Incorporating sulfur atoms into the polymer main chain rationally introduces unique material properties and functions compared to their all oxygen counterparts. In particular sulfuration renders polymers more susceptible to photolysis and oxidation although investigations are usually limited to artificial conditions involving irradiation of polymer solution with intense UV light or exposure to excess chemical agents. Sulfur containing polymers such as polythioesters are usually synthesized via polycondensation routes or ring-opening polymerisation (ROP), yet these methods only give access to a limited number of polymer structures. Relatedly, ring-opening copolymerisation (ROCOP) of a strained heterocycle with heteroallenes is a popular method for the synthesis of heteroatom containing oxygenated polymers such as polycarbonates and polyesters which offers modular access to a wide range of polymer structures. Sulfur analogous are also accessible such as polythiocarbonates from the ROCOP of CS2 and polythioesters from the ROCOP of cylic thioanhydride (Figure (a)). Most relevant to our study are reports on phthalic thioanhydride (PTA)/epoxide ROCOP yielding semi-aromatic poly(thioester-alt-esters). Although alternating ROCOP was achieved, transesterification sidereactions, particularly at higher reaction temperatures, were occasionally observed to reduce the sequence selectivity and related side-reactions can reduce the maximally achievable molecular weights. This notion helps to explain why maximum obtainable molecular weights were limited to 65 kg/mol (Ð = 1.7). Relatedly, CS2/epoxide ROCOP was likewise plagued with numerous sidereactions. 34 Tackling this problem, we recently reported that a heterobimetallic Cr(III)K catalyst LCrK (Figure ) enables the selective copolymerisation of CS2 and oxetane (OX) to poly(trimethylenedithiocarbonate) in 99% polymer and up to 96% linkage selectivity. Unfortunately, the CS2/OX copolymers show low thermal stability and start to decompose at ca. 160 ºC and are highly susceptible to depolymerisation to cyclic dithiocarbonates limiting their use in material applications. Nevertheless, the suppression of sidereactions demonstrates that heterobimetallic catalysis could be a promising tool for the synthesis of sulfurated ROCOP polymers. This prompted us to investigate the copolymerisation of OX with PTA employing LCrK which we report in this contribution. Accordingly, we performed PTA/OX ROCOP with LCrK (Table ) at a loading of 1 eq. LCrK: 1000 eq. PTA: 1000 eq. OX at 80 ºC for 140 min producing a highly viscous mixture which solidified upon cooling. NMR analysis of the crude mixture reveals 99% PTA and OX consumption and a symmetric spectrum indicative of a highly regular polymer. 1D and 2D NMR as well as IR spectra conclusively show alternating esters ((C q ) = 166.5 ppm; ṽ = 1720 cm -1 ) and thioesters ((C q ) = 194.6 ppm; ṽ = 1660 cm -1 ) links making up the poly(esteralt-thioester) (ESI Section S3). No signs of ester-altester or thioester-alt-thioester links from trans(thio)esterification reactions as well as ether links from OX homopolymerisation can be detected. GPC analyses shows a bimodal molecular weight distribution with a Mn = 52.1 kg/mol (Ð = 1.4) which is somewhat lower than the theoretical Mn,theo. of 111.2 kDa assuming initiation from both OAc coligands of LCrK. MALDI-ToF mass spectrometry of a low Mn oligomer confirms the strictly alternating sequence and reveals -OAc, -OH functional chains (from OAc initiation and termination via chain-end protonation during work-up). Furthermore, the 31 P end-group test also confirms the formation of primary alcohol chain ends. However, chain-transfer processes with protic impurities as often observed in ROCOP cannot be ruled out to reduce molecular weights forming -OH functional chains which tentatively explain the bimodal molecular weight distribution. Next, we surveyed the effect of catalyst choice employing the sodium derivative LCrNa and the rubidium derivative LCrRb and found these to be less active but equally selective than the potassium derivative in the order LCrK (TOF 435 h - 1 ) > LCrRb (TOF 275 h -1 ) > LCrNa (TOF 112 h -1 ). To mimic the electronic and coordinative situation in LCrK albeit as distinct components, we then employed a bicomponent catalyst comprising a bismethoxy substituted (MeO)2SalCyCrOAc complex L'Cr (see ESI Section 2 for the structure) with KOAc@18-crown-6. This combination likewise showed excellent selectivity and activity. In contrast L'Cr or KOAc@18-crown-6 by themselves show no appreciable activity. Taken together our results indicate that ensuring heterobimetallic cooperativity employing Cr and K in combination rather than fixing both metals within the same scaffold is needed to achieve PTA/OX ROCOP. Having identified LCrK as a potent catalyst we decreased the catalyst loading to obtain higher molecular weight materials and indeed obtained an Mn of 65.8 kg/mol (Ð =1.2) from a run at 1 LCrK: 2000 PTA: 2000 OX and a maximum Mn of 139.0 kg/mol (Ð =1.3) from a run at 1 LCrK: 4000 PTA : 4000 OX. Note this Mn,max represents a more than two-fold improvement over ROCOP thioesters, while even for ROCOP in general few examples exist with Mn's exceeding 100 kg/mol. The materials are in all cases obtained as brittle powdery solids after precipitation. Differential scanning calorimetry reveals a melting point at Tm = 88.3 ºC in the first heating cycle, yet no crystallisation in the following heating/cooling cycles is observed indicating slow crystallization; glass transition occurs at Tg = 23.2 ºC. Previously semi-crystalline poly(thioester-alt-esters) from