## 1 Introduction  
Shape memory polymers (SMPs) have gained much attention owing to their special ability to return to their original shape upon external stimulation such as heating, light and chemicals [1,2]. The superior properties of lightweight, large shape recovery rate, low cost and easy processing as compared to shape memory alloys have promoted their rapid development and commercialization [3, 4, 5]. Shape memory polyurethane (SMPUs) are one emerging class of the SMPs [6, 7, 8]. They exhibit remarkable shape memory effect (SME), attributed to micro-phase segregation, which is interrelated to the chemical nature and processing methods used for synthesis. SMPUs typically consist of two phases: \"fixed phase\" and \"reversible phase\" which can also be termed as hard and soft segments, respectively [9,10]. The hard segments normally contain long sequences of hydrogen bonding sites and also serve as the physical cross-links [11]. These cross-link points prevent the neighbouring chains slipping from each other when it is subjected to deformation and consequential stress build-up [12]. These physical cross-links also act as net points during the shape recovery process. Therefore, the location of the cross-links is one of the most important factors affecting the SME and mechanical properties of the SMPs. Literature study shows that physically cross-linked SMPUs possess better thermal and tensile properties [13]. [PERSON] _et al._ found that SMPs with higher contents of cross-link points between the hard segments have higher shape recoverability than those with more contents of cross-link points at soft-hard segments [14]. The number of available physical cross-link points is related to the molecular weight, M\({}_{\rm w}\), and the high hydroxyl number, N\({}_{\rm OH}\), in the molecules (polyols and diisocyanates _etc._), more specifically the polyols used in synthesizing the SMPUs. The polyols with lower molecular weights and higher hydroxyl number (N\({}_{\rm OH}>50\)), normally have a larger number of cross-link points and stronger bonding owing to the short molecular chains, hence higher intermolecular interactions at soft segments, and _vice versa_ for the polyols with high molecular weights due to the limited number of cross-link points [18]. Hydroxyl number of polyols is another important parameter which could affect the shape memory effect of the SMPUs significantly. Polyols with a higher functionality are more reactive, and can produce more cross-links, hence better mechanical properties, and _vice versa_. Up to now, most SMPUs were synthesized using polyols with low molecular weight ranging from a few hundreds to a few thousand (200\(\sim\)3,000) g/mol and high functionality (N\({}_{\rm OH}>50\)). Although some SMPUs have been synthesized using high molecular weight polycaprolactone (PCL) (M\({}_{\rm w}\): 7,000\(\sim\)8,000 g/mol) polyol [15,16], polyethylene glycol (PEG-3400) and polytramethylene glycol (PTMG-2900) as soft segments [17] and other SMPUs with varying molecular weight with good shape memory effect [18,19], limited work has been done on SMPUs with high molecular weight polyols and low hydroxyl number, and little is known on how these affect the properties of the SMPUs. It is, therefore, scientifically interesting and important to investigate SMPUs with high M\({}_{\rm w}\) and low N\({}_{\rm OH}\) polyols. Furthermore, as the crystallinity, which affects the phase segregation significantly [13], of polyols withhigh and low molecular weight are different from each other, it is necessary to clarify how this affects the shape memory effects of the corresponding SMPUs due to their different crystalline natures. Several SMPUs were recently synthesized with five types of polyols all having M\({}_{\mathrm{w}}\sim\)2,000 and N\({}_{\mathrm{OH}}\) = 55-58 [20]. At optimal concentrations and combinations of diisocyanates hard segments, these SMPUs have shown excellent properties with a maximum strain up to 1,000% and a shape recovery ratio of 95%.