in the solid state. [38,45] In general, the terminal relaxation behavior of PBT vitrimers above the melting temperature was rarely observed at angular frequencies down to 10 −2 s −1 in the temperature range from 250 to 270 °C. [37,38,45] Considering that typical shear rates during injection molding are as high as 10 000 s −1 and the viscosity of PBT vitrimers can be up to 10 7 Pa s at 250 °C, these PBT vitrimers cannot be processed like neat PBT during the short residence times typical for extrusion and injection molding. [52] Here, we propose a controllable way to process PBT vitrimers (based on transesterification exchange reactions) via controlling the network formation with the help of protection-deprotection chemistry. In this way, we can overcome the relatively long relaxation times and high viscosities associated with PBT vitrimers and maintain the high production rates of final parts by injection molding as for neat PBT. The strategy is schematically shown in Scheme 1: pentaerythritol is first converted into 5,5-bis(hydroxymethyl)-2-phenyl-1,3-dioxane (BPO, melting point 135-137 °C) via benzaldehyde protection chemistry. [53] Subsequently, this diol is incorporated into the PBT backbone via solid-state (co)polymerization to form a linear copolyester in line with earlier work from our group on PBT modification in the solid state. [54][55][56][57][58][59][60] The linear polymer chain is then transformed into a network via deprotection of the benzal group to afford a linear polymer chain with pendent-free hydroxyl groups for further thermal transesterification of the linear copolyester catalyzed by the Zn(II) catalyst. If deprotection and subsequent cross-linking takes place during processing, we can combine an initial low viscosity with final vitrimer characteristics.Normally, the deprotection step to obtain the linear copolyester with pendent hydroxyl groups, as shown in Scheme 1, involves an acid-promoted deprotection of the benzal group at room temperature and an acid used often is trifluoroacetic acid (TFA). [53] Accidentally we discovered that compression molding of the linear copolyester with benzal protection groups resulted in a cross-linked material with vitrimer characteristics, even without deprotection with TFA. This observation is in contrast to results previously reported by Collard et al., [53] who obtained thermoset materials after incorporation of BPO into PBT and poly(ethylene terephthalate) (PET) and repeated subsequent cycling to 300 °C in differential scanning calorimetry (DSC). Driven by this intriguing result and its practical relevance, we investigated the in situ network formation in more Vitrimers Although the network dynamics and mechanical properties of poly(butylene terephthalate) vitrimers can to some extent be controlled via chemical and physical approaches, it remains a challenge to be able to process PBT vitrimers with the same processing conditions via, for example, injection molding as neat PBT. Here, it is shown that the use of protected pentaerythritol as a latent cross...