PTA could only be generated via stereocomplexation of variants derived from enantiopure epoxides. Thermogravimetric analysis reveals an increase in thermal stability with increasing Mn from Td,5% ca. 285ºC (Mn = 17.1 and 52.1 kg/mol) to ca. 330 ºC (Mn = 139.0 kg/mol). This improvement is more pronounced when considering the onset of thermal degradation revealing some decomposition to already occur at 150 ºC for the lower Mn samples while no decomposition occurs until 300 ºC for the highest Mn copolymer. In comparison, the related propylene oxide copolymers of PTA exhibit a thermal decomposition onset at ca. 200 ºC revealing an improvement of ca. 100 ºC compared to our PTA/OX copolymers which is enabled by the increased in Mn,max. compared to those. In terms of other material properties, uniaxial tensile testing of copolymer films reveals an increase in ultimate strength with molecular weight from σb = 2.4 MPa for 17.1 kg/mol to σy = 8.3 MPa (y = 5% strain) for 52.1 kg/mol to σy = 9.3 MPa (y = 8% strain) for 139.0 kg/mol. Furthermore, the samples become somewhat more brittle with increasing molecular weight showing decreasing b at 560%, 234% and 137% strain respectively. For comparison, commodity LDPE exhibits a σb = 12 MPa at b = 385% strain. We infer that the increase in strength in the higher Mn samples is a consequence of higher degrees of crystallinity which is also apparent from the increasing opaqueness of the highest Mn film. The latter could be also tentatively confirmed by WAXS revealing more features in the higher Mn samples although the broad nature of these prevents quantification of the crystallinity. Previously, sulfur containing links lead to oxidative and photodegradability benefits due to the lability of the C-S bond. Accordingly, we suspended the high Mn PTA/OX copolymer (Table run #8) in aqueous H2O2 for 1 week and observed complete degradation with no insoluble products remaining. H NMR in D2O reveals desymmetrisation of the initially highly symmetric starting spectrum due to the oxidative cleavage producing short chain oligomers. Likewise broadband UV irradiation of a CDCl3 solution of the copolymer for 1 week led to degradation into oligomers with Mn < 2.5 kDa; related all-ester ROCOP polymers remain intact under these conditions. As photooxidation and -degradation represent some of the primary breakdown pathways of polymer waste in nature prior to biodegradation we hypothesized that the thioester links could also enhance the degradability of our materials in a scenario closer to what happens with uncollected waste in the environment. Note that although management plans exist, approximately 22% of plastic is leaking into the environment as uncollected litter after use. Therefore degradability due to natural weathering factors (UV, heat, humidity, rain) was investigated using artificial weathering employing films of the PTA/OX copolymers (see ESI Section S4). The scenario being that the respective material was exposed outdoors to natural weathering lying on the ground. A cycle that incorporated UV, rain and dry phases and changing temperatures with a maximum of 30 °C was applied (adapted from ISO standard 4892-3:2016). The equivalent of the 336 hours artificial weathering resulting in an UV radiant exposure of 54 MJ/m 2 is about 4 months of natural weathering in Central Europe based on the UV radiative exposure. Note that lamps that don't show spectral contributions below the solar cut-off around 290 nm were employed to ensure comparability with irradiation by sunlight. Weathering led to a brown discolouration (Figure (a)) as well as increasing brittleness which caused cracking of some of the polymer films, the latter being potentially beneficial for the mechanical breakdown of the material in a real-world scenario. GPC analysis of the film (Table run #8, Mn = 139.0, Ð = 1.3) after weathering shows a decrease in molecular weight down to Mn ca. 9 kDa (Figure ) and a significant broadening of the weight distribution to Ð = 5.8 and together with the increase in brittleness this could indicated some cooccurring photo crosslinking of chains alongside degradation. Nevertheless, the samples remain soluble in organic solvents such as THF after weathering. The surface IR spectra (Figure ) reveal thioester groups (ṽ = 1660 cm -1 ) on the film surface are more affected by weathering than the ester groups (ṽ = 1720 cm -1 ). Accordingly control samples of commercial semi-aromatic polyesters (PET and PBT) show negligible weathering under identical conditions clearly highlighting the degradability benefits thioester links introduce. In conclusion, we have developed a new synthetic methodology to yield semi-crystalline poly(ester-alt-thioesters) from the copolymerisation of phthalic thioanhydride and oxetane under heterobimetallic Cr(III)K catalysis. Hereby we obtain a high maximum Mn of 139.0 kg/mol corresponding to improved thermal stability compared to lower molecular weight and related literature known polymers. Metal choice and cooperativity are key to maximize catalytic performance as individually Cr(III) and K complexes do not produce copolymer. Furthermore, moving from the lower to the higher end Mn materials reported in this study results in an approximate fourfold increase in maximum strength. Lastly, we found that the PTA/OX copolymers clearly benefit from the lability of the thioester bonds in terms of degradability under laboratory and environmental weathering conditions. Therefore, polythioesters could potentially be considered more sustainable materials than their all-oxygen counterparts which continue to contribute to plastic pollution.