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We demonstrate here seven pure cyclic samples, which belongs to two series of oligoesters, cyclic oligo(trimethylene terephthalate)s (COTTs) and cyclic oligo(butylene terephthalate)s (COBTs), showing odd-even effect of degree of oligomerization (repeating units number) on properties. The pentamer of COTT and trimer of COBT show much lower melting temperature, and the trimer of COTT and trimer of COBT show significant lower refractive index, which can be ascribed to the low packing density of cyclic oligoesters with odd number of degree of oligomerization. Our results reveal the discrete property change as a function of cyclic size of oligomers, which can be used to design materials with special properties.Recently, cyclic polymers have attracted great attentions due to their special topological structure with no chain end groups [17] . However, most of the reported cyclic polymers show no supreme property over their linear counterparts except some solution properties (viscosity etc.) [812] . This is due to the chain end groups' influence on the linear polymer properties (if not correlated to topology) can be neglected for polymers with relatively high molecular weights. As a result, the linear polymers show similar properties as cyclic polymers. Since the synthesis of linear polymers is much easier than that of cyclic polymers, finding where the cyclics show significant difference to linears is important for their applications.We believe the cyclic polymers with low molecular weights, i.e., cyclic oligomers, should be one of the key research directions due to the following reasons: 1) the difference between cyclic and linear oligomers should be significant, since the end-groups have significant influence on the properties of the latter [1318] ; 2) by careful purification process, pure cyclic oligomers with different degree of oligomerization (cyclic ring size) can be obtained. This excludes the effect of molecular weights distribution, which exists in all traditional polymers. These pure cyclic oligomers can be looked as the perfect samples to reveal the structure-property relationship in cyclic polymers, besides their "endless" topological structure which excludes the influence of chain ends.

A portion of gastric cancer patients with negative lymph node metastasis at an early stage eventually die from tumor recurrence or advanced metastasis. Occult lymph node metastasis (OLNM] is a potential risk factor for the recurrence and metastasis in these patients, and it is highly important for clinical prognosis. Positron emission tomography (PET)/computed tomography (CT) is used to assess lymph node metastasis in gastric cancer due to its advantages in anatomical and functional imaging and non-invasive nature. Among the major metabolic parameters of PET, the maximum standardized uptake value (SUVmax) is commonly used for examining lymph node status. However, SUVmax is susceptible to interference by a variety of factors. In recent years, the exploration of new PET metabolic parameters, new PET imaging agents and radiomics, has become an active research topic. This paper aims to explore the feasibility and predict the effectiveness of using PET/CT to detect OLNM. The current landscape and future trends of primary metabolic parameters and new imaging agents of PET are reviewed. For gastric cancer patients, the possibility to detect OLNM non-invasively will help guide surgeons to choose the appropriate lymph node dissection area, thereby reducing unnecessary dissections and providing more reasonable, personalized and comprehensive treatments.

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