2,4:3,5-Di-O-methylene-d-mannitol, abbreviated as Manx, is a d-mannitol-derived compound with the secondary hydroxyl groups acetalized with formaldehyde. The bicyclic structure of Manx consists of two fused 1,3-dioxane rings, with two primary hydroxyl groups standing free for reaction. A homopolyester made of Manx and dimethyl terephthalate as well as a set of copolyesters of poly(butylene terephthalate) (PBT) in which 1,4-butanediol was replaced by Manx up to 50% were synthesized and characterized. The polyesters had M_w in the 30 000–52 000 g mol^–1 range and a random microstructure and were thermally stable up to nearly 370 °C. They displayed outstanding high T_g with values from 55 to 137 °C which steadily increased with the content in Manx. Copolyesters containing up to 40% of Manx were semicrystalline and adopted the crystal structure of PBT. Their stress–strain parameters were sensitively affected by the presence of carbohydrate-based units with elongation at break decreasing but tensile strength and elastic moduli steadily increasing with the degree of replacement.

Two series of bio-based PBT copolyesters were obtained by polycondensation in the melt of 2,4:3,5-di-O-methylene-D-glucitol (Glux-diol) or dimethyl 2,4:3,5-di-O-methylene-D-glucarate (Glux-diester) with 1,4-butanediol and dimethyl terephthalate. The glucose-based bicyclic compounds used as comonomers were synthesized from commercially available 1,5-D-gluconolactone. The prepared PBT copolyesters had weight-average molecular weights in the 30 000–50 000 range; they had a random microstructure, and they were stable above 300 °C. The copolyesters containing less than 30% of sugar-based units were semicrystalline and were found to adopt the triclinic structure of PBT. These copolyesters with low contents in Glux were able to crystallize from the melt but at lower rates than PBT. The T_g value of PBT steadily increased with the incorporation of Glux units in the polyester chain with an increasing ratio of ∼1.7 °C or ∼1 °C per %Glux point, depending on which unit, the diol or the diacid, was replaced. The copolyesters hydrolyzed at higher rates than PBT, and those containing glucarate units displayed an appreciable susceptibility towards biodegradation.

2,4:3,5-Di-O-methylene-D-glucitol (Glux-diol) and dimethyl 2,4:3,5-di-O-methylene-D-glucarate (Glux-diester) have been copolymerized with ethylene glycol and dimethyl terephthalate by polycondensation in the bulk to produce PET copolyesters as well as their respective homopolyesters. These sugar-based bicyclic monomers were synthesized from 1,5-D-gluconolactone, a commercially accessible compound derived from D-glucose. The PET copolyesters with either the diol or the diacid counterpart partially replaced by Glux had molecular weights in the 20 000–40 000 range and a random microstructure, and they were stable well above 300 °C. The PET copolyesters containing more than 10–15% of sugar-based units were amorphous while those displaying crystallinity were observed to crystallize from the melt at much lower rates than PET. The glass transition temperature of PET dramatically increased with the incorporation of Glux, whichever unit, diol or diacid, was replaced, but the enhancing effect was stronger in the former case. A preliminary evaluation of the mechanical behaviour of these copolyesters indicated that the genuine properties of PET were not substantially impoverished by the insertion of Glux. Compared to PET, the copolyesters exhibited a higher hydrolysis rate and an appreciable susceptibility towards biodegradation. The homopolyesters made of these sugar-based monomers could not be obtained with high enough molecular weights so as to be comparatively evaluated with copolyesters.

The extensive use of oral dosage forms for the treatment of diseases may be linked to deficient pharmacokinetic properties. In some cases the drug is barely soluble; in others, the rapid transit of the formulation through the gastrointestinal tract (GIT) makes it difficult to achieve therapeutic levels in the organism; moreover, some drugs must act locally due to a gastric pathology, but the time they remain in the stomach is short. The use of formulations capable of improving all these parameters, as well as increasing the resident time in the stomach, has been the target of numerous research works, with low-density systems being the most promising and widely explored, however, there is further scope to improve these systems. There are a vast variety of polymeric materials used in low-density gastroretentive systems and a number of methods to improve the bioavailability of the drugs. This works aims to expedite the development of breakthrough approaches by providing an in-depth understanding of the polymeric materials currently used, both natural and synthetic, their properties, advantages, and drawbacks.