## 1 Introduction  
The control of a material's mechanical properties with light, called the photomechanical effect, is becoming the focus of intense research due to its promise in directly converting light to work [1]. The origins of the idea can be traced to [PERSON]'s 19 th century work, who used photomechanical materials that convert light into sound to demonstrate a photophone [2; 3]. Photomechanics saw a rebirth a century later when [PERSON] used ceramic materials to make small light-actuated \"walkers\" [4; 5; 6; 7]. In addition to the promise of converting light directly into work--for example, to make motors [8; 9; 10; 11; 12; 13; 14]--materials that combine logic and actuation can underpin ultra-smart morphing technologies that leapfrog present-day capabilities [15; 16; 17]. As such, many investigations are focused on finding more efficient materials by studying the underlying mechanisms with the goal of using the fundamental knowledge that results to engineer better materials [18; 19; 20; 21; 22; 23]. The observation that azo dyes can fuel a photomechanical response [24] has made dye-doped polymers an attractive material class for applications. [PERSON] and [PERSON]'s classic experiments showed that photoisomerization of azo dye-doped elastomers is responsible for the photomechanical response [25], which has since been used to power such effects in more sophisticated materials [26]. All of these effects can be understood in models developed in the early 1990s [27]. Our work asks the question as to whether photoisomerization fuels the photomechanical response in dye-doped polymers. The answer is sought by determining the heating contribution, using parameters extracted from temperature-dependent stress measurements that are used as an input to our model of the photothermal heating mechanism. We apply an independent measurement of the tensor components of the photomechanical response, which differentiates between heating and photoisomerization-induced angular hole burning/reorientation. In addition, the measurements are repeated as a function of chain transfer agent (CTA) concentration during polymerization, which controls the length of the polymer chain to determine if chain length matters. The mechanical, physical and optical properties of polymeric materials depend on their molecular weight, which changes with chain length [28; 29]. Thus, the ability to control the chain length of the polymer in a polymer optical fiber (POF) is critical to its performance as a waveguide and its durability in the field [29]. We focus our thermomechanical/photomechanical studies on azo dye-doped polymers that are drawn into fibers to study these effects in the geometry used in POF technologies, using poly (methyl methacrylate) (PMMA) as the host polymer and the azo dye Disperse Red #1 (DR1) as a representative material commonly used in POFs [17]. Chain transfer agents (CTA) limit the chain length with increased concentration as quantified by the average molecular weight of the polymer [28; 29; 30; 31]. The best class of chain transfer agents for making PMMA by polymerizing methyl methacrylate (MMA) monomer are the thiols [28; 29; 31; 32], due to the weakness of the S-H bond [28; 29; 31; 33; 34] and the high reactivity of the thiyl radicals [28; 29; 31; 35; 36; 37; 38; 39]. The CTA molecule halts the growth of the polymer radical during polymerization [32; 40; 41], decreasing the molecular weight of the resulting polymer with increased CTA concentration. Many studies have shown the effect of chain transfer agent on methyl methacrylate-based polymers. [PERSON] et al. investigated the molecular weight and optical properties of PMMA samples using different types of CTA, including isopropyl alcohol(IPA), n-butyl mercaptan (n-BMC), and pentamethyldisilane (PMDS) [42]. They showed that an increase of the CTA concentration decreased the molecular weight of the resulting bulk PMMA material [42] The optical transmittance was observed to not be affected with varying CTA concentrations, nor was the refractive index [42].