethane (PUS) were synthesized and then biodegraded, and evidence was found to support this hypothesis. The polyurethanes were synthesized by a polycondensation reaction with yields up to 97%. It is important to note that both polyurethanes were constituted only by the recalcitrant hard segment and showed low crystallinity and molecular weight. Spectroscopic, thermal, and chromatographic techniques were used for physical and structural characterization. Both polyurethanes were biodegraded by the BP8 microbial community and the Cladosporium tenuissimum A3.I.1 fungus during a two-month period. A postbiodegradation characterization revealed the detriment of properties in both materials, indicating successful biodegradation. As a general trend, more efficient biodegradation was observed by the Cladosporium tenuissimum fungus A3.I.1 than by the BP8 microbial community. Specifically, with the fungus, the infrared analysis showed a decrease in the characteristic bands as well as the appearance of new carboxylic acid signals (approximately 1701 cm-1), suggesting the enzymatic cleavage of the urethane group. By comparison to polyurethanes, PUU showed superior biodegradation; using the fungus, a 51% decrease in molecular weight (Mw) for PUU was achieved, in contrast with 36% achieved for PUS. Despite the low crystallinity and molecular weight, the determining factor in biodegradation was the presence of unsaturations along the main chain. Thus, a more efficient oxidative attack is carried out by microorganisms on double bonds. The novel PUU showed similar biodegradation to the known polyester-type PU with highly hydrolysable groups. Consequently, PUU represents a green alternative to conventional polyurethanes and is a key material to achieve biorecycling.

Metathesis transformations of natural products: cross-metathesis of natural rubber and mandarin oil by Ru-alkylidene catalysts.

This study reports on the degradation of natural rubber (NR) via crossmetathesis with mandarin oil and d-limonene, an abundant compound in essential oils; that were used as chain transfer agents (CTAs) and green solvents. Reactions were performed in the presence of the ruthenium-alkylidene catalysts (PCy3)2(Cl)2Ru=CHPh (I) and (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) (PCy3)Cl2Ru=CHPh (II), respectively. Catalyst II bears an N-heterocyclic carbene ligand (NHC) bounded to the ruthenium atom, which has a strong basic character; therefore it is more active toward trisubstituted olefins in comparison with catalyst I. In both cases, isolated monoterpene-terminated isoprene oligomers were obtained as products of the cross-metathesis degradation of NR. In the presence of catalyst II molecular weight values around M(n) × 10² and yields of 80% were obtained; whereas with catalyst I, the molecular weights of products were about M(n) × 104 with yields ranging 70 to 74%. The composition and yield of NR degradation products were determined by GC/MS (EI) analysis and it was found that the oligomers obtained have primarily one vinyl group and one terpene-monocyclic group at the chain end, with isoprene units A(m) = 2, 3 y 4